# Dormer Ridge Board or Beam



## Francis Vineyard

Have plans reflecting this photo (taken from web) office discussing what's required here a 1x or 2x ridge board? We have 25 psf snow load

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/monthly_2010_12/Remodeling_015.jpg.74c190b4e0b15e81b3b5354d400ee588.jpg

/monthly_2010_12/Dormer018t.jpg.f80c730752c690d914cd5470c2d3c662.jpg


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## mtlogcabin

i think you have a bigger problem

 Where the roof pitch is less than three units vertical in 12 units horizontal (25-percent slope), structural members that support rafters and ceiling joists, such as ridge beams, hips and valleys, shall be designed as beams.


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## TJacobs

Picture does not look like a dormer to me...


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## Code Neophyte

Shed dormer?

I agree that it is a beam or girder requiring engineering, per §R802.3.1 "...where ceiling joists or rafter ties are not provided, the ridge formed by these rafters shall be supported by a wall *or girder designed in accordance with accepted engineering practice*


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## Jobsaver

Francis:

If the pitch on either side of the ridge is equal to or greater than a 3/12 pitch, the ridgeboard is sufficient. If less than 3/12, the "ridgeboard" needs to be designed as a beam.


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## steveray

Jobsaver....what is controlling roof thrust? I have seen alot of these...structural ridge required or other engineering..(diaphragm for the roof system?)


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## Jobsaver

Per R802.3.1 (2006IRC), collar ties are required, or, as you say, a structural ridge.

It is hard to determine, based on the submitted picture, what the plan is under and to the right of the ridge.


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## mtlogcabin

The original roof line looks like a 6/12 pitch. The shed dormer looks to be about 1/3 of that (2/12 pitch). I see a minimum 12 ft span on the ridge board. Beam required


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## dhengr

Francis:

This is my first post here, so I hope I’m not stepping on anyone’s toes, but my 8 cents worth....  I think you need engineering help on this roof job, and I don’t mean the loose talk you get on forum where no one knows what you’re really doing, want or need.  Get a local Structural Engineer involved, so you don’t end up in trouble with one of the regular members on this forum, on down the line, or worse yet make a real mess of someone’s house.

You’re way short of providing sufficient meaningful info. to have an intelligent discussion on your roof design problem.  Your thumbnails show three different things: the std. symmetrical gable dormer in the left sketch has identical pitch on both sides of the ridge board and ceiling joist as ties to take the rafter thrust at the top of the side walls; the shed dormer in the right sketch has an exterior wall and a significant header beam up near the main roof ridge to take the reactions of the low slope rafters which act much more like horiz. spanning fl. joists; the photo is a real bastardization which needs a ridge beam and its reaction posts and then, special engineering attention because the thrust from the two roof slopes is different and they don’t cancel each other out at the ridge, thus the ridge has a net lateral load, plus the vert. loads.  Each of these requires special attention to different engineering details by someone who can look at the building your starting with and properly advise you what needs to be done.  And, however complex and detailed and unintelligible we have made our building codes, of late, they just don’t cover all of these conditions in separate subsections, one at a time.


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## KZQuixote

From this simple carpenter's perspective here's what I see.

The one by ridge has been there since before Methuselah's time. Someone embarked on an addition of a shed dormer and added a few recycled 2X4's as braces to support the ridge, in addition, of course, of the new rafters.

Solution: My bet is that the 2X4 braces are bearing on a bearing wall below. How about require that each rafter be so braced and add some A35's to the ancient rafter to ridge connection. Cornsiderin that the ancient ridge is a one by, some power blocking ( Oregon definition ) would provide the necessary purchase for the hanger nails. This design would depend on a cantilevered support of the new roof, no problem from my perspective.

Bill


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## DRP

Shed dormer. Reference 802.3, Rafter ties/ ceiling joists in the lower third of roof height. If it has them, on the shed side they are at the bottom, on the right side they are in the upper third. I've built this with ties and a ridgeboard many times and in regions with a much higher snow load with no problem, we also used deeper rafters than structurally necessary. It works but it is not to current prescriptive code. If I were to do another it would be with a ridgebeam sized by the lvl supplier.

Just saw dhengr's response, and since i can't delete mine... yup, what he said.


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## brudgers

KZQuixote said:
			
		

> From this simple carpenter's perspective here's what I see. The one by ridge has been there since before Methuselah's time. Someone embarked on an addition of a shed dormer and added a few recycled 2X4's as braces to support the ridge, in addition, of course, of the new rafters.
> 
> Solution: My bet is that the 2X4 braces are bearing on a bearing wall below. How about require that each rafter be so braced and add some A35's to the ancient rafter to ridge connection. Cornsiderin that the ancient ridge is a one by, some power blocking ( Oregon definition ) would provide the necessary purchase for the hanger nails. This design would depend on a cantilevered support of the new roof, no problem from my perspective.
> 
> Bill


...and that's why there is a code which often requires a licensed design professional.


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## GHRoberts

I will agree that pictures of different areas would help. The plans would also help.

There might be an argument made that the original roof used the floor joists as ties - allowing a ridge board. Might even be able to continue the argument to say that the new rafters allow for a ridge board.

But without more knowledge of the construction, who is to say.


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## DRP

This was drawn by an engineer, built in 20 psf snow country... although he also specced the let in bracing, we beefed that with ply corners later and didn't tell;







The missing rafter bays have logs sitting on the plates and around the steep rafters at the 8' level as ties. The shed rafters sit on flattend section on the log ties' tops and framing angles secured them. Ties were bolted w/ 1" bolts and the connection from tie to wall was a heavy angle bolted down. 2x ridgeboard. The one above was built around '95 and we spoke with the owner a couple of nights ago. The house is still doing fine. Wind on the steep roof of that one is probably much higher loading than the vertical loads. I have done plenty with a conventional stick framed ceiling though. I believe those were all drawn by designers. I can think of two with lvl ridgebeams. One had tji rafters, the other hung trusses from the ridge. Neither of those had ties. I suspect the shed roof diaphragm is sufficient to take care of the lateral loads.


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## TimNY

As long as the rafters are directly opposing I don't think the board is the issue.  As suggested by others, the ridge needs to be supported as a beam.  I don't think 2x would suffice as a beam any more than 1x.

Hangers don't look to be skew hangers.. fail.

No comment on the sidewall framing for the shed dormer..


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## Code Neophyte

Maybe if they shot a few dozen more nails into the tops of those.... king...posts....brace...things...???  Think that would help??


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## Yankee

DRP said:
			
		

> Shed dormer. Reference 802.3, Rafter ties/ ceiling joists *in the lower third of roof height*. If it has them, on the shed side they are at the bottom, on the right side they are in the upper third. I've built this with ties and a ridgeboard many times and in regions with a much higher snow load with no problem, we also used deeper rafters than structurally necessary. It works but it is not to current prescriptive code. If I were to do another it would be with a ridgebeam sized by the lvl supplier.Just saw dhengr's response, and since i can't delete mine... yup, what he said.


I don't see the bolded wording in your quote in the code. Please instruct.


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## texasbo

I agree ties required or ridge beam. Are the ceiling joists adequate as floor joists, or were they replaced?


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## TimNY

Yankee said:
			
		

> I don't see the bolded wording in your quote in the code. Please instruct.


Prescriptively speaking, Tables R802.5.1 footnote a limits the tables to the rafter ties/joists being in the bottom 1/3 of the span.

Not to say you can't do it some other way, but you are outside the prescriptive design. I'm looking at our code based on the 2006 IRC.

Pitch is still a problem.


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## brudgers

DRP said:
			
		

> This was drawn by an engineer, built in 20 psf snow country... although he also specced the let in bracing, we beefed that with ply corners later and didn't tell;
> 
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> 
> The missing rafter bays have logs sitting on the plates and around the steep rafters at the 8' level as ties. The shed rafters sit on flattend section on the log ties' tops and framing angles secured them. Ties were bolted w/ 1" bolts and the connection from tie to wall was a heavy angle bolted down. 2x ridgeboard. The one above was built around '95 and we spoke with the owner a couple of nights ago. The house is still doing fine. Wind on the steep roof of that one is probably much higher loading than the vertical loads. I have done plenty with a conventional stick framed ceiling though. I believe those were all drawn by designers. I can think of two with lvl ridgebeams. One had tji rafters, the other hung trusses from the ridge. Neither of those had ties. I suspect the shed roof diaphragm is sufficient to take care of the lateral loads.


"We've been doing it that way for years, and nobody has been killed."


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## Mule

dhengr said:
			
		

> Francis:This is my first post here,


Welcome to the forum!

Please continue to give us your input. A fresh set of brains is always welcome. And don't worry about toes.......we are used to people stepping on more than our toes!


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## Yankee

TimNY said:
			
		

> Prescriptively speaking, Tables R802.5.1 footnote a limits the tables to the rafter ties/joists being in the bottom 1/3 of the span.Not to say you can't do it some other way, but you are outside the prescriptive design. I'm looking at our code based on the 2006 IRC.
> 
> Pitch is still a problem.


Just so it is clear, all ceiling joists acting as rafter ties attached anywhere above the rafter support wall need their span shortened by the amount in the table referenced at the bottom of table R802.5.1, , , and that table allows the adjustment a tie to a maximum of 1/3rd the height to the ridge.

Contractors use that "1/3rd" statement a lot without making the span adjustment required.


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## KZQuixote

Francis Vineyard said:
			
		

> Have plans reflecting this photo (taken from web) office discussing what's required here a 1x or 2x ridge board?We have 25 psf snow load


Hi Francis,

"...Photo ( taken from web )..." Am I to understand that this is an academic exercise or is the photo of a job you're actually dealing with?

Thanks

Bill


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## Francis Vineyard

Apologize for not posting drawings from the plans; I’m not computer savvy enough yet to reduce resolution to the download limit. I kicked back the plans for not giving the existing roof detail and ceiling floor joist size and wanted to be prepared for this next step.

The 8 ft. wide dormer is for headroom of back to back tub/shower in the center of a 64’ x 40’ house.

R802.3 in part says “Where the roof pitch is less than three units vertical in 12 units horizontal, structural member that support rafters and ceiling joists, such as ridge beams, hips and valleys, shall be designed as beams.

However;

R802.3.1 “Where ceiling joists or rafter ties are not provided, the ridge formed by these rafters shall be supported by a wall or girder designed in accordance with accepted engineering practice. 

I was in agreement with the majority posted here but talking with a couple of local hammerheads (a new word I learned for the trade dictionary) they routinely attach to the 1x ridge board in this manner which is telling me other jurisdictions have been approving this installation. I need proof not just because that's how we’ve always done it! 

Reference: Southern Pine Maximum Spans guide;

"Ridge beams must be installed at roof peaks with rafters bearing directly on the ridge beam or supported by hangers or framing anchors. Ceiling joists are not required when properly designed ridge beams are used.

A ridge board may be substituted for a ridge beam when the roof slope equals or exceeds 3 in 12, except that ridge beams are required for cathedral ceilings. Rafters must be placed directly opposite each other, and ceiling joists must be installed parallel with rafters to provide a continuous tie between exterior walls".

Guess really what I was asking if this type of dormer would be framed similar to floor openings as what is shown in the R/H drawing sourced from “Building Construction Illustrated; Francis D. K. Ching and Cassandra Adams? Where a header is sized as a beam supported by the double rafter trimmers. But is drawing is not showing this.

Welcome dehngr, ditto what Mule said. I must be a glutton for punishment since I cannot stop learning my lessons here.


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## Francis Vineyard

KZQuixote said:
			
		

> Hi Francis,"...Photo ( taken from web )..." Am I to understand that this is an academic exercise or is the photo of a job you're actually dealing with?
> 
> Thanks
> 
> Bill


Bill,

This is as close of a picture I could find to try and show what's on the plans.  Found it on Fine Home Construction Forum (I think) but I wasn't looking for the picture as I was doing the research.


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## texasbo

Francis Vineyard said:
			
		

> Apologize for not posting drawings from the plans; I’m not computer savvy enough yet to reduce resolution to the download limit. I kicked back the plans for not giving the existing roof detail and ceiling floor joist size and wanted to be prepared for this next step.The 8 ft. wide dormer is for headroom of back to back tub/shower in the center of a 64’ x 40’ house.
> 
> R802.3 in part says “Where the roof pitch is less than three units vertical in 12 units horizontal, structural member that support rafters and ceiling joists, such as ridge beams, hips and valleys, shall be designed as beams.
> 
> However;
> 
> R802.3.1 “Where ceiling joists or rafter ties are not provided, the ridge formed by these rafters shall be supported by a wall or girder designed in accordance with accepted engineering practice.
> 
> I was in agreement with the majority posted here but talking with a couple of local hammerheads (a new word I learned for the trade dictionary) they routinely attach to the 1x ridge board in this manner which is telling me other jurisdictions have been approving this installation. I need proof not just because that's how we’ve always done it!
> 
> Reference: Southern Pine Maximum Spans guide;
> 
> "Ridge beams must be installed at roof peaks with rafters bearing directly on the ridge beam or supported by hangers or framing anchors. Ceiling joists are not required when properly designed ridge beams are used.
> 
> A ridge board may be substituted for a ridge beam when the roof slope equals or exceeds 3 in 12, except that ridge beams are required for cathedral ceilings. Rafters must be placed directly opposite each other, and ceiling joists must be installed parallel with rafters to provide a continuous tie between exterior walls".
> 
> Guess really what I was asking if this type of dormer would be framed similar to floor openings as what is shown in the R/H drawing sourced from “Building Construction Illustrated; Francis D. K. Ching and Cassandra Adams? Where a header is sized as a beam supported by the double rafter trimmers. But is drawing is not showing this.
> 
> Welcome dehngr, ditto what Mule said. I must be a glutton for punishment since I cannot stop learning my lessons here.


Technically, Ching's illustration does not comply with the code, unless the ridges are designed as beams. By the way, Ching's BCI was required for my freshman year in architecture school.


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## Mule

Yankee said:
			
		

> Just *so it is clear, all ceiling joists acting as rafter ties attached anywhere above the rafter support wall need their span shortened *by the amount in the table referenced at the bottom of table R802.5.1, , , and that table allows the adjustment a tie to a maximum of 1/3rd the height to the ridge. Contractors use that "1/3rd" statement a lot without making the span adjustment required.


Not necessarily true. If you calculate out some of the installations, sometimes the rafters are adequate without increasing their size.


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## Mule

I don't know if I accidently closed this thread. If I did I screwed up! I am going to open it up because I see no reason to close it. If it was closed by another moderator I apologize for overstepping my authority but I do not see a reason for closing it!


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## DRP

> "We've been doing it that way for years, and nobody has been killed."


The "we've" in this instance is the industry as far as I can tell, I've gotten this detail many times and as I pointed out this was engineered. Please explain the reason or math that makes it fail in your opinion. This was prescriptive to my understanding up until the '06 cycle.

Very often the rafter depth is determined by insulation requirements, that was my comment on them often being deeper than structurally necessary. In the pic I posted  also notice where the tie is joining the main rafter. I'd agree with Mule here.


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## Francis Vineyard

Mule said:
			
		

> I don't know if I accidently closed this thread. If I did I screwed up! I am going to open it up because I see no reason to close it. If it was closed by another moderator I apologize for overstepping my authority but I do not see a reason for closing it!


Mule, there has been much worst things said, so I don't think an apology is needed from you.

I'm of the mind that the IRC is prescriptive and when it states "designed in accordance with accepted engineering practice" means what is accepted as standard in the field, not to "support your local engineer".

brudgers what are those notches on top of the rafter?


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## dhengr

Francis:

Reread my earlier post; the shed dormer roof is framed more like a floor, at or around a large opening in the main roof, with the shed rafters acting more like fl. jsts.  The low slope rafters would cause too much thrust if we expected them to act in the trussing fashion as we normally think of for a normal pitched roof; that is rafters for top chords and ceiling jst. tension ties as bottom chords.  The shed rafters are supported on the exterior wall on one end, and on a properly designed header on the other end, up near the main roof ridge.  This header then spans to the dormer side walls which are picked up by the new doubled or tripled main roof rafters (you said rafter trimmers) which support both the side wall and the new header and span up to the main roof ridge.  Someone has to make a conscious design decision about sizing these new primary spanning members and carrying their new concentrated reactions down to the found., and this all has to be done paying attention to what this does to the existing structure.  The 3/12 rafter slope is not a magic number, it’s just a good place to draw the line in a prescriptive code.  Think of it this way: studs and columns are one extreme, infinite pitch, and no lateral thrust on the wall below; a 24/12 pitch rafter acts mostly like a column, but causes some thrust; fl. jsts. are beams, zero pitch, and no thrust on the wall below; and as you start to increase the pitch, the member still acts as a beam, and the reaction seats must be horiz. so the reactions are vert., but if we try to make it act as a normal stick framed gable roof the thrust is almost infinite, or the tie tensions are almost infinite, this the 3/12 cutoff.

Because I’ve done enough of these my rule of thumb is, but NOT to be construed as your absolute certified design criteria: on an 8' wide dormer, I generally think in terms of moving the old rafters I’m removing out under the side walls of the dormer, I still have basically the same roof LL, but some additional DL for the new constr., so I must need at least the same amount of framing (rafter type beams) in the main roof.  For an 8' wide dormer, with 16" o/c main rafter framing that would mean removing 5 old rafters, so being the conservative old guy that I am, I’ll put 3 ganged rafters under each side wall.  Every experienced carpenter know and does this.  Except, there are a hundred little details which come into play; what are the existing roof conditions, we have unbalanced rafter thrusts up at the main ridge now, when does that new upper header start to get to long and when do the various concentrated reactions get to large?  Does my rule of thumb start to go to hell at 10 or 12' wide dormers?  Someone has to make that determination, and while you shouldn’t be doing the builder’s engineering for him, and the code doesn’t tell you everything for every situation, you have to have enough structural intuition, if you’re going to be a good builder or bldg. inspector to know when to call for help.

George and I can’t compete with the lumber yard when they give away member sizing to sell the material.  But, read the fine print on your bill or on the computer printout they give you, they assume absolutely no responsibility for your design.  You gave them all the loads, or maybe not, you know the framing details and connections, or maybe not, they don’t, so ya pays your money and takes your chances.  A good structural engineer is your best friend, and they like talking about this stuff; hire one once in awhile, or take one out for lunch, they can really be helpful without necessarily breaking the bank.  You young inspectors and builders would do will to develop a meaningful relationship with a good engineer.  We can’t give it all away, after all we make our living selling our services, knowledge and experience, but a few minute question on the phone or some advice over a beer or two usually doesn’t cost you much.


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## dhengr

DRP..... The idea that ‘we’ve been doing it that way for years’ doesn’t make it right.  It might just be that they have been very lucky doing it wrong, or at least not the best way.  Likely, they have never seen the max. design wind, EQ, or LL, and gotten by on the code’s factors of safety.  That may be that builder’s norm, but that doesn’t make it an industry std., and many times it’s far from a good industry std.  I would say the IRC is the industry std. without much need for additional engineering.  I can push thing further, many times, when I use IBC on your project.  I must say I’m not sure exactly what detail you’re talking about,  although I’ve heard “we’ve always done it that way” hundreds of times when I commented on a bad construction detail or condition.

Francis....  The IRC is most certainly a prescriptive code.  It has been written and shows tables, values and sketches, etc. which have been fairly conservatively engineered to keep you and builders out of trouble, in most cases; as long as the follow it, without stretching it, and read and understand all the footnotes, commentary and fine print.  It generally follows the IBC and well tested construction and engineering practice but is somewhat conservative, because it is recognized that there might not be much engineering oversight on most of the projects.  My interpretation of "designed in accordance with accepted engineering practice" when used in the IRC means engineering beyond that already provided in the IRC, and that you do need an engineer’s involvement.  Std. industry practice and details usually don’t require much additional engineering.  The notches in the top of the gable rafter are to receive look-out cantilever members to frame the rake ladder framing in the same plane under the roof sheathing.

Mule....   Thank you for the warm welcome, and now you see what was blocked, thanks for the unlock.  Certainly no apology required, I’m just a confused beginner on your forum, and not a particularly savvy internet and computer user.  I do hope to be a constructive contributor.


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## Francis Vineyard

dhengr

I apologize for not making it clearer that the picture is not the job I was working on but very similar to what was shown on the plans and questioning how to determine the proper size of the ridge board base on what is standard practice.

Your first post through me off a little calling out the R/H sketch as a beam when it shows a board; but as texasbo stated the sketch is not code. I believed it was because of what I heard in the office before posting. 

I understand your explanation about rule of thumb; it applies with the prescriptive method as where not all construction is back and white. One area the code does not address to my knowledge is headers for non-bearing walls and exterior non-bearing walls supporting more than one story. An inspector turned these down and an engineer called back wanted to know what the code prescribes and then approved them. That does not mean I can take this as standards of practice. But I can use better judgment next time on what an engineer may approve. This involves judgment as you say on where to draw the line.

You’ve presented an opportunity for another topic I’ve wanted to bring up, appreciate all the inputs thus far.


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## Jobsaver

dhengr: Welcome to the forum! Already, you are a constructive contributor.

As you say, developing an appropriate sense of when to seek help is a very important part, both of building, and, of performing building inspections. I think this conceptualization of our jobs, (municipal and independent inspectors), defines our place accurately. There are posts in these forums that suggest that our jobs as inspectors do not, or rather, should not include offering building solutions. But, I offer solutions all the time based on my knowledge of the building code, and, repetitive exposure to engineered solutions.

The idea you express concerning supply sizing members, and, "so ya pays your money and takes your chances" best defines in this forum the design professional.

The design professional takes legal responsibility for his building advice. The rest of us are simply giving our building advice.


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## dhengr

Francis.....    I did understand that the photo in your OP was not from your actual job, but rather an approx. look-alike.  I do need to see the real plans to start to comment meaningfully, rather than in generalities, I won’t normally do engineering over the phone or on a forum.  With either you or a contractor, once we have worked together for awhile, so I know what you know, I know the quality of your workmanship, and know how you interpret what I tell you to do, by my struct. plans and details, a quick sketch or verbally, then I may do some quick engineering by phone to keep you working, but I will still want a follow up hard copy of what you are doing, for the file and so I know you understood.  There’s no other way for us to run our respective businesses these days, and to stay on the straight and narrow.  I believe I said, or meant to say, the left sketch you posted, a std. symmetrical gable dormer can usually get buy with a ridge board, as long as the rafters line up and there are ceiling jst. ties.  And, the R/H sketch, a shed dormer does need a beam, a header up, near the main roof ridge to pick up the shed rafters, which at that low slope ack like simple beams or fl. jsts.

Regarding ‘headers for non-load bearing walls’....   headers, are headers, are beams, whatever you call them.  I need to know the span length and loading to design a beam; and then I am concerned about bending and shear stresses, proper beam bracing, reactions and supports below, and deflection for serviceability.  You’ve got to develop an eye for the loads and load paths.  Most header tables assume generally uniform loads from above, if you call cripple studs and the fl. or roof above as a uniform load, not completely unreasonable in my world.  What you need to look for is that the jambs from openings above all line up vertically so one of them doesn’t put a large concentrated load at the middle of a header below.  On gable ends, smaller headers often can be used, but most carpenters tend not to do that out of expediency; they have one guy making up headers, jack studs and cutting cripples by the hundreds, and they don’t want to fart around with different depth headers, or don’t have the smaller 2x’s on the job.  But, on lightly loaded headers they could use a single 2x8 or 2x10 instead of double 2x’s.  But, this presents a problem for them too, since now they need some different sized ripped blocking for sht.rk. or siding nailing at the header.  If headers is your new question, lets broach that in a new thread.  Ten years of experience provides a whole bunch of “better judgement,” so keep at it.

Building is no longer black and white, and you young code inspectors have a hell of a lot to learn about the practical nature of the process; and then you have the cook book, we call it a code, to memorize, letter for letter, without a good basic understanding of where the letters really came from, or a good understanding of the general structural concepts.  This is not to damn you, it is to damn the excessive complexity that is showing up in our codes today.  You must understand the basics, because there is no way we can possibly write a cook book to cover everything, all the time.  But, that is what they seem to be trying to do.  Half the time we are arguing on these forums about how much is a pinch of salt, but nobody knows what the hell we are really cooking.  These considerations are what make older engineers who’ve actually designed bldgs. such good plan checkers and older builders such good inspectors, that experience and basic knowledge; otherwise we’re all kinda in the same boat trying to interpret the complexity of the latest codes.

Jobsaver....   I don’t compete with you guys, most of my bldg. inspecting is after the fact, after the problems have developed, and now how to fix them or who’s at fault and gets to pay to fix them.  Knock wood, I’ve never had to defend any of my own work anywhere near a court.  I think you do have the right idea, we are paying you to keep it safe and done correctly, and to use your experience and judgement, and knowledge of the code, to be as helpful as you can while enforcing the bldg. code in a pleasant way when that is possible and practical.  Sometimes you do have to be a dink to get your point across, but that or playing God should not be your general rule.  You should offer solutions, within the code and your experience, why not, why make life miserable for everyone; you should not be doing much real engineering if you aren’t one; but you do have one advantage as you suggest, and that’s the difference btwn. my “legal responsibility” and your “advice.”  It really boils down to the fact that none of us should be giving advice if we are not qualified in that area.  You should get to know a good Structural Engineer, they can really be your best friend, when you’re not fightin with them.


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## mtlogcabin

dhenger

Welcome

Well said

WOW


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## jar546

mtlogcabin said:
			
		

> dhengerWelcome
> 
> Well said
> 
> WOW


I was so impressed with his reply that I promoted it to an article.


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## GHRoberts

dhengr mentioned diaphragms in another thread.

Many people have mentioned the thrust of the rafters against the shed roof.

One aspect of a diaphragm is that it can be used to control thrust. In this particular instance the shed roof diaphragm transfers much of the thrust to the dormer end walls and then to (I guess) the floor joists.

It is east to say a certain construction is not-prescriptive. It is much harder to say the construction is poor. (I would like to be critical about the attachment of the shed roof to the ridge board, but I am having a hard time in finding something that provably wrong. Seems to be a common position I find myself in.)


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## dhengr

Thank you to everyone for the warm welcome.  And, Jar546 (Jeff, is it?), thank you for the tip-of-the-hat, I hope that doesn’t end up in the Federal Register, and/or ever come back to haunt me.

GH....    I often find myself in the same situation, and you said;    “(I would like to be critical about the attachment of the shed roof to the ridge board, but I am having a hard time in finding something that provably wrong. Seems to be a common position I find myself in.)”    After years of experience, gut feeling, intuition, etc. make some things pop out when I see them, ‘I don’t like that, there is something wrong with that detail, I can’t put my finger on it at the moment, let me think about it for awhile.’  And, if I can’t rationalize how it works and put some numbers on it, to my own satisfaction, I won’t sign off on it.  After all, I am the one who will have to defend my work and detail if things don’t work, so irrespective of the IRC or IBC, I have to be confident of my solution, by ‘applying rational and acceptable engineering methods and standards’ (from another thread).   We might change it, to our mutual satisfaction, maybe not really to your liking, or you have the option of finding someone else to approve it and sign off on it.  In my mind, it’s got to be ‘provably right’ or it’s wrong; to that extent I would change your wording.

The photo that came with the OP was a cobbled mess, about a 20' long change in a light ridge board and roof system.  I’d have to look long and hard at that structure, and we’d probably change some things based on what I could see.  The later photo by DRP, of new construction, certainly looks like I could make that work.  As he suggested, added shear panels at corners, good new roof sheathing diaphragms, etc., we have a chance to make adjustments to the structure to account for all of the loads and reactions and unbalanced roof load conditions.  The std. shed dormer we’ve been talking about, in the 8 - 12' wide range, I’ve seen work often enough that it’s pretty routine.  Wider than that requires a bit more design and attention to detail.  We all have a fair handle on how a std. gable roof works; imagine a cross section through the bldg., matching rafters aligned put no lateral bending in the ridge board, thrusts cancel each other out at the ridge; ridge board primarily holds rafters aligned, braced and properly spaced, and that’s about all; clg. jsts. act as tension ties for thrusts at lower brgs. of rafters.  It’s a simple triangular trussed framing system, very efficient and stable.  Now, plumb cut the existing rafters on the left roof plane, 18 or 24" below the existing ridge and install an appropriate header at this elevation, and tie this into new rafter/girders at the dormer side walls.  Quality of joinery and connections is very important here, this new framing and attachment to existing framing should not be a cobbled mess.  If you attach the new sheathing, only 24" wide, to the ridge and the new header and the other framing, we have a 24" deep beam or diaphragm, in the plane of the old roof.  We will try to make this take the unbalanced thrusts at the ridge out to the dormer side walls and back into the original roof system.  But, by making this 8 - 12' wide cut, out of the existing roof plane, we are concentrating all of the various loads and forces from the original and the new roof systems at the side walls of the dormer.  It’s not always easy, and if you do a crappy job of plumb cutting the old rafters, so they don’t fit-up to the header, or get yanked around, and loose, up at the ridge, you are starting to defeat what we have to accomplish.

It takes a fair amount of thought to really go through this problem a step at a time and really follow the load paths.  The fact that many cobbled together jobs are still standing is more a testament to code factors of safety, material allowable stress FofS’s, and the fact that we don’t often see the max. design loads, than it is proof that poor construction or bad details do work, just because we’ve always done it that way.

Merry Christmas to all,   Dick


----------



## DRP

If Francis doesn't mind a hijack, I think a sketch might help move us along;
	

	
	
		
		

		
			





This is a 12/12 main roof and a 3/12 shed dormer. The building is 24' wide, the peak is centered. The tie is well connected.Why does this require a ridge at all?I plan on sheathing the dormer support wall and main roof but plan to skip sheath the shed roof. I'll let you plug in your own variables, please show me why this wouldn't work or what you would do differently.

View attachment 311


View attachment 311


/monthly_2010_12/572953bc65c93_sheddormer.jpg.cccb7703ab3c57e700e76c6bade6f465.jpg


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## Francis Vineyard

DRP said:
			
		

> If Francis doesn't mind a hijack,


You're wrong, that is not a hijack!  Just taking it a step further on the path to enlightenment.

I believe that is showing a conventional framing of a dormer shed roof similar to one the AWC Framing Manuals


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## steveray

Your 12/12 rafters need to take a span reduction according to the tables in the code(don't have it here on Xmas morning)  if they work....they work! They usually don't...at least not around here!  Also around here, the roof insulation drives the rafter size most of the time......


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## peach

that could have been my prior home.. very typically done to get a little more room in the "attic" space.. usually for a bathroom, or walk in closet.


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## Yankee

steveray said:
			
		

> Your 12/12 rafters need to take a span reduction according to the tables in the code(don't have it here on Xmas morning)  if they work....they work! They usually don't...at least not around here!  Also around here, the roof insulation drives the rafter size most of the time......


How does the span reduction table work in this case?


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## peach

in 12/12 probably not, yankee.. the new dormer has a flattened pitch, so it does.. the floor joints (which are the ceiling joists before you build the dormer) are the stablizing force. For the "new" joists, collar ties or ceiling joists are required.. unless the ridge board is acting as a beam (which is probably is). Nothing says the beam needs to be anything more than a 1x but you need to consider it.


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## DRP

steveray, I agree, insulation is almost always driving the main rafter depth, for new construction this is always a 2x12 from my experience.

Thinking more;

The main rafter is tied at its' foot. The dormer rafter is tied at its' foot. The footnote table for raised ties doesn't really apply, there is no raised tie in the sense the table was meant to address. Prescriptively I could argue the dormer tie forms a collar tie on the main rafter in this case but that would be outside of the intent. The dormer tie is adding bending stress to the main rafter that the rafter span table does not address but less than the raised tie footnote is trying to resist, and we are higher than the current table's prescription.


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## GHRoberts

DRP said:
			
		

> Prescriptively I could argue the dormer tie forms a collar tie on the main rafter in this case but that would be outside of the intent. The dormer tie is adding bending stress to the main rafter that the rafter span table does not address but less than the raised tie footnote is trying to resist, and we are higher than the current table's prescription.


In applying the prescriptive codes one does not mention stress or strain. They are only used after a design is determined to not be prescriptive.


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## DRP

Lets get back on track, I'm more interested in structure.

This is a 12/12 main roof and a 3/12 shed dormer. The building is 24' wide, the peak is centered. The tie is well connected.







Why does this require a ridge at all?

I plan on sheathing the dormer support wall and main roof but plan to skip sheath the shed roof. I'll let you plug in your own variables, please show me why this wouldn't work or what you would do differently.


----------



## GHRoberts

DRP ---

No ridge is needed. The usual reason for a ridge board is to provide some stability during construction and a surface to hold the nails.

The problem with giving an answer is that I don't do designs like that and I have no prepared engineering that would indicate what issues are important. I would look for people who have engineered salt-box designs (New England).


----------



## Yankee

DRP said:
			
		

> Lets get back on track, I'm more interested in structure.This is a 12/12 main roof and a 3/12 shed dormer. The building is 24' wide, the peak is centered. The tie is well connected.
> 
> 
> 
> 
> 
> 
> Why does this require a ridge at all?
> 
> I plan on sheathing the dormer support wall and main roof but plan to skip sheath the shed roof. I'll let you plug in your own variables, please show me why this wouldn't work or what you would do differently.


If there is no ridge board then each set of rafters will be considered a (separate) truss, and engineering is required for trusses.


----------



## DRP

Lets take care of structure first. I was establishing whether there needs to be a structural ridgebeam.  A conventional stick framed rafter set is a truss. A ridgeboard, as George pointed out, is a framing aid. A ridgeboard is not structural. I have almost always used a ridgeboard to ease assembly, but not always. R802.3, first sentence, a ridgeboard is not required.

I don't want to get too far off track, but a little history. Light timberframe "common rafter" construction often used an open mortise and tennon or half lap joint at the peak, obviously with no ridgeboard. This later became a butt joint with a nailed gussett. Since they were using board sheathing they didn't need a ridgeboard, one or two rows of sheathing held things in place as they assembled each pair. From observation I know these were sometimes tipped up as an assembly. Our code will still allow me to gusset a butt jointed rafter pair and board sheath it.

Francis was correct, the detail I drew is also in the WFCM and AWC's "Details for Conventional Wood Frame Construction"... it is industry standard construction. I've been rather suprised that it seems to have fallen off of everyone's radar.


----------



## dhengr

I think Yankee has it just about right.  And, I think DRP is the one who is getting off track here because he is not taking care of or understanding the structure first.  Despite the history lesson there is very little acceptable engineering thinking going on above.  All George said was he doesn’t think a ridge board is needed, and that he probably wouldn’t be inclined to do a design like that.

Francis....  I would say DRP is taking things much further than one small step beyond a std. shed dormer.  Why are rafters to be treated differently when the pitch is below 3/12, and is 3.08/12 really any different than 2.92/12 in this respect?  Show me the code sec. on that issue, and explain the problems associated with low pitch rafters.  Should the ridge line brg. on this rafter be plumb cut or have a horiz. seat cut, and what’s the diff.?  Jobsaver was just asking about the need for more rules of thumb, to expand the limits of the IRC a little or to cover things the IRC doesn’t explicitly cover.  DPR’s stretch here is a fine example of why poorly explained rules of thumb without some very specific limitations are dangerous, in the wrong hands.  The idea that this roof geometry works in a 10,12 or 14' wide shed dormer, and then still works without any changes in thinking when you extend the dormer width to 30 or 40' and then only partially sheath the dormer roof, so it may not be a particularly good diaphragm, takes a real leap of faith in the nonexistence of gravity.

DRP.....  Yes,   That’ll work just fine, if done right.  I’m sorta paraphrasing here, but you probably want to look in the mirror and see what engineering knowledge and experience is looking back at you, then you want to sit down and actually engage your thinking cap, and apply all of your best accepted engineering practices to the problem.  Answer the questions below, I’d have to if I were doing the design, and you can save some engineering costs by gathering this kind of info. for me.  Your answers will help us determine how acceptable your accepted engineering practices are, and then we’ll talk about whether you need a licenced engineer or not, before you go for a permit, because it’s a stretch for the IRC to cover this, I suspect.  By the way, I kinda think you took it on the chin from both the engineer and the BO on that deck beam, but in their defense, decks have been such a liability of late that everyone is skittish.  If we had worked together long enough, so I knew what you know, and I worked in that area so I knew the BO, I would think we could have worked that out with a phone call or a letter from me.  But, you also have to understand that you are asking me to take on a potential million dollar liability for a few hundred dollar fee, and you grumble at that, and my insur. co. wants all the fee as premiums.  The questions:

1.  What are your roof LL’s (snow and drifting, I assume) and what does your code say about snow drifting and load reductions as a function of roof. slope?  Wind and EQ loads?

2.  What is the makeup of the roof system, starting with the sht.rk. and working outward in order, and what is the DL in #/sf in the plane of the roof?  Any other roof or clg. loads?

3.  What species and grades of lumber are you using?

4.  What sizes and spacing are the various members in your bldg. section, per your accepted engineering practices?  And, show me how you arrived at those sizes.  The 17' rt. rafter looks too shallow in comparison to the shed rafter.

5.  What does your code say about rafter ties higher than 1/3 the rafter length, and why, show me the code sec.?

6.  What’s the elevation of the top of the fl. deck, top of top pl. on left wall, top of ridge?  I assume 2' overhang on the left and 1' on the right?

7.  Why are you skip sheathing the shed roof and what exactly does that mean?  Show me a roof plan with the size and location of the roof openings and the framing around them.

8.  This is similar to your earlier photo, but a new bldg., I assume, without the center wall and logs, right.  How are you going to frame the gable end walls and the left brg. wall?

9.  What are your horiz. and vert. reactions at each end of each rafter, and what’s the tension in the tension tie?

10.  What is you connection detail at each end for each rafter brg.,  and at each end of the tension tie?  You may be will connected, but I’m not sure your tension ties or the rafters are.  Is there someplace in the IRC which defines ‘well connected’ or do you actually design those connections on a crazy roof like this?

11.  Assuming the rt. rafter is well connected to the fl. deck and that its deflection is zero, what are the deflections of the left rafter at the top of the wall?

12.  Are you a builder, where?

There is a little needling mixed in here fellows, but not real much.  The idea that engineers mostly aren’t needed, and then you’re not real sure when they are, and then for a low fee they should, in effect, sign-off on it and act as the insurer of last resort, doesn’t appeal to me.  It’s a heck of a way to make a living, since I am not your insurance co.  But that is kinda the way it shakes out when you and the BO get us involved at the last minute, because you two have chickened out or won’t take the responsibility.  If you stick to std. framing and the IRC you don’t need us, but if you and the Arch. want to go crazy, then you probably need an engineer’s help to keep it stable and standing.  This is not std. framing even though you are talking rafters and ridge boards.

So far I haven’t seen anything that looks like acceptable engineering practice.  What’s really frightening is that these darn drafting programs will let you draw anything, irrespective of the fact that it may or may not be workable, and then by some wild interpolation from a completely different condition, pretend it’s O.K., and that it’ll work.  I would much rather discuss and critique these kinds of things one on one, rather than in front of the whole world.  And, I would really enjoy meeting 10 or 12 of you at a time, with a blackboard, a few six packs, and one or two items on an agenda to get the ball rolling, then just let it be open to the next question.  And, it’s entirely possible that I wouldn’t be able to answer every question, on the spot, but I’d find a reasonable explanation for the next meeting.


----------



## steveray

The shed dormer tries to break the front rafters at the tie, which is the reason for the span reduction, because the thrust is being controlled so far from the point of attachment below. I believe the ratio is the height of the ties divided by the height of the ridge and it gives you a max 50% reduction...which on 2x10's I think brings you somewhere under 10 feet...not getting too specific with no books here. Contractors here always confuse rafter ties and collar ties...the rafter ties are for thrust, collar ties are more for uplift and tearing the roof off the ridge IMHO...but I am not an engineer...


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## GHRoberts

Looking at the comments by dhengr and steveray reminds me of the importance of employing an engineer who can base an answer on previous experience rather than one who has little experience. Employing the later will result in a much higher bill. (Not to imply that either poster is deficient in any manner. It is just that there are a lot of issues that a person with experience would throw out. I claim no experience in structures like this and would submit a large bill.)

---

Since steveray mentioned the rafter on the right. Not knowing if bending is imprtant ...

Tip the rafter so it is level and looks like a beam. The beam is simply supported at the floor joist and the "tie". The end of the beam is cantilevered. The loading is 25# snow load adjusted for the angle and spacing. There is also a point load on the end due to the ridge board connection. 25# uniform load must be less than 53#/ft and allow a 15' span (2x10") between supports. I don't know what the load on the end of the cantilever is but it does subtract from the 25# uniform load so I suppose we are allowed about 7.5' span there.

There is compressive load on the beam so the numbers are wrong but close enough for me. I guess working those numbers out were a waste of time ($$$). Just look up the span from the rafter tables.


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## Rio

steveray said:
			
		

> The shed dormer tries to break the front rafters at the tie, which is the reason for the span reduction, because the thrust is being controlled so far from the point of attachment below. I believe the ratio is the height of the ties divided by the height of the ridge and it gives you a max 50% reduction...which on 2x10's I think brings you somewhere under 10 feet...not getting too specific with no books here. Contractors here always confuse rafter ties and collar ties...the rafter ties are for thrust, collar ties are more for uplift and tearing the roof off the ridge IMHO...but I am not an engineer...


Collar Ties vs. Rafter Ties

by Nick Gromicko, Rob London and Kenton Shepard

Collar ties and rafter ties are both horizontal roof-framing members, each with different purposes and requirements.

Collar Tiescollar rafter tie

Collar ties are designed to tie together the tops of opposing rafters. This helps brace the roof framing against uplift caused by wind. Collar ties must be placed in the upper third of the roof.

Other facts about collar ties:

    * They may or may not be required, depending on jurisdiction. InterNACHI inspectors should not call out lack of collar ties as a defect unless they know that collar ties were required in the jurisdiction in which the home is located at the time the home was built.

    * Collar ties are probably not needed if metal connectors were used to fasten the rafters to the ridge.

    * Where they are required, they should be installed on every other rafter where rafters are on 24-inch centers.

    * The 2006 International Residential Code (IRC) requires they have a minimum nominal dimension of 1-inch x 4-inch.

    * Collar ties, contrary to popular belief, do not prevent walls from spreading.

Rafter Ties

Rafter ties are designed to tie together the bottoms of opposing rafters. This helps keep walls from spreading due to the weight of the roof. When the walls spread, the ridge will sag. A sagging ridge is one clue that the home may lack adequate rafter ties. Rafter ties form the bottom chord of a simple triangular roof truss. They should be placed as low as possible in the roof framing.

Other facts about rafter ties:

    * Rafter ties are always required unless the roof has a structural (self-supporting) ridge, or is built using engineered trusses. A lack of rafter ties is a serious structural issue in a conventionally framed roof.

    * In most homes, the ceiling joists also serve as the rafter ties.

    * Where rafters are oriented perpendicular to the ceiling joists, rafter ties should be installed just above the ceiling joists. The ties usually rest on the joists.

    * When rafters are installed on 24-inch centers, rafter ties are typically installed every other rafter.

    * It’s not unusual to see rafter ties of either 2-inch by 4-inch or 2-inch by 6-inch. The 2006 IRC requires them to be at least 2-inch by 4-inch.

In summary, collar ties and rafter ties perform different functions but are both essential roof-framing members.

 Found the above on the net while reading the thread and thought it looked good enough to post.  Regarding the 3/12, 12/12 roof, the first thing that jumps out at me when I see it is that it's not symmetrically loaded which is a red flag to me that means proceed with caution.  As pointed out above in the thread if the spans are small it's probably no big deal.  If they start getting large then there is more grounds for concern.  As I'm still learning (a long way to go) wood engineering there's no way I'd proceed on this one without consulting one of the engineers I work with on a regular basis.


----------



## Rio

steveray said:
			
		

> The shed dormer tries to break the front rafters at the tie, which is the reason for the span reduction, because the thrust is being controlled so far from the point of attachment below. I believe the ratio is the height of the ties divided by the height of the ridge and it gives you a max 50% reduction...which on 2x10's I think brings you somewhere under 10 feet...not getting too specific with no books here. Contractors here always confuse rafter ties and collar ties...the rafter ties are for thrust, collar ties are more for uplift and tearing the roof off the ridge IMHO...but I am not an engineer...


Collar Ties vs. Rafter Ties

by Nick Gromicko, Rob London and Kenton Shepard

Collar ties and rafter ties are both horizontal roof-framing members, each with different purposes and requirements.

Collar Tiescollar rafter tie

Collar ties are designed to tie together the tops of opposing rafters. This helps brace the roof framing against uplift caused by wind. Collar ties must be placed in the upper third of the roof.

Other facts about collar ties:

    * They may or may not be required, depending on jurisdiction. InterNACHI inspectors should not call out lack of collar ties as a defect unless they know that collar ties were required in the jurisdiction in which the home is located at the time the home was built.

    * Collar ties are probably not needed if metal connectors were used to fasten the rafters to the ridge.

    * Where they are required, they should be installed on every other rafter where rafters are on 24-inch centers.

    * The 2006 International Residential Code (IRC) requires they have a minimum nominal dimension of 1-inch x 4-inch.

    * Collar ties, contrary to popular belief, do not prevent walls from spreading.

Rafter Ties

Rafter ties are designed to tie together the bottoms of opposing rafters. This helps keep walls from spreading due to the weight of the roof. When the walls spread, the ridge will sag. A sagging ridge is one clue that the home may lack adequate rafter ties. Rafter ties form the bottom chord of a simple triangular roof truss. They should be placed as low as possible in the roof framing.

Other facts about rafter ties:

    * Rafter ties are always required unless the roof has a structural (self-supporting) ridge, or is built using engineered trusses. A lack of rafter ties is a serious structural issue in a conventionally framed roof.

    * In most homes, the ceiling joists also serve as the rafter ties.

    * Where rafters are oriented perpendicular to the ceiling joists, rafter ties should be installed just above the ceiling joists. The ties usually rest on the joists.

    * When rafters are installed on 24-inch centers, rafter ties are typically installed every other rafter.

    * It’s not unusual to see rafter ties of either 2-inch by 4-inch or 2-inch by 6-inch. The 2006 IRC requires them to be at least 2-inch by 4-inch.

In summary, collar ties and rafter ties perform different functions but are both essential roof-framing members.

 Found the above on the net while reading the thread and thought it looked good enough to post.  Regarding the 3/12, 12/12 roof, the first thing that jumps out at me when I see it is that it's not symmetrically loaded which is a red flag to me that means proceed with caution.  As pointed out above in the thread if the spans are small it's probably no big deal.  If they start getting large then there is more grounds for concern.  As I'm still learning (a long way to go) wood engineering there's no way I'd proceed on this one without consulting one of the engineers I work with on a regular basis.


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## DRP

1. Let's use Francis' numbers 25 psf, no reductions, seismic C, 90mph. Feel free to use other numbers.

2. 10 psf will cover DL, no don't pick up any diaphragm action here just load it

3. Let's use #2 SPF, snow, repetitive member

4. 2x12, for insulation, keep it simple 2' oc spacing. How did I arrive at those sizes, typical,... academic exercise, pulled out of air feel free to use something other. Different depth plumb cuts are ok, ridge must simply be deeper than the deepest rafter to support bottom edge of plumb cut. If that offends drop the dormer rafter a size,we'll insulate the level ceiling there, it'll still check on this building.

5. Not allowed, Footnote Table R802.5.1, Why, the tie tension and bending moment get out of hand if they are restraining the rafter thrust from high on the rafter. Is that the situation here, no.

6. top of fl deck, anything, put the ridge at 35'. You have width and pitches, sufficient info for general plate and ridge heights, I used 12/12 and 24' wide, 3/12 to make it easy on you. I sketched quickly, go grand call both overhangs 2'.

7. I was tempted to say sheathed with a tarp. What I'm asking for is that no diaphragm action be used in your analysis of the dormer roof. 50' long, call this the entire roof. IIRC the engineered dormer in the photo I posted was about 24' long, ridgeboard. The framing under the ridge is temporay construction support, it was removed. I stated that the shed roof diaphragm was probably sufficient to take care of lateral loads but upon thinking more I don't believe we even need to bring that into the equation, I'm calling it gravy for the purposes of this discussion, don't use it.

8. There was no center wall in the original photo of that engineered shed dormer. For what its worth that build was the subject of a feature article in a trade mag. The article was on design rather than structure. However over 350 home companies were advertising in that magazine at that time. I know many of their design depts read those articles. Not one wrote in commenting that they were concerned with the design. Having built basically the same detail for a number of them over the years, neither am I. Walls conventionally framed 2x6- 16 oc fully sheathed.

9. Your baby, the moment I break out my calculator here it's going to get off track but I do have a fair idea, let's see what you come up with. I've been showing what has worked and been standard among many thousands of homes. Go back to the beginning, I didn't really disagree with you did I? Show your calcs.

10. Why do you think the connections are anything higher than the heeljoint table would cover? If this were a 3/12-3/12 would the tension be higher or lower? Now look at the heeljoint table, it's using 16 commons, we'll be shooting .131's. Make the conversion, I can do it from a referenced table. You can go round the horn and use the yield limit equations if you want to do it the hard way. Double check the heeljoint table, I'm not factoring diaphragm action, now check the table again, it's light isn't it? "Rule of thumb"... pull the trigger at least half again.

11. Vertical deflection of rafter over a wall is zero. Horizontal deflection of the wall does occur, I was wondering who would make it that far. For the loads above it is within acceptable limits though isn't it. Then we do have the shed roof diaphragm in the "real world".

Actually scooting around a plan with these drawing programs give me headaches, I learned with slide rule, pencil and parallel bar. I'm perfectly capable of drawing a phantom clevis with paper and pencil   . No interpolations from different conditions has occured here, standard practice. I've simply asked for you to show proof.

12. Third time's a charm, I've got others depending on my income. Y'all take care.


----------



## brudgers

DRP said:
			
		

> The "we've" in this instance is the industry as far as I can tell, I've gotten this detail many times and as I pointed out this was engineered. Please explain the reason or math that makes it fail in your opinion. This was prescriptive to my understanding up until the '06 cycle. Very often the rafter depth is determined by insulation requirements, that was my comment on them often being deeper than structurally necessary. In the pic I posted  also notice where the tie is joining the main rafter. I'd agree with Mule here.


"The industry" built homes in New Orleans below the flood level for many years and nobody got killed.

And then one day it started killing people.

That's why it's called "a recipe for disaster."


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## brudgers

peach said:
			
		

> Nothing says the beam needs to be anything more than a 1x but you need to consider it.


_"Framing members shall be at least 2 inch nominal width"_ [2305.1.2.1]


----------



## brudgers

Yankee said:
			
		

> If there is no ridge board then each set of rafters will be considered a (separate) truss, and engineering is required for trusses.


And the truss would not be statically determinate.


----------



## Francis Vineyard

I could probably cut and paste all the good points made on this thread and write an article very similar to one I found written in JLC 9/93 by Robert Randall, a structural engineer from Mohegan Lake. N.Y. He noted observations of homes eventually showing the effect of undersize framing and poor construction. I’ve notice sagging roofs without dormers when I did home inspections. Wish I had taken notes of the rafter size to confirm this observation.  During my search online many of the articles resulted with an engineer dispensing the same advice as dhengr; in fact they sound just like Dick on this message board. Quoting from this article in reference to my OP; “Except for very small dormers, I would not recommend using a header between doubled rafters to support the top of drop-ridge dormer rafters. *Although this practice is common* these header are usually undersized and a sagging roof or cracked rafters can result.” *Emphasis added*. PM me and I’ll send a copy of this article from JLC Troubleshooting Guide; Archive Members can read it online. The image is very poor owing to resolution limits to post. My learning curve shot up again from this, thanks to everyones input and dhengr for bringing your expertise from Eng-Tips Forum to here; I'm a frequent visiter at that site too.

View attachment 313


View attachment 313


/monthly_2010_12/Dormer.jpg.0073e738993a4addfd8ca0f5ee9de043.jpg


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## peach

brudgers said:
			
		

> _"Framing members shall be at least 2 inch nominal width"_ [2305.1.2.1]


IRC R802.3 "Ridge board shall be at least 1" nominal thickness and not less in depth than the cut end of the rafter"


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## alora

peach said:
			
		

> IRC R802.3 "Ridge board shall be at least 1" nominal thickness and not less in depth than the cut end of the rafter"


Ok, so ...

IRC R802.2:

"Design and construction. The framing details required in Section R802 apply to roofs having a minimum slope of three units vertical in 12 units horizontal (25-percent slope) or greater."


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## Rio

alora said:
			
		

> Ok, so ...IRC R802.2:
> 
> "Design and construction. The framing details required in Section R802 apply to roofs having a minimum slope of three units vertical in 12 units horizontal (25-percent slope) or greater."


Just for clarification, a  3:12 pitch is equal to a 14% slope, not a 25% slope.  A 25% slope will have a 6:12 pitch ( about 5-5/8" units vertical to 12 units horizontal).


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## alora

Rio said:
			
		

> Just for clarification, a  3:12 pitch is equal to a 14% slope, not a 25% slope.  A 25% slope will have a 6:12 pitch (6 units vertical to 12 units horizontal).


Just for clarification, you may not know what you're talking about.


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## Rio

I've attached a diagram showing what I believe is of a 3:12 and the resulting slope.  Please let me know what is incorrect about it so I don't repeat the error.  14 degrees in 90 degrees is equal to 15.56 in 100 so the slope is, by my calculations and per the diagram, 15.56%  
	

		
			
		

		
	

View attachment 678

	

		
			
		

		
	
Thanks!

View attachment 316


3_12 PITCH.pdf

3_12 PITCH.pdf


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## Francis Vineyard

Rio

25 percent slope equals 14.056 degrees! alora quoted from the code.


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## Darren Emery

It looks like your mixing degrees and percentages.

To convert rise:run to percentages, divide the first by the second.

3/12 = .25

4/12 = .33

5/12 = .416

6/12 = .50

etc...

It's percentage of rise to run as a ratio, not percentage of angle of degree to angle of degree.


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## Rio

Thanks to all for the clarification of the clarification, lol.


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## peach

alora said:
			
		

> Ok, so ...IRC R802.2:
> 
> "Design and construction. The framing details required in Section R802 apply to roofs having a minimum slope of three units vertical in 12 units horizontal (25-percent slope) or greater."


your point alora exactly is what?

Really low slope roofs are sometimes treated as walls.. that's why the reference to Chapter 6.


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## GHRoberts

peach said:
			
		

> Really low slope roofs are sometimes treated as walls.. that's why the reference to Chapter 6.


Perhaps like floors.


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## peach

exterior walls.. not floors.. Chapter 6, not 5


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## Francis Vineyard

Rafter table & slopes revisit: http://www.google.com/url?q=http://www.inspectpa.com/phpbb/showthread.php%3F1054-Rafter-tables-and-slope&sa=U&ei=HgoeTe7LN4Wclgeq3oT_Cw&ved=0CB0QFjAG&usg=AFQjCNFFcZ9S2y5yw9RKlKKY5p_54Pswbg

it was almost a year ago!


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## Francis Vineyard

.........deleted


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## brudgers

peach said:
			
		

> IRC R802.3 "Ridge board shall be at least 1" nominal thickness and not less in depth than the cut end of the rafter"


You said "beam" - see the quoted text in my previous post.


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## alecponting

Hello.The ties are required or ridge beam. Is the ceiling beam is suitable for it....


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