# Approving Hip Rafter Sizes



## jar546

Educational:You are given a set of prints for a hip roof.  The hip roof is typical construction.  The specifications are as follows:Building length= 28'Building width= 28'No measurable ridge, center point meeting of hip rafters.Roof Pitch 12/12Dead Load 10psfGSL 30psf#2 SPF Lumber usedWhat size would the 4 equally sized hip rafters have to be prescriptively?

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## jar546

If I were building this to minimum prescriptive standards I would have to use (3)2x12's as a beam to make that span for each hip rafter.


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## GBrackins

I'd use (3)2x12's per Table 3-28 of the 2012 Wood Frame Construction Manual for a 14'x14' area


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## jar546

GBrackins said:
			
		

> I'd use (3)2x12's per Table 3-28 of the 2012 Wood Frame Construction Manual for a 14'x14' area


And prescriptively, that is exactly how I got my answer.  Looks like we have some consistency in answers.

Anyone disagree or got a different answer?


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## Glenn

All answers reference hip "beam".  The table in the WFCM is for hip "beam".  How do you believe this meshes with IRC R802.3 and the last sentence:

"Where the roof pitch is less than three units vertical in 12 units horizontal, structural members that support rafter and ceiling joists, such as ridge beams, HIPS and valleys, shall designed as beams."

That sort of says when the slope is more than 3:12 that the hips would NOT need to be designed as beams.

For the sake of discussion.  I have a 1958 home.  It is full hip and 26 ft wide.  It is 4:12 pitch.  I have a single 2x8 hip board with 2x6 rafters w/purlins.  It's still standing, and has under 3 ft of snow.

I don't have a better answer, as the prescriptive methods in the IRC are horrible.  However, I'm not sure I would push (3) 2x12's unless it was a vaulted ceiling with no rafter ties.


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## Jobsaver

We are assuming a clear span. With consideration for R802.3 and Table R802.5.1(3), (2006 IRC), based on a common rafter length of a little under 20', it is plausable that 2x10 common rafters attached to 2x12 hip rafter can be used prescriptively where common rafters are 12" O.C.

If the building has interior walls or beams centrally located suitable for bracing off of,  a number of prescriptive configurations may be achieved using 2x6 common rafters attached to 2x8 hip rafters.


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## jar546

Glenn said:
			
		

> All answers reference hip "beam".  The table in the WFCM is for hip "beam".  How do you believe this meshes with IRC R802.3 and the last sentence:"Where the roof pitch is less than three units vertical in 12 units horizontal, structural members that support rafter and ceiling joists, such as ridge beams, HIPS and valleys, shall designed as beams."
> 
> That sort of says when the slope is more than 3:12 that the hips would NOT need to be designed as beams.
> 
> For the sake of discussion.  I have a 1958 home.  It is full hip and 26 ft wide.  It is 4:12 pitch.  I have a single 2x8 hip board with 2x6 rafters w/purlins.  It's still standing, and has under 3 ft of snow.
> 
> I don't have a better answer, as the prescriptive methods in the IRC are horrible.  However, I'm not sure I would push (3) 2x12's unless it was a vaulted ceiling with no rafter ties.


This is interesting (I love starting controversy to get things going).  2 of us get the same answer and you point out that the pitch of the roof does not make us go to the WFCM.  Well, I agree with you but we still don't have a 2nd opinion on what size hip we would need to make this prescriptively compliant.

Wow, what's going on here?  Such a simple, everyday life question for code enforcement.


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## jar546

Jobsaver said:
			
		

> We are assuming a clear span. With consideration for R802.3 and Table R802.5.1(3), (2006 IRC), based on a common rafter length of a little under 20', it is plausable that 2x10 common rafters attached to 2x12 hip rafter can be used prescriptively where common rafters are 12" O.C.If the building has interior walls or beams centrally located suitable for bracing off of,  a number of prescriptive configurations may be achieved using 2x6 common rafters attached to 2x8 hip rafters.


Wait a minute.  If you use 2x10 jack rafters to be compliant with the tables at 10# dead and 30# snow, then I agree that the jack rafters must be a minimum of 2x10.  But, if they are cut at a 45 degree angle for the 12/12 pitch, the cut ends are now almost 13.5" so the use of a 2x12 cannot be compliant with R802.3

So per the IRC this cannot be prescriptively done unless we use the WFCM?


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## Jobsaver

jar546 said:
			
		

> the cut ends are now almost 13.5" so the use of a 2x12 cannot be compliant with R802.3


Ah! Well done!


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## mjesse

From a framers perspective, you've designed a pyramid. About the strongest shape there is.

2x8 rafters, 16" o.c

Single 2x12 hips

(the above assumes standard ceiling joists, and a point to bear a center post)


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## STB

On the 4 sided pyramid, which way do the ceiling joist or rafter ties run, N to S or E to W?  Also in the above example, what size brace would be required?


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## rleibowitz

Unless I'm mistaken the total load on the hip is 3,920 pounds. Here's how I got that number... the hip area is 14' x 14' x 40lbs= 7,840 lbs. divided by 2 =3,920 lbs the span is about 20 ft ...looks like 3 2x12's Not sure where to look for this prescriptively without doing the math.


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## mjesse

STB said:
			
		

> On the 4 sided pyramid, which way do the ceiling joist or rafter ties run, N to S or E to W?  Also in the above example, what size brace would be required?


Ceiling joist perpendicular to a center bearing wall if there is one, otherwise doesn't matter. Roof is symmetrical, add look-outs [stub joist/cripples] and a stiff-back [strong-back], and you're all tied in.

Post/brace = 2-2x4


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## rleibowitz

Just in case your interested the load for a hip looks like a paper kite with most of the load at the top.


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## mjesse

rleibowitz said:
			
		

> Unless I'm mistaken the total load on the hip is 3,920 pounds. Here's how I got that number... the hip area is 14' x 14' x 40lbs= 7,840 lbs. divided by 2 =3,920 lbs the span is about 20 ft ...looks like 3 2x12's Not sure where to look for this prescriptively without doing the math.


I'm no engineer, but it seems many are looking at the problem two-dimensionally.

In three dimensions, there is much less load on the hip, the jacks actually help hold it up. On a valley rafter, it's the opposite, increased load. That's when the kite/diamond come into play.

How about some SE input??


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## STB

Partially correct.  The comment regarding the brace size and the ceiling joist location is a non-prescriptive response.  Most building codes require ceiling joist and/or rafter ties to provide resistance to the thrust reaction at the plate connection.  This in turn "defies the gravity loads" that are on the roof system, that are increased when subject to snow loads.  So per prescriptive requirements of a building code, the rafters must be tied at the plate connection (or close to it) with the ceiling joists and the perpendicular rafters must be tied with rafter ties located above the ceiling joists to form an effective and code compliant connection to resist the rafter thrust.  As far as not being an engineer, you sort of just became one (not legally) because your explanation above cannot be found in any prescriptive building code.

"Ceiling joist perpendicular to a center bearing wall if there is one, otherwise doesn't matter. Roof is symmetrical, add look-outs [stub joist/cripples] and a stiff-back [strong-back], and you're all tied in.

Post/brace = 2-2x4 "

Please provide the code section that allows what you refer to as joist/cripples that will typically have a drywall ceiling attached to them to resist the rafters thrust (not sure if drywall can provide resistance), the size, type,quanity, spacing and connection details for your "stiff-back", and what table was used to determine that an approximately 14' Post/brace is an adequate brace.


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## mjesse

STB said:
			
		

> Please provide the code section that allows what you refer to as joist/cripples that will typically have a drywall ceiling attached to them to resist the rafters thrust (not sure if drywall can provide resistance), the size, type,quanity, spacing and connection details for your "stiff-back", and what table was used to determine that an approximately 14' Post/brace is an adequate brace.


Let me first admit, I'm playing both sides of the net.

As a Code Official, I completely agree with your statements above. If it's not in The Code, It can't be allowed (prescriptively).

As a Carpenter, I would say wait a sec.

28' square building

12/12 pitch

4 common rafters at 19' 9-9/16 (no ridge needed, drop 3/4" from 2 side commons to sandwich the first 2 mains)

Commons set at 13'-11-1/4" from corner

4 equal hips at 24' 3"

4 left, 4 right of each jack;

17-10

16-0

14-1

12-3

10-4

8-5

6-7

4-8

2-9

((approx.))

Stiff-back at the commons perpendicular to the joist.

Done.

Carpenter: "Now, Mr. Code Official, you want me to put a stiff-back on EVERY rafter?! Even the last (shortest) 4? There's no outward thrust there! The jacks meet at the hips, 1/2 of each pair is tied to the cj. Where's it gonna go?! ..and a triple 2x12 hip!!!!?"

There needs to be some common sense in the Code interpretation. The Code can't outline every situation, nor should we expect an engineered solution every time.

The stick frame methods I learned, were passed down for generations. The homes my great-grandfather built are still standing (they're a bit drafty, and they don't have fire sprinklers...but I digress)

The Code guys (myself included) like to slam the "we've always done it that way" response. But some things work regardless of what the Code says.

Rant over. Happy Independence Day. Independence from tyranny. Thinner Codes are better Codes.

mj


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## mtlogcabin

R802.3 Framing details.

Rafters shall be framed to ridge board or to each other with a gusset plate as a tie. Ridge board shall be at least 1-inch (25 mm) nominal thickness and not less in depth than the cut end of the rafter. At all valleys and hips there shall be a valley or hip rafter not less than 2-inch (51 mm) nominal thickness and not less in depth than the cut end of the rafter. Hip and valley rafters shall be supported at the ridge by a brace to a bearing partition or be designed to carry and distribute the specific load at that point. 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.

I believe you would have to go to a SP SS or #1 for the rafters to get them down to a 2x8 because anything bigger on a 12/12 roof would create an end cut larger than 12" and would not meet 802.3. I don't think you can easily find a 2x17

Then again I am pretty rusty with the charts since I haven't seen a framed hip roof since 1996.

Anything over a 3/12 pitch is acting as a ridge board, correct? Don't the rafters have to sit on top of the hip in order for the hip ridge to considered a beam?


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## mjesse

mtlogcabin said:
			
		

> I believe you would have to go to a SP SS or #1 for the rafters to get them down to a 2x8 because anything bigger on a 12/12 roof would create an end cut larger than 12" and would not meet 802.3. I don't think you can easily find a 2x17Then again I am pretty rusty with the charts since I haven't seen a framed hip roof since 1996.
> 
> Anything over a 3/12 pitch is acting as a ridge board, correct? Don't the rafters have to sit on top of the hip in order for the hip ridge to considered a beam?


2x8 spf #2, 16" o.c. DL 10psf, GSL 30psf Table 802.5.1.(3) max span 15' 1"

*R802.5 Allowable rafter spans. Spans for rafters shall be in accordance with Tables R802.5.1(1) through R802.5.1(8). The span of each rafter shall be measured along the horizontal projection of the rafter.*



*12/12 cheek cut on a 2x8 (45º) is +/- 10-5/8" - 2x12 = 11" + It works*



*mj*


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## mtlogcabin

Thanks.

I was at the wrong Table, 50 psf ground snow and using 24' OC


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## jar546

So to prescriptively meet the code without engineering and according to the IRC, in order to make this roof code compliant for a #10 dead and 30# gsl, we would need: (no purlins in this design)

2x8 rafters and a 2x12 hip.  Correct?


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## mjesse

jar546 said:
			
		

> So to prescriptively meet the code without engineering and according to the IRC, in order to make this roof code compliant for a #10 dead and 30# gsl, we would need: (no purlins in this design)2x8 rafters and a 2x12 hip.  Correct?


That's my story, and I'm sticking to it!


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## jar546

mjesse said:
			
		

> That's my story, and I'm sticking to it!


OK, I will buy it.

Now on to an acceptable method to control rafter thrust in this installation.  Any takers?


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## STB

> "Carpenter: "Now, Mr. Code Official, you want me to put a stiff-back on EVERY rafter?! Even the last (shortest) 4? There's no outward thrust there! The jacks meet at the hips, 1/2 of each pair is tied to the cj. Where's it gonna go?! ..and a triple 2x12 hip!!!!?"


Part of the language you have used is a typical everyday response, "YOU WANT ME TO PUT", no, the code requires it.  My typical response would be "You couldn't build the house the way I want it"!

If you tie the rafters to ceiling joists (both directions) and install a properly sized post/brace, 3 2x12's wouldn't be needed.  And furthermore, if there is no thrust or downward weight, why would you need the post/brace that you originally suggested?

If the thrust in both directions is resisted by CJ and Rafter ties and the hip or valley is braced to bearing below, then the rafters are not treated as load carrying members.  I have been around the block with this for many years and every organization that is affiliated with the codes has agreed with the above statement.  I have recently contacted the AF&PA who is referenced in the roof code sections and that is their stand also.  So understanding that you as well as I have a framing and a code official background, as a code Official, how can you allow non-prescriptive design and sign off on inspections and CO's which states that it is in conformance with the adopted codes?

There are things that I know as a framer that can be done that are not addressed by code, but without my 16+ years as a plan reviewer and inspector, but as a everyday framer, I cannot provide the proper paperwork for the file of a non-prescriptive design.  I think the point of this post was to show that although we may think that something that is not prescriptive can or may work, without the required documentation, how are we as BCO's compliant with the codes.


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## jar546

STB said:
			
		

> Part of the language you have used is a typical everyday response, "YOU WANT ME TO PUT", no, the code requires it.  My typical response would be "You couldn't build the house the way I want it"!If you tie the rafters to ceiling joists (both directions) and install a properly sized post/brace, 3 2x12's wouldn't be needed.  And furthermore, if there is no thrust or downward weight, why would you need the post/brace that you originally suggested?
> 
> If the thrust in both directions is resisted by CJ and Rafter ties and the hip or valley is braced to bearing below, then the rafters are not treated as load carrying members.  I have been around the block with this for many years and every organization that is affiliated with the codes has agreed with the above statement.  I have recently contacted the AF&PA who is referenced in the roof code sections and that is their stand also.  So understanding that you as well as I have a framing and a code official background, as a code Official, how can you allow non-prescriptive design and sign off on inspections and CO's which states that it is in conformance with the adopted codes?
> 
> There are things that I know as a framer that can be done that are not addressed by code, but without my 16+ years as a plan reviewer and inspector, but as a everyday framer, I cannot provide the proper paperwork for the file of a non-prescriptive design.  I think *the point of this post was to show that although we may think that something that is not prescriptive can or may work, without the required documentation, how are we as BCO's compliant with the codes.*


Bingo my friend, Bingo


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## Glenn

I think Mjesse already touched on this, but let's look one more time.

This hip is 24 ft. long.  How will you expect the splices to be handled, assuming they don't special order a 24' 2x12?

How would you handle the triple 2x12, if you were still to require it?

Great thread, Jeff!


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## jar546

Hip rafters would be just under 20'

A triple 2x12 is NOT required but it may look like it prescriptively if you don't read the entire code section in the IRC (hence my point in starting this)


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## mjesse

STB said:
			
		

> I think the point of this post was to show that although we may think that something that is not prescriptive can or may work, without the required documentation, how are we as BCO's compliant with the codes.


Well, I took the long way there, but that was my point. I agree with you by the way.

It wasn't until the last couple years I was in the trades, that I even owned a Code book. It was a well versed and "picky" inspector that got me into the books. After some great discussions with him on the interpretation of the Codes, I realized I found my next career.

Whether we like it or not, we can only approve what's prescriptively written, or get an architect/engineers design. I have good success with contractors by explaining my past and the reasons for requesting additional information.

mj


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## Glenn

jar546 said:
			
		

> Hip rafters would be just under 20'


If you're talking about a 2D flat roof it would be...

14 x sqrt of 2 (1.41) = 19.74'  That is the diagonal FLAT distance of the hip.  That is the "span"

Now raise that hip across that 20' distance to a height of 14' to find the "length"

sqrt [(19.74 x 19.74) + (14 x 14)] = 24' or so.

So back to the question...how do you handle splices?


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## jar546

Glenn said:
			
		

> If you're talking about a 2D flat roof it would be...14 x sqrt of 2 (1.41) = 19.74'  That is the diagonal FLAT distance of the hip.  That is the "span"
> 
> Now raise that hip across that 20' distance to a height of 14' to find the "length"
> 
> sqrt [(19.74 x 19.74) + (14 x 14)] = 24' or so.
> 
> So back to the question...how do you handle splices?


I was simply finding the hypotenuse of a right triangle.  sq root of 14' run sq. plus 14' rise sq , just under 20'

Educate me.


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## peach

span does not equal length. the span is.. well just that.. the horizontal distance.. the hypotenuse is the length of the member..

having said that.. that's a LONNNNNNNNNNNNNNNNNNNNNNG hip.. a hip truss set is a better way to go.


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## jar546

peach said:
			
		

> span does not equal length. the span is.. well just that.. the horizontal distance.. the hypotenuse is the length of the member..


Oh great, now we have a PE in here slinging around all that technical jargon!!


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## pyrguy

The top cut of the jack that goes against the hip rafter is 12/17 not 12/12. The cut is different for jacks. I agree that the load is self supporting (basically) as an old carpenter.


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## MASSDRIVER

pyrguy said:
			
		

> The top cut of the jack that goes against the hip rafter is 12/17 not 12/12. The cut is different for jacks. I agree that the load is self supporting (basically) as an old carpenter.


The cut angle of a jack rafter is the same as the common, with the saw set at a 45 degree angle. That sets the side cut, which is on the scale on the framing square body. On a regular roof, the only thing that gets cut on the 17 is the hip or valley.

Brent.


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## pyrguy

Been too long since I had a saw in my hands. You're right I guess i had a case of CRS,


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## jar546

pyrguy said:
			
		

> Been too long since I had a saw in my hands. You're right I guess i had a case of CRS,


20 lashes with a wet, half cooked lasagna noodle for you!


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## MASSDRIVER

pyrguy said:
			
		

> Been too long since I had a saw in my hands. You're right I guess i had a case of CRS,


Ha! You have an excuse.

Back in the 90's the sacramento valley was awash in these giant ranch house hip roofs. Hips and valleys everywhere.

I was the cutter on a roof crew and had done maybe 20 of these monstrosities, so I could do them in my sleep. But for whatever reason I decided one day that all the jacks had to be cut on the hip scale. Imagine my surprise when the stackers swore nothing was working. Total mental fart.

Then the time on the fine home using forum, or Badger Pond, when I offered up the lamest stupidest way to find the sides of an octagon. 100 percent wrong. Got maybe 20 pm's telling me what a retard I was.

Ego shattered again.

Brent.


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## Glenn

jar546 said:
			
		

> I was simply finding the hypotenuse of a right triangle.  sq root of 14' run sq. plus 14' rise sq , just under 20'  Educate me.


That is the first part and it is 20'.  But then you need to find it on a right triangle formed between the 14' height of the roof assembly and the 20' distance.  For that, use the old "A squared + B squared = C squared"  That becomes "20 squared + 14 squared = X squared"  Then take the square roof of "X squared".  This becomes 24'.  Considering room for bearing and a tail, you'd need about 26 ft.


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## jar546

Glenn said:
			
		

> That is the first part and it is 20'.  But then you need to find it on a right triangle formed between the 14' height of the roof assembly and the 20' distance.  For that, use the old "A squared + B squared = C squared"  That becomes "20 squared + 14 squared = X squared"  Then take the square roof of "X squared".  This becomes 24'.  Considering room for bearing and a tail, you'd need about 26 ft.


Thanks, I forgot about the distance to the corner for the hip vs distance to the sides.


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## MASSDRIVER

jar546 said:
			
		

> Wait a minute.  If you use 2x10 jack rafters to be compliant with the tables at 10# dead and 30# snow, then I agree that the jack rafters must be a minimum of 2x10.  But, if they are cut at a 45 degree angle for the 12/12 pitch, the cut ends are now almost 13.5" so the use of a 2x12 cannot be compliant with R802.3So per the IRC this cannot be prescriptively done unless we use the WFCM?


The hip is angled too so the jack will fit. The plumb line depth increases at the same rate for the jacks as well as the hip.

Brent.


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## jar546

MASSDRIVER said:
			
		

> The hip is angled too so the jack will fit. The plumb line depth increases at the same rate for the jacks as well as the hip.Brent.


May fit.  Excellent point.  Looks like this thread is really turning into something.  I appreciate all the input.


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## MASSDRIVER

jar546 said:
			
		

> May fit.


May fit? Au contraire.

You're framer is not amused





Buts that's totally cool, cause I have this 2 X 12 here in the garage to play with.

So in the interest of accuracy:





You get about 16" out of this one as it's shrunk to 11 1/8", but it should be 16.97 on the diagonal. But hey...who's counting.

Brent.


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## jar546

Damn you!  I was too lazy to do the math not knowing you had the lumber and framing square right in your possession the entire time and you are 3 hours behind me in time with daylight.

I am wrong, humbled and ashamed at my laziness.

Nice to see someone use a real framing square for a change btw.


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## jar546

The Inspector is Humbled  (back at ya)  LOL

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## MASSDRIVER

Holy Crap! you ARE a Marine!

The furrowed brow, the withering stare, the hair that won't grow more than 1 1/2" lest it be smoked by pushups.

You ever just hit someone with a roll of plans?

And yes...my methods are sneaky and underhanded.

:devil

Brent


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## mjesse

Glenn said:
			
		

> This hip is 24 ft. long.  How will you expect the splices to be handled, assuming they don't special order a 24' 2x12?


we don't need no steenkin splices!LVL to the rescue
	

		
			
		

		
	

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## jar546

My back is hurting looking at that hip


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## Jobsaver

Not to veer off center of topic, but, is there any objection to substituting engineered lumber for solid lumber for hip or valley rafters, or ridge boards . . . to increase dimension or avoid spilces? Also, what are acceptable methods to splice a hip or valley rafter?


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## jar546

Jobsaver said:
			
		

> Not to veer off center of topic, but, is there any objection to substituting engineered lumber for solid lumber for hip or valley rafters, or ridge boards . . . to increase dimension or avoid spilces? Also, what are acceptable methods to splice a hip or valley rafter?


To answer question 1, anytime we have someone specify an LVL or Glu-Lam, they must be accompanied by a spec sheet/cut sheet from the manufacturer for the specific application that they are used.  An exception would be a ridge board since it is just for nailing.

Q2 I can answer as follows.  If it is not prescriptive, which a splice is not, then we need a splice design from an RDP.


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## mjesse

I would have no problem accepting an LVL in place of a specified/approved dimensional lumber member. Work would be "exceeding" the minimum Code in my mind.

I would accept a piece of solid blocking/gusset for a ridge splice, and likely accept the same for a hip over 3/12 pitch.

As I've stated before, the valley rafter carries more load, so I would request and RDP spec for a splice for that.

mj


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## MASSDRIVER

At some point you start to exceed the capabilities of a 2x board to free span. I can't remember going over 28 feet before getting into a lam with a backing cut on top. Fortunately typ. residential has plenty of walls to hit a mid span support somewhere.

As a cutter I would rather use a nice strait long lvl so I have some consistant dimensions for layout.

Brent


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## MASSDRIVER

And to mjesse's comment on valleys, they seem to always have walls underneath to get strut on. It seems open roofs are more pleasing to see the underside of hips rather than valleys, but those usually carry a load from a ridge beam or other support.

Brent


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