# Roof frame pics to make you smile



## Sifu (May 30, 2012)

http://i1269.photobucket.com/albums/jj584/raspicher/HPIM0086.jpg

http://i1269.photobucket.com/albums/jj584/raspicher/HPIM0085.jpg

This guy got his list of framing corrections on the 16th.  I went back for the scheduled reinspect on the 23rd to find a completely sheetrocked house.  These pics are just a sample of what he didn't address.


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## righter101 (May 30, 2012)

Doesn't simpson make a product to fix this??


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## TheCommish (May 30, 2012)

harder to fix with the board up


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## ICE (May 30, 2012)

Are you sure this isn't another one of Jeff's sheds?


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## steveray (May 31, 2012)

Maybe he used structural drywall???? GR could get that to work with some engineering....that first pic MIGHT work under the prescriptive notching.....that valley rafter is way not good...although I have seen them stamped that way.....


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## Sifu (May 31, 2012)

http://i1269.photobucket.com/albums/jj584/raspicher/HPIM0077.jpgHow would the rafter work prescriptively?  Max notch is 25%, this one is 50%.  Are you employing the exception because if that permits it then I need to back off this correction if he has 4".  I was under the impression that this is not what the exception addressed.


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## ICE (May 31, 2012)

Sifu said:
			
		

> http://i1269.photobucket.com/albums/jj584/raspicher/HPIM0077.jpgHow would the rafter work prescriptively?  Max notch is 25%, this one is 50%.  Are you employing the exception because if that permits it then I need to back off this correction if he has 4".  I was under the impression that this is not what the exception addressed.


It is not possible to make a determination without all of the details.  It could be that the rafter is twice as large as would be required.  A picture from 10' away would help.  There doesn't appear to be a connection of the roof diaphragm to the wall because there are no frieze blocks or wall top plates visible, etc.


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## Sifu (May 31, 2012)

The rafter is over-spanned and there is no rafter tie (other corrections noted on the inspection) but how is that relevant to the prescriptive code?  The code limits the notch (without exception) to 25% without mention of required rafter size or rafter ties.  I was taught that it didn't matter since the danger of splitting along the grain on a notch could potentially reduce the member to nothing.  This thread now has a kind of sister thread concerning the span issue, sorry I didn't want to mix the issues but it looks like it happened anyway.


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## codeworks (May 31, 2012)

remove drywall for reinspection of framing corrections not cleared through proper inspection process. refer to R109.4 2006 IRC.


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## tmurray (May 31, 2012)

Sifu said:
			
		

> The rafter is over-spanned and there is no rafter tie (other corrections noted on the inspection) but how is that relevant to the prescriptive code?  The code limits the notch (without exception) to 25% without mention of required rafter size or rafter ties.  I was taught that it didn't matter since the danger of splitting along the grain on a notch could potentially reduce the member to nothing.  This thread now has a kind of sister thread concerning the span issue, sorry I didn't want to mix the issues but it looks like it happened anyway.


If your code language is the same as ours here in Canada it will say something along the line of "notching shall not exceed x in size unless the depth of the member is increased by the size of the notch"


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## north star (May 31, 2012)

*+ + +*

I agree with "codeworks" suggestion also, ...remove all drywall

for proper inspection of all of the framing.....For a "fix" for the

overnotched roof rafter, I'm guessing that requiring a

competent RDP is out of the question.

What we have allowed in the past was to have custom made

pieces of plate steel, ...3.8" thick, that covered the entire width

of the rafter, ...on both sides and bolted thru the rafter.....It

was not an RDP design......We have since started using RDP's to

submit a compliant design......This practice / requirement has

stopped most of the obscenely overnotched framing members

[ for the most part ].......Once the contractor community knows

that you will not approve their discrepancies and move the

project forward, they have [ here ] tended to improve their

practices.

I'm pretty sure I know the answer ahead of time, but can

you require the removal of the drywall and possibly an

engineered fix for the "overnotching" & any other issues?

Also, "thanks" for your continuing transparency!

*+ + +*


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## mtlogcabin (May 31, 2012)

> The code limits the notch (without exception) to 25% without mention of required rafter size or rafter ties. I was taught that it didn't matter since the danger of splitting along the grain on a notch could potentially reduce the member to nothing


If the rafter ended at the top plate with no overhang would you approve it where it is bearing on the top plate or should the rafter be bearing closer to the heel than the toe (sheathing side) of the cut.

Don't get hung up on the knotching requirements it may not be a problem but hard to tell from the picture.


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## north star (May 31, 2012)

*$ $ $*

*From Section R802.7.1 - Sawn lumber [ `06 IRC ]:*

Notches in solid lumber joists, rafters and beams shall not exceed one-sixth

of the depth of the member, shall not be longer than one-third of the depth

of the member and shall not be located in the middle one-third of the span.

Notches at the ends of the member shall not exceed one-fourth the depth

of the member........The tension side of members 4 inches (102 mm) or

greater in nominal thickness shall not be notched except at the ends of the

members.........The diameter of the holes bored or cut into members shall

not exceed one-third the depth of the member........Holes shall not be

closer than 2 inches (51 mm) to the top or bottom of the member, or to

any other hole located in the member........Where the member is also

notched, the hole shall not be closer than 2 inches (51 mm) to the notch.



*Exception:* Notches on cantilevered portions of rafters are permitted

provided the dimension of the remaining portion of the rafter is not less

than 4-inch nominal (102 mm) and the length of the cantilever does not

exceed 24 inches (610 mm).

*$ $ $*


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## steveray (May 31, 2012)

Sifu said:
			
		

> http://i1269.photobucket.com/albums/jj584/raspicher/HPIM0077.jpgHow would the rafter work prescriptively?  Max notch is 25%, this one is 50%.  Are you employing the exception because if that permits it then I need to back off this correction if he has 4".  I was under the impression that this is not what the exception addressed.


I don't think that notch is 50%.....it may be more than 25%.....it should be measured from the bearing point perpendicular to the grain in that case...I hate when they mess up the seat cut like that....


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## DRP (Jun 1, 2012)

The "notch" appears to be the unsupported 1-1/4"  that is beyond the 6" seat cut. The code language does a poor job here, it should prescriptively say "the seat cut should not extend beyond the plate". Right now it says it can extend inboard off the bearing till it erodes 25% of rafter depth, he is golden but it's not right. No different than a sharp 2x2 ledger notch, specifically allowed and causes splitting.


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## Sifu (Jun 1, 2012)

OK, I'll readily admit I'm not the sharpest chisel in the tool box so I can ask these questions and not feel too much more uneducated.

-If a rafter of 7 1/4" depth is notched with a birdmouth/seat cut, and the depth of that cut measured perpendicular to the grain of the rafter is 3 1/2" to 4", and that notch is on the end/bearing point of the rafter, is it not notched 45% to 50%?

-Doesn't the exception refer to the tail of the rafter, in other words the cantilever portion is the portion extending out after the bearing point?  Thus limiting the tail to

4"x24" and having nothing to do with the depth of the notch.

-Therefore is not this rafter excessively notched?  How would he be golden?

-Am I measuring the depth of the notch from the wrong point?  In the photo the outer edge of the seat cut is 3 1/2" to 4" but the inner edge may only be 1 1/4".  Is that where I am not getting this?

-Doesn't this tie into into the location of the bearing point brought up previously?

-Finally, I don't know what NorthStar means by me being transparent, is that good or bad?

BTW, I have informed the owner that for me to sign anything the corrections will need to be made or a DP will need to sign off and that for the corrections to be observed the drywall will need to be removed.  Some of the corrections I don't have prescriptive corrections for, like the rafter notch, thats why I'm trying to make sure of my understanding of the actual code language.  If I am wrong it will cause him a lot of time and money and I don't want to do that. (every rafter in the house)


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## steveray (Jun 1, 2012)

Everyone is being positive with you Sifu, no worries there.......The notch depth is measured square off of the bottom edge of the rafter and ends at the bearing point...it is not the length of the seat cut.....I agree it is hack work, but it looks like it might be close to being compliant...(tough to tell from here)....the valley rafter is a whole different different ball game however...


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## Big Mac (Jun 1, 2012)

Nifty notching.  Rafter load carrying capability severely compromised.  Much more susseptible to shear that if it had been left full sized.


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## DRP (Jun 1, 2012)

Sifu,

Northstar paid you a big compliment. Mr Edwards who was aquitted today, not very transparent.

I saw mention of these being finger jointed rafters. I've never used finger jointed lumber. My first question is whether this is an appropriate use and whether the design values are the same as the solid sawn dimensional values you are using in your tables. I would call the grading agency listed on the grade stamp unless someone here knows the answer to that.



> If a rafter of 7 1/4" depth is notched with a birdmouth/seat cut, and the depth of that cut measured perpendicular to the grain of the rafter is 3 1/2" to 4", and that notch is on the end/bearing point of the rafter, is it not notched 45% to 50%?


dhengr covered this bit in his response on the other thread, Look at your picture above the rafter is 7-1/4 wide the tape is showing 6" remaining, or a 1-1/4" notch. dhengr also mentioned the notch splitting along the grain out of the re-entrant corner of the notch. I've drawn a red line following the grain out of the notch. He then went on to explain that the grain can be highly variable and the split can run up into the board and cause serious problems. Imagine if the knot in this picture was a bit closer to the notch and then the split began. I followed that grain in green, see how the split would then run up deeply into the rafter. Slope of grain is very critical.

I've drawn in the repair I would do in the field if I drove up on this job.







 I would get out the sawzall and chuck up a long stiff blade. Make a full level seat cut on the black line dropping that dangling tip off. Slide in another long 2x4 flatways under the seat cut giving it full bearing. Have a pack of shims in your bags and make that snug. Secure both plates well to the ceiling joists and run a framing angle alongside the rafter nailing to the plate and to the rafter.


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## TheCommish (Jun 2, 2012)

nice fix, easy I would accept without engineering


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## Sifu (Jun 3, 2012)

OK, I see that I misunderstood the point at which the notch should be measured.  I was taking it from the tip of the seat cut at the outside.  This being the case I agree that it is much closer to compliant than I thought.  The irony is I attended a day long seminar taught by a local PE on framing last week, two days after this inspection.  He brought up the problem of over-notched rafters, explaining how detrimental it can be to the structure.  I guess it was just my preconception of the requirement and my (mis)understanding of his explanation that caused this.  I am going to email him the picture and get his opinion, I'll let you know what he thinks.

I did take a good picture of the grade stamp so I can check on the lumber but my understanding about FJ lumber is that it is acceptable as a replacement for non FJ lumber.  I have seen stress tests on it in the past and the failures occured at the solid portions while the FJ remained intact.  This is something I have taken for granted for some time but if I am wrong and someone knows better please pipe in.  I will do some better research as well.


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## Sifu (Jun 3, 2012)

Having read DRP and Dhenger's posts I think I finally get it but just to be sure, -the re-entrant notch is the inside cut that starts at the red line in DRP's corrective drawing and is across from the 6" on the tape in the picture?  That is the critical point where failure will start and potentially split along the grain and that is where the notch refered to in the code should be measured from in relation to the bottom of the rafter.....correct?  Thus any notch on a 7 1/4" rafter at this point deeper than 1 13/16" from the bottom of the rafter would exceed the limit of the code?

Please say yes.


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## DRP (Jun 3, 2012)

Yes, I think he is fine by code. I told you what I would do and why.

That is the re-entrant corner. Stress concentrates, splits and cracks lead out of, inside corners.

I think your instructor was referring to thexception to R802.7.1, if there is an overhang beyond the birdsmouth there must be a 2x4 strap of rafter over the outboard corner of the plate, measured perpendicular to the rafter to support up to a 2' overhang.


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## ICE (Jun 3, 2012)

I'm not saying that it's all wrong because it might be a regional way of framing but nothing about the framing in the picture looks right from here in CA.  No connection of the roof diaphragm to the foundation.  No frieze blocks.  No solid blocking between the joists.  No rafter tie.  The 2"x that the rafter bears upon isn't nailed to the ceiling joist and if it were nailed the joist would probably split.  And then there's that goofy seat cut.

A seat cut isn't a notch, it's the end of the rafter.


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## DRP (Jun 3, 2012)

There is always the possibility I was fixating on a dangling tit.


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## ICE (Jun 3, 2012)

DRP said:
			
		

> There is always the possibility I was fixating on a dangling tit.


That's happened to me.  Not that I'm complaining but sometimes I think they do it on purpose.


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## Sifu (Jun 4, 2012)

If memory serves, in addition to the possibly over-zealous over-notched condition, my list for the roof framing contained over-spanned rafters, missing and improper rafter ties, over-cut and unsupported hip and valley cuts at the top and bottom and incomplete load path to the fdtn.  I did not list missing attachment of the roof to the foundation but as far as I know we have no prescriptive code for that here other than the hold down nails, I will re-visit that one though to be sure.  I also did not list missing frieze blocks.  I am not 100% sure what you refer to but if it is the blocking between the CJ I don't think they are required on a 2x8 rafter.  I don't know if the 2x4 is nailed to the CJ or not, I will check on that if given the opportunity.  Personally, I don't think this type of framing is particularly neat and it leads to multiple connection points that I think make an inspection less clean but I have to take baby steps and tackle the less ambiguous items.


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## globe trekker (Jun 4, 2012)

IMO, I would not be comfortable with cutting more out of the rafter as *DRP*

suggested. I DO like the idea of placing additional framing / bracing next to

the overnotched one though, and securely & correctly nailing it to the ceiling

joists.

Because the roof rafter is overnotched, are you comfortable with telling

them how to repair it (as you or the Forum members think), or would

requiring a RDP be more prudent? Just asking...


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## Sifu (Jun 4, 2012)

As a rule I don't recommend corrective repairs like that.  I will recommend a correction when it is supported by the code but when it reaches the engineering stage and/or a stage about which I don't know enough (obviously, like this one turned out to be) I stay clear if I can.  I don't have the training nor the insurance nor the income of a DP.  At first I thought EVERY rafter was notched 50%, now it may be that some of them are slightly over-notched.  If it were the only problem he had I might try to help out by giving him an option I would accept, though keep in mind I was wrong to start with!  Since he has multiple other issues and he has already supposedly retained a DP I will let the DP handle it.  There were corrective actions he could have taken to cover a lot of his issues had he chosen that path.  He chose a different path, he ignored the failed inspection, ignored the required insulation inspection, ignored the stop-work order and claims to have retained a DP.  And, unfortunately I have too many other similar issues to deal with to chase him around and tell him how to build his house.  Like I always tell the owner/contractor; when you elect to be your own contractor you may save money but the trade off is the knowlege you might miss out on from a pro, and it may cost you more in the long run.


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## dhengr (Jun 4, 2012)

Globe trekker:

The dangling tit that DRP suggests cutting off serves no purpose and hurts nothing.  The reentrant corner is already there and that’s the danger point.  By removing the dangling tit he allows the 2x4 to be inserted, which now does a better job of supporting the heel of the rafter, as long as the rafter bears tightly on the new 2x4 at the heel; and there is no over cutting at the current notch.  The disadvantage is that this moves the rafter bearing further out on the clg. joist, instead of out over the bearing wall, and now this must be rationalized.

Sifu:

That whole roof framing effort is a botched-up mess, which shows that the guy really doesn’t understand what he is doing.  He seems to have no concept of clean load paths or connection details; or why things are done the way they are normally done.  Because of the messy details, every step of the way is a challenge to rationalize with the code or good sound design practice.  You are right to make the builder’s engineer rationalize and explain all of these goofy details, just hand him the most complete list of problem areas that you can come up with.  And, I would enforce that stop-order until he does his work and you have a meeting-of-the-minds.

What do you mean by ‘over-spanning rafters’?  That the span length, roof load, or spacing are to great for the rafter size?  You’ve not told us any of these, except the rafter size.  Remember that roof DL has to be adjusted to a horiz. projection and then added to the LL to use the rafter tables, maybe the tables do already do this.  Incomplete and complicated load paths are a real critical condition.  The most direct connection btwn. the rafter and the clg. joist must transmit the rafter thrust into the clg. joist.  Reread my other posts, I mentioned blocking btwn. the ceiling joists, or a rim joist, this is to prevent the joists from just rolling over, like a house of cards, just parallelograming onto the top plates.  Otherwise, ICE offers a good starter list of things that need attention in that one photo.


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## Big Mac (Jun 4, 2012)

The safest route to a code compliant design is an engineer.  Oh excuse me RDP


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## Sifu (Jun 4, 2012)

dhengr said:
			
		

> What do you mean by ‘over-spanning rafters’?  That the span length, roof load, or spacing are to great for the rafter size?  You’ve not told us any of these, except the rafter size.  Remember that roof DL has to be adjusted to a horiz. projection and then added to the LL to use the rafter tables, maybe the tables do already do this.  Incomplete and complicated load paths are a real critical condition.  The most direct connection btwn. the rafter and the clg. joist must transmit the rafter thrust into the clg. joist.  Reread my other posts, I mentioned blocking btwn. the ceiling joists, or a rim joist, this is to prevent the joists from just rolling over, like a house of cards, just parallelograming onto the top plates.  Otherwise, ICE offers a good starter list of things that need attention in that one photo.


Going from memory here:  He has a 2x8 rafter with a horizontal span of 20'2, the code span table indicates 17'11.  If I take it straight out of the code he is over-spanned.  The WFCM does indicate an increase of span multiplied by 1.07 for the 8/12 pitch.  (I utilize the WFCM because I try to find ways to pass folks, not find ways to fail them)  In this case he is still over-spanned, though by the WFCM the over-span is less.  To be quite honest I don't typically use the increase in the WFCM because I'm not entirely sure I understand it.  It may be legit but I don't like pulling little pieces of it out to justify something the code doesn't permit.  The increase may be what you say about the span taking into consideration the adjustment but I don't know for sure.  If the code intended that adjustment, it would be in there wouldn't it?  BTW the WFCM shows the same allowable span as the code table.  By my eyes he has an over-spanned rafter with a max notched or over notched condition, those two conditions together don't seem to be a good idea.  Seems like he will have maximum load and deflection with minimum resistance at the heel joint to withstand the resulting force at the heel joint.


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## Sifu (Jun 4, 2012)

As far as the parrallelograming of the CJ to give it lateral stability I agree, I just need code to back it up.  The way I read the code, it requires blocking for lateral stability if the member exceeds 6 to 1, which a 2x8 doesn't meet.  Have I missed a code that says otherwise?  I hate to beat a dead horse but if I don't have code, specifically IRC 2006, I have a tough road.  I will get slapped down hard and fast by my BO if I go outside of it.  I was told today that if I was going to require verification of things like individual member reports for LVL's or truss drawings that the BO was going to recommend dissolving the department to the county commision and that stuff is in the code!


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## ICE (Jun 4, 2012)

So what you are looking for is what you might have enforced had you been able to enforce anything.


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## GBrackins (Jun 5, 2012)

Sifu said:
			
		

> As far as the parrallelograming of the CJ to give it lateral stability I agree, I just need code to back it up.  The way I read the code, it requires blocking for lateral stability if the member exceeds 6 to 1, which a 2x8 doesn't meet.  Have I missed a code that says otherwise?  I hate to beat a dead horse but if I don't have code, specifically IRC 2006, I have a tough road.  I will get slapped down hard and fast by my BO if I go outside of it.  I was told today that if I was going to require verification of things like individual member reports for LVL's or truss drawings that the BO was going to recommend dissolving the department to the county commision and that stuff is in the code!


I don't know if this will help or not, I don't know if much can help you in your situation except a change of venue ....

In Massachusetts we adopted the 2009 International Energy Conservation Code. When air permeable insulation is used with soffit vents the insulation must be sealed at the exterior wall to prevent wind-washing of the insulation. I use 2x blocking to aid in accomplishing this. Of course there are other methods that can be used to seal with, but blocking is met with less resistance from local builders. This way I get the blocking I want for the roof diaphragm and justify it by the energy code no matter the size of the rafter/ceiling joist.

Section 402.4 Air Leakage (Mandatory) #12 of 2009 IECC requires air sealing of other sources of air infiltration. See Section N1102.4 #10 of the 2006 IRC, requires sealing of other sources of air infiltration.

Sounds like you are fighting an uphill battle without any ammunition when it comes to your BO, especially if he slaps down requiring documentation of LVL's and trusses.


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## DRP (Jun 16, 2012)

I remembered a pic I posted a few months ago. Here the step is fully supported and used to resist the horizontal thrust of the rafter. Not a notch in the code sense, the dangling tit is doing serious work, so much so that it is checked for crushing and shearing off of the relish beyond the notch in the bottom chord.







This is a step lapped rafter. The "notch" is the area removed by the gently curved cope on the underside. This predates the NDS by a couple of hundred years but the old carpenters understood the part where it says that it's a good idea to gently taper out of a notch rather than making them square cornered. The 90 degree "notch" is the rafter end bearing, is fully supported and locked in against thrust very well. It is not a "notch" in the sense of the code useage.






This is the document the above drawing came from, an excellent free download;

http://ncptt.nps.gov/pdf/2004-08.pdf

In my modification look in the document above at pg 10 fig 8, a raising plate.


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## Sifu (Jun 17, 2012)

Pretty cool way to install rafters.  I can't even begin to imagine someone doing that today in standard light frame construction, though I imagine timber frames use something like it.  After the discussion on the rafter notches I took a real close look at them and based on my new-found rafter notch education they were not as bad as originally thought.  I had him pack them out in a few places where they might have been a little much as was shown and I had him add purlins everywhere to relieve the loads on the ends of the rafters.  As for the OP, I issued corrections which included seeing a DP.  I have not heard back since that last meeting which means he'll probably finish the house without making any corrections and call for a final.


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## globe trekker (Jul 27, 2012)

Continuing to beat on this horse, I have read and re-read this topic &

discussion several times.    Did Sifu ever get an actual answer to his

original question of: Is the rafter (in the pics.) overnotched,

according the letter of the code (max = 25%)?

Not saying that it is not a fixable situation, but is the rafter

overnotched by more than 25%? (RE: Section R802.7.1 in

the 2006 IRC)?

.


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## DRP (Jul 28, 2012)

It does not appear to be overnotched by code.

Code also allows a joist to be notched up to 25% of depth to sit on a ledger.

Notching like this in a material that is weak in tension perpendicular to grain is asking for trouble though.

This is another sketch I had on photobucket.






It is an old roof framing method called a "raising plate". This is basically the repair I was suggesting. In this situation the load does move inboard in doing so and dhengr pointed that out. I'll counter that in this situation the load would not shift more than the depth of the joist and would then pass without review.


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## globe trekker (Jul 28, 2012)

Continuing on with my question, ..as an example, if a 2" x 6" is

used as a roofing rafter, and that rafter is cut (notched) more

than 1.375" ( 1 3/8" ), is it a violation of Section R802.7.1 in

the 2006 IRC? Also, suppose the a 2" x 6" rafter is notched at

1.50", 1.675", ? When does it become an actual violation of

Section R802.7.1?     Yeah, I am a having difficult time with

this section and Section 2308.8.2 in the 2006 IBC.

.


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## DRP (Jul 28, 2012)

The birdsmouth is not a notch in the sense of this section. Think of it as the end cut on a stud. However once the seat cut slides inboard off the top plate that bears its' tension edge, the underhanging section, measured perdendicular to grain, becomes the unsupported notch.  If there is a tail extending beyond the wall, then you must leave at least a 2x4 section of wood remaining above the birdsmouth.

Look at my raising plate rafter, you've seen the same level cut on trusses, it goes all the way down to nothing. That is not a notch it is the end of the beam/column. The birdsmouth is the same, you're good till you slide the cut on the tension edge off the bearing.


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## Sifu (Jul 29, 2012)

I am glad I am not the only one that had trouble with this.  I understand it better now, (I think) seeing the underhanging section as the notched portion where I was looking at the entire seat cut and/or vertical cut.  I went back and looked long and hard at these rafters and all rafters since then.  It makes better sense the way DRP explains now than it ever has.


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## globe trekker (Jul 30, 2012)

Thanks DRP!

Forgive me for being slow of mind on this one. When Section R802.7.1,

Sawn Lumber (from the 2006 IRC), states that: Notches at the ends of the

member shall not exceed one fourth the depth of the member, I do not

understand how a birdsmouth is not a notch.

This particular section, and Section 2308.8.2 in the 2006 IBC, IMO, may

need some additional wording as clarification.

.


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## steveray (Jul 30, 2012)

GT...the birdsmouth is only a notch once it exceeds the bearing point.....once it is past the point of bearing, it can be cut all the way through (crosscut) and considered a notch....if it is supported, the notch does not really matter...


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## dhengr (Jul 30, 2012)

The birds mouth on a rafter bearing, on a wall top plate, is not a notch in this discussion because of the way the stress act on the member at the point of what obviously looks like a notch to all of us.  The eave overhand puts the member in compression at the reentrant corner of the notch, that is the effect of the overhang, a cantilever moment, and the shear stresses are fairly low.  Thus, this notch does not have a tendency to split with (along) the grain, also the tension edge is on the top of the member at this location.  The bearing surface of the birds mouth is supported by the top wall plate, in bearing or compression, so its likelihood of splitting along the grain is minimized.  The birds mouth is o.k. as long as the horiz. seat cut is not longer than the width of the 2x4 or 2x6 wall top plate which supports it.

When the rafter has a long horiz. seat cut, so the tension edge of the rafter substantially overhangs the inter edge of the wall top plate, then the tension edge of the rafter is not supported and can split along the grain.  If you extended the tension edge of the rafter to the wall, it might actually meet the wall below the wall top plates.  This forms the notch DPR and I are talking about, and it is very likely more than 25% of the member depth, measured perpendicular to the lower edge of the rafter.  The reentrant point of this notch is right at the upper/inner corner of the top plates, even if the seat cut is just a long horiz. cut.  It is akin to a 2" notch at the bearing end of a 2x8 floor joist.

This type of notch is dangerous for several reasons.  It is in the tension edge of the spanning member which is inherently dangerous, even though the bending stresses are very low at this location.  The bigger problem at the member bearing has to do with shear stress, which is max. at the bearing or member reaction.  Wood is notorious for splitting with (along) the grain of the member.  Wood is actually quite strong w.r.t. a shear stress (shearing force, vert. reaction force) which tries to cleave it across the grain.  But, at this notch point the vert. reaction force also produces what we call horiz. shear, equal in magnitude to the vert. shear stress, but acting along the grain, which tends to split the member with the grain.  The horiz. shear stress is the killer at these notches, it is happening in a region of tensile stresses; and is maximized because it is acting on a smaller member at the bearing due to the notch.  In my example above the 2" notch has effectively turned the 2x8 into a 2x6 which also means the shear stress effectively increases because of this loss in member area.  Thus a shear stress of 135psi on the 2x8 becomes 180psi on the 2x6, a 33% shear stress increase, right at the notch.

On the other hand, the notch in the tension edge of a beam in the middle of its span is a killer because it interrupts the uniform (smooth) flow of the tensile stresses where they are at their max.  It also reduces the depth of the member or its stiffness, and causes a reentrant corner which is prone to splitting along the grain.  Although, in this area the shear stress is usually fairly low.  So the notch at mid span is controlled by one set of stresses and conditions, while the notch at the beam bearing is controlled by another set of stresses and conditions.


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## globe trekker (Jul 30, 2012)

Thanks to all for your input!

I came across the following link in my search for some

more information on notches, birdsmouth cuts, rafters,

and etc. Check it out!

*http://www.builderbill-diy-help.com/rafter-cuts.html*

Hope this helps!

.


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