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Roof Truss Bird-Mouth Cuts

Roof Truss Bird-Mouth Cuts

Roof Truss Bird-Mouth Cuts

(OP)
Take a look a the attached wall detail from the original drawings for 1970's Low Income Housing Project.  The current remodeling project calls for the 3'-3 3/4" overhang to be removed in some locations and in other areas the roof of a new addition will bear on the existing roof with the two ridges at 90 degrees to each other.

When we opened up the existing roof and cut off the overhange as called for, we discovered the bird-mouth cuts at all the roof trusses.  At first, we thought it was a field fix to a dimension error to make the trusses fit.  Well, when I went back to the original details (see attached) sure enough, they were designed that way.  Is this very common? What about the reduced section at the wall line, how does it affect the overhang strength?  Bye the way, these trusses are all 2x4 members with a clear span of about 25'-6" in the Mid-West with a ground snow load of 25 psf.

I've never seen a detail like this and I'm just looking for some input, is this still done today and why?







 

RE: Roof Truss Bird-Mouth Cuts

I never detail things this way, but if the designer was thinking solid rafters with flat bearing notches (bird mouth) due to an unfamiliarity with trusses, the obvious alternate would probably not have occurred to him.

FYI, my first thought, as yours, was a dimensional error too.

Yes, with ovefr a three foot overhang, it will affect the overhang for shear, bending and deflection, hands down.

Mike McCann
MMC Engineering
Motto:  KISS
Motivation:  Don't ask

RE: Roof Truss Bird-Mouth Cuts

You would need to find the truss design/details to see who took the responsibility of the overhang. Back in the early 80's (when I started designing trusses). We designed only the specialty trusses. The common trusses were all covered by standard designs. With standard overhang and bird-mouth details. Some manufacturers would make the trusses per these common truss standards and use the plans details for the bearing and overhangs details. Assuming the building designer was taking the responsibility for the overhang and bird-mouth. Unfortunately, many of the building designers were assuming the truss engineer would be taking the responsibility, but failed to check the truss designs to make sure. I did do some specialty designs showing the maximum bird-mouth/overhang we would allow without a add-on and what they could get with an add-n(s). But they seldom match the plans.
Please note the the design is from the 70's and without a failure, due in part to the safety factor in wood values and the fact that the detail probably never saw the full code design load.

Garth Dreger PE
AZ Phoenix area

RE: Roof Truss Bird-Mouth Cuts

Another thought...  

If there is an extra horizontal member at the soffit level  that is connected to either the truss end or rim board and the wall, this could be what has prevented the eave end of the truss from failing in any high load condition.   

Mike McCann
MMC Engineering
Motto:  KISS
Motivation:  Don't ask

RE: Roof Truss Bird-Mouth Cuts

I think it just indicates ignorance on the part of the architect who developed the wall section.  The truss provider was a puppet.

RE: Roof Truss Bird-Mouth Cuts

I wonder if there was a structural involved here at all.  Remember that sometimes Architects think that they are Structural Engineers.  What's even more dangerous, is that up to a point, the code allows them to be such.

Mike McCann
MMC Engineering
Motto:  KISS
Motivation:  Don't ask

RE: Roof Truss Bird-Mouth Cuts

Agree, Mike.  No structural engineer, at least not a competent one.

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