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?
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
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
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
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
RE: Roof Truss Bird-Mouth Cuts
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Roof Truss Bird-Mouth Cuts