wood roof truss porch overhang - ceiling diaphgram
wood roof truss porch overhang - ceiling diaphgram
(OP)
If you have a wood roof truss system, 6:12 pitch, that continues outside the exterior wall of a residential structure over an 8ft porch area and bears both on the exterior wall and a post and beam at the edge of the porch, does the porch ceiling need to be a diaphragm? I think the roof sheathing adequate to provide the cantilever resistance but I typically specify rated sheathing and a dense regular nail pattern for the porch ceiling anyway and figure they could put the finish on top of that.
Instead the contractor put 5/8" T111 and finish nails (don't know the nail spacing). I think this is adequate to brace the bottom chords...but don't know it diaphragm capability...and I doubt the finish nails provide adequate resistance for any C&C loads (we are in a high wind area)...
Any thoughts on suitability of what was constructed?...how do you typically detail the ceiling of a truss porch overhang?
Instead the contractor put 5/8" T111 and finish nails (don't know the nail spacing). I think this is adequate to brace the bottom chords...but don't know it diaphragm capability...and I doubt the finish nails provide adequate resistance for any C&C loads (we are in a high wind area)...
Any thoughts on suitability of what was constructed?...how do you typically detail the ceiling of a truss porch overhang?






RE: wood roof truss porch overhang - ceiling diaphgram
The lateral wind load (parallel to the porch and perpendicular to your trusses) will flow down through your diaphragm towards the ends of the trusses.
If you have a brace wall at the main wall of the house then the diaphragm would want to be tied into that wall with some kind of transfer wall from the top chord down to the bearing wall. This is difficult to do as you have to sheath the connecting "attic" wall between each truss.
If you don't provide that lateral tie, then the diaphragm force will continue down to the end of the truss (edge of the porch) and then have to flow back under the trusses to the braced wall at the back of the porch via the ceiling. Not the best scenario but perhaps the only reasonable way to do it.
RE: wood roof truss porch overhang - ceiling diaphgram
Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.
RE: wood roof truss porch overhang - ceiling diaphgram
RE: wood roof truss porch overhang - ceiling diaphgram
RE: wood roof truss porch overhang - ceiling diaphgram
I do a number of clear story - 2 story atriums. Either I build a steel moment frame or use the ceiling as a diaphragm. Guess which is cheaper.