wood roof truss porch overhang - ceiling diaphgram

[eng003]

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?

 

[JAE]

Think of your load path for lateral loads on the roof diaphragm.

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.

 

[woodman88]

I try to never consider the ceiling diaphragm as a structural component of the building system, except for the bottom chord bracing of the trusses. If you are concern with transferring the lateral forces from the roof diaphragm to the bearing wall. I typically detail a shear panel (four pieces of 2x with 3/8” OSB/plywood nailed to them) between the trusses. Nail the diaphragm to the top 2x and the verticals to a verticals in the truss and the bottom to the wall. Use clips on the truss to wall connect for the shear panel uplift. This way I only consider the roof sheeting as my diaphragm.

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.

 

[eng003]

I actually took care of transferring the roof diaphragm to the exterior wall by detailing extended shearwall panels up to the roof elevation (however the contractor got me to revise the detail to some X brace instead). So I think I have lateral transfer taken care of...and I think the ceiling assembely installed provides adeqauate bracing of the truss bottom chords...the only thing left is to verify the nailing is sufficient for any wind suction loads it will see...but how am I going to quantify withdrawl of finish nails?

 

[ToadJones]

Can you put 2x4 bracing on the top of the bottom chords of the cantilever portion of the truss and bring the diaphragm loads back to wall of the house this way?

 

[MiketheEngineer]

My thoughts are that any help you can get - is WORTH it.

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.