Is a roof system considered a "diaphragm" for seismic design purposes?

Any horizontal structural diaphragm is part of the lateral resisting system whether it be wind or seismic induced, and this DEFINITELY includes the roof structure. Mainly the plywood roof diaphragm, but also the chords, drag struts, and edge connections to the shear walls or moment frames.

Mike McCann
MMC Engineering

Msquared is right

ASCE 7-05 does not clearly state design provisions for roof trusses or roof systems and I think it is largely ignored by truss designers. What method/section of ASCE 7-05 would you use to design the roof system? My interpretation of the code makes me think it should be treated as a diaphragm. There is also an argument that it should be treated as a non-structural component. Any thoughts?

I think you are comparing apples with oranges here.

The "roof diaphragm" is the plywood membrane over the roof truss system, not the roof trusses themselves. Now the trusses have to be able to withstand wind and seismically induced loads applied in certain ways, but the structural diaphragm is the plywood. The roof trusses and intermediate eddge blocking serve to provide support to the diaphragm to stiffen it as necessary, but they are really not a part of the structural diaphragm per se.

Am I missing your question here?

Mike McCann
MMC Engineering

msquared48, I want to thank you for your input. It is greatly appreciated. I don't think you're missing my question. I think this may be an iterative process because I am trying to interpret the code. I understand that the "roof diaphragm" is the plywood or deck material, and not the trusses themselves, but I am trying to design the trusses and blocking for seismic loads. Since there are no specific design provisions for trusses or blocking, I figured that their main purpose is to trasnfer the diaphragm loads to the structure. By calculating the diaphragm loads, that will give me the seismic design loads for my trusses and blocking. Is this incorrect? Am I missing something?