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Re: Plywood Diaphragms

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Richard Dahlmann,

My understanding of the idea is to install a new plywood diaphragm over
existing roofing, forming a roofing-sandwich between the new diaphragm and the
old roof structure?

If I have that right, I don't like it.  How will the installer know what the
nails are being driven into: a sheathing board near a board edge, the space
between sheathing boards,  close to (or off) the end of a board at a butt
joint over a rafter, or well into a sound sheathing board with the nail well
centered in a rafter?  It seems that most nails are likely to have inadequate
penetration.  Nail values assigned by the Code are based on proper
penetrations, as in UBC Table 23-I-G.

Appendix I (eye) of the NDS for Wood Construction illustrates the six 2-member
yield modes considered in developing the strength values for connectors in 
wood -- all have the the connected wood members directly in contact.  I think
that an assembly with a 1/2" gap of roofing would be an untested kind of
assembly with unknown strength values.  

Nels Roselund
Structural Engineer