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[SEAOC] Re: [SEAOC] Re: [SEAOC] Re: Reroofing (Was: Basic Roof Rafter Analysis Question )

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I, personally, don't recommend the use of plywood over the existing space sheathing because of the additional weight it imposes.
There is a benefit to stiffening the roof by the addition of plywood, however, it may be detrimental to the existing structure to increase the demand to walls that don't have the capacity to resist it.
This is the same issue that we faced when we designed URM buildings.  Los Angeles City determined in their RGA that there was a benefit to increasing the stiffness of the diaphragm by installing plywood over the existing 1x straight sheathing. I refer you to Guideline 9 of the City of Los Angeles Department of Building and Safety Earthquake Safety Divisions. This references Section 91.8809(c) of the city code that provides an increase shear value for the "nailing of new plywood sheathing over existing straight sheathing".  The demand was increased from 100 plf for straight sheathing to 225 plf for plywood installed.
I believe, since we are discussing flexible diaphragms, that the analogy is the same whether the shear resisting element is masonry or wood.
The difference lies in the fact that we were *required* to check the in-plane capacity of the piers, while in re-roofing, we generally don't. By increasing the demand of the roof, you may, in fact, be creating a greater risk to the structure. You may be reducing the risk of damage to interior walls by directing the shear to the exterior walls, but you create a greater risk to exterior resisting walls that may not be prepared for the increased stiffness.
Hope this helps, but I think it raises more questions.

Dennis Wish PE