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Plywood Diaphragms

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Just off the top of my head, I would not allow a contractor to do this.

For one thing, if you increase the nail size, you increase the chances of
splitting the existing framing.

For another, if the existing roofing material is 1/2" thick wood, don't you
already have a legitimate diaphragm?  If it is in poor shape, placing the
ply sheathing over it is going to damage it further, and you still won't
have a real diaphragm because you will not get a good connection between
the new sheathing and the framing.

My 2.3 cents worth at 3AM

Kate O'Brien, P.E.
Simi Valley

 >>The contractor wants to leave the existing roofing (1/2" thick roofing)
place. I am looking for opinions on increasing the nail size and allowing
to do this. The plywood will not carry any gravity loads. Has anyone seen
pullover problems or anything else which may suggest problems with this?<<