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Help, the contractor that is redoing my roof wants to cap the existing osb board with plywood. I've always heard that shingles are heavy and having two layers of wood instead of just one will only increase that weight. Should I let him put these two layers on with the shingles or just go with a metal roof. I want to have an overhang so that is why he needs to overlay the existing board. Removing the old would just be time consuming in his view.

Removing is time consuming, but is he framing new eaves or just relying upon the sheathing overhang to create it? And how much of an overhang?
If you have 1/2 decking up there already, I wouldn't go back with anything that would make it thicker than 3/4" - therefore about 1/4" sheathing.

I would want any water damaged sheathing replaced, meaning pull the old ones off and replace with new. Plywood alone of any thickness would not make a very good overhang. I am sure it will warp. An overhang should be framed as Mark shows in his books. If this was going to be a metal panel roof, you can overhang those an inch or so. I have seen them stick out 6" and more, but I wouldn't do it. Climbing a ladder on a roof edge like that could create damage or leaks. A person working on top of the roof could step on that area and fall off the roof. Shingles should overhang the edge of the sheathing (at the nose edging) by 3/8" to help that water fall away from the eave.

I would really advise against layering sheathing over sheathing.