theyre needing to put on a roof and the plywood is shot can we add a layer of plywood over the old and reroof?

not a good idea is it? get all the bad ply off!

Depends on the extent of the rot, Both the square footage of the affected sub-roof and the severity of the rot itself. If the roof joists are mostly intact, the bad portions of sheathing can be removed and patched with new plywood of the same thickness. You can shingle will 3 tab tiles at this point or you can go ahead and add a second layer of plywood and then shingle.

Don't do it. First off you will have a roof that is all wavy...plus the added weight is not good on the rafters. I would pretty much bet city code would not allow you to put a layer of board and more shingles over that.

They had to replace our ply board too when we needed a new roof. There were rotten spots and holes in the old ply board so nailing on the tar paper would have been a chore let alone the shingles. It took just one day to cut and install the new board.

It is not a stupid question! You should never put new materials over old ones, because the old ones might have termites and other bugs, or the old material might contain mold. Anyway to answer your question - DO IT RIGHT! the first time. Tear everything off and start fresh for the very best looking and securest roof in town!

I am no expert, but my gut tells me that you shouldn't do that at all. Time would be saved by doing it right the first time. You will have more trouble in the long run doing it the way that you want to do it. Why throw good money after bad. Meaning why do it in the first place. Do it right the first time and then you will not have to worry about it again.

This is really far out. I wouldn't even think of this. It's crazy. Do the job right or your going to be running into trouble.

There are a bunch of fungus spores in the rotted wood waiting to rot more wood. Throw out the rotted wood.