I'm having my tar and gravel roof replaced, and as part of the contract I was having my old 3/8 plywood removed and replaced with 5/8. Turns out the old plywood is still in good shape ie no rot at all. The roofer would like to just put the new plywood on top of the old one. I understand it's easier for him--is there any downside to this for me? As a possible negative, the roof will be heavier, but will a 3/8+5/8 (i.e. I inch roof) be any better than a 5/8?