I've got a old 8 x 20 shed with a flat roof in good shape except the old tar paper is pretty beat and leaks in one place. I saw a roll of material that's crack and peel water proofing and was told it's terrific. Can't recall the name. It's got an adhesive "built in". Can that be put on a relatively flat existing surface or do I need to take up the old stuff down to the plywood? Also, I'm guessing tar paper or some other material has to eventually go over it? True? Thanks very much.