Part of the roof on the house we live in has a very low sloped roof over it. About 15' x 40'. Presently there is rolled roofing that is leaking, again. I have patched seams, holes,, cracks and am tired of it. I am considering installing a one piece rubber roof. I talked with retailer that can get all the supplies I'll need and he gave me an application guideline brochure I've looked through. Any one have any experience installing these rubber roofs on a low sloped roof? I am not certain of how many layers of the rolled roofing are already up there and there is some un-evenness where it looks like the sheathing is sinking between the rafters. How much variation can I get away with from 'totally flat'? When should I consider a complete tearoff and re-sheath before applying the rubber roof? I have done ashphalt and fiberglass three tab singles but not a rubber roof.