My house is built into a hillside, such that the roof of the garage serves as the deck next to the living room. The garage roof was metal, and over time developed rust and leaks, so I ripped it up, replaced underlaying damaged wood, recovered with new plywood, then used several layers of tarpaper along with roofing sealer, with a top layer of the heavy asphalt shingle type material that comes on a roll. But since that is not an attractive surface to walk on or set patio furniture on, I set 2x 4s on edge, on 16" centers, then covered with manmade plastic/wood decking. I was pleased with the result. But after 7 years I have leaks again, so lifted some of the decking and was surprised to see that the 2x4's had cut through the asphalt shingle material at several places. I'm scratching my head about what to use to prevent the 2x4s from repeating the cutting over the next 7 years. I've been told that if any weight, like patio chairs & table, is put onto rubber roofing membrane,that voids the warranty. So I'm looking for a good idea as to how to prevent future damage to the roof.