For 99% of the flat roofs (or low-sloped roofs below 2/12 pitch) that we replace, we use 0.60 Mil TPO. TPO is a welded seam flat roof system. It comes in white and is a smooth vinyl-type material and is installed over 1/2" insulation. The use of TPO instead of rolled roofing/ modified bitumen base-sheet/top-sheet is beneficial because the seams are heat welded together to prevent water penetration.