ROOF SYSTEM with more than one layer. MULTIPLE PLY MEMBRANE roof systems are also called BUILT-UP ROOFS and hot tar roofs. They are usually made from ROLL ROOFING materials that are BONDED together with asphalt. A three-ply roof has a BASE SHEET, PLY SHEET, and CAP SHEET. A five-ply roof has a BASE SHEET, three PLY SHEETS, and a CAP SHEET. Hot tar (asphalt) is used to BOND the plies and make the roof water-tight.
