How to Lay a PVC Roof over a Torch-Down Roof

Nail 2-by-4s around the perimeter of the roof as a fascia board to secure the roof edging. Allow one inch of the 2-by-4 boards to protrude above the roof edge. Starting from one edge, lay down 4-by-8-foot sheets of one-inch-thick rigid foam insulation.

Install one screw and washer at each corner of a foam insulation sheet and one screw and washer centered at each edge. Use screws long enough to penetrate through the roofing into the substrate. Fasten odd-shaped fill-in pieces at corners only.

Lay a 3-foot-wide perimeter strip of PVC roofing around the edges of the roof so that the outside edge droops 5 inches over the roof edge. Lay with the white side up. Fasten the inside edge with screws and washers spaced 6 inches apart. Fold the outside edge of the roofing down over the fascia board, and nail it to the underside of the fascia board.

Secure the metal drip edging to the roof perimeter with roofing screws set 12 inches apart. Lay a 6-inch-wide strip of PVC roofing over the metal drip edge, and use the hot-air heat gun to weld it to the PVC material on the roof and the fascia board, forming a leak-proof edge seal.

Starting at one edge, unroll the 6-foot-wide PVC roofing, white side up, across the roof so that it overlaps the screws holding down the edge pieces. Secure the inner edge with screws and washers set 6 inches apart. Heat-weld the outer edge to the perimeter piece so that it covers and seals the row of screws holding the perimeter piece.

Roll out the next section, overlapping the screws holding down the previous section. Repeat the screw-down of the inner edge and heat-welding of the outer edge to cover the row of screws holding the previous piece. Continue this process until the roof is covered.

Cut the roofing to fit flat around roof hatches, skylights and other low roof projections. Install PVC flashing around each roof projection so that the flashing rises to the top of the projection. Shape the flashing to fit the projection, and heat-weld it into place. Heat-weld the bottom edges of the flashing to the roofing. Install the cap piece on the projection to cover the top edge of the flashing.

Install PVC flashing around tall projections such as chimneys, vent stacks and parapet roof curbs. Seal the flashing to the projections according to manufacturer directions. Heat-weld the bottom edges to the roof.