A rooftop deck leaks into the room below after the crew screwed the boards through the membrane

As the owner described it, the assembly runs from a plastic sheet and insulation up to 9 mm cement board, a liquid-applied waterproofing coat and a sheet membrane, with wood-plastic composite decking on top. The boards were screwed down through the membrane, and the surface is dead flat. Each screw is a hole in the one layer meant to stop water, and with no fall the water sits on the membrane long enough to find them.

The International Residential Code requires low-slope membrane roofs, including modified bitumen, EPDM, TPO and PVC, to have a design slope of at least 1/4 inch per foot (2%) and to be installed to the manufacturer's instructions. Doug Horgan of BOWA, a Virginia design/build remodeler, wrote in the Journal of Light Construction that his crews never go below that slope under a deck, and that even at 1/4 inch, seam buildups can hold puddles. Carlisle, a membrane maker, tells owners that standing water should be gone 48 to 72 hours after a rain.

Horgan's firm floats its decks. It sets tapered pressure-treated sleepers or adjustable pedestals on a loose-laid EPDM protection layer and screws the boards only into the sleepers, using screws shorter than the sleepers are deep. Carlisle's EPDM specification has pavers sitting loose on rubber pedestals on top of a cured-membrane slip sheet. Horgan also leaves lift-out deck sections at the high and low ends so drains can be cleaned, and notes that some composite decking makers advise against floating sleepers.

Fixing this deck means lifting the boards to reach every fastener hole. A roof with too little slope can be corrected with tapered insulation over the sheathing, Horgan wrote, before the deck goes back down on sleepers or pedestals.