What is a low-slope roof?

What it is

A shingle roof works by gravity: each course laps the one below and water runs off before it can find a way in. Below a certain slope water lingers, wind drives it uphill under the laps, and shingles fail. The IRC therefore forbids asphalt shingles below two in twelve and requires two layers of underlayment between two and four in twelve, and it requires the membrane systems — built-up, modified bitumen, thermoset and thermoplastic single-ply — to have a design slope of at least one-quarter unit in twelve for drainage. That quarter-inch per foot is the "flat" roof's slope: enough to move water to a drain, scupper or gutter, not enough to see.

The IBC's definition of positive roof drainage captures the intent: the roof is designed and built so that water drains within a reasonable time after rain, allowing for deflection of the structure. A roof that holds ponds for days is not draining positively, whatever its drawings say.

What it costs and who does it

Low-slope replacement is priced by the square, with the system, the insulation beneath it (which may be tapered to create the slope the deck lacks), the number of penetrations and drains, the edge and wall flashing, and the tear-off of the old roof and any wet insulation as the variables. Commercial roofing contractors do most of it; residential roofers with single-ply certification handle porches, additions and flat-roofed houses. The directory's flat-roof replacement cost guide explains how those estimates are built.