Flat and Low-Slope Roofs:
Ponding, Open Seams, Blisters and Shingles Where They Don’t Belong

Matching the roof to the slope

Roof slope is measured in rise per 12 inches of run. A 2:12 roof rises 2 inches every foot. Below about 3 or 4 inches, roofers call it low-slope, and the material has to change.

Shingles work by overlapping and shedding water downhill. On a shallow roof, water moves slowly enough to back up under them, especially in wind-driven rain. The residential code allows asphalt shingles on slopes of 2:12 and steeper, and from 2:12 up to 4:12 it calls for a double layer of underlayment underneath.

Russ found both sides of that line crossed. An Aloha roof at about 2:12 had shingles without the proper underlayment. A roof in The Dalles had shingles on a pitch under 2:12. Battle Ground and Portland low-slope shingle sections, at about 3:12 and 2.5:12, had only a single layer of synthetic underlayment. The shingles look normal from the yard. The difference is underneath.

Below 2:12 the roof needs a membrane or roll roofing. Common ones on Portland-area houses:

• Modified bitumen, often torch-down, in rolls bonded with heat
• Single-ply membranes like EPDM (black rubber) and TPO (usually white)
• Asphalt roll roofing, the cheapest option, with the shortest life
• Built-up roofs (tar and gravel), mostly on older flat roofs

Seams, edges and penetrations

On a membrane roof, the seams are the weak point. Every lap has to be bonded or welded watertight, because water may sit on it for days. Open, lifted or uneven seams were part of several of Russ’s membrane findings, including the Oregon City roof with uneven seams and protruding fasteners.

Penetrations are next. Anything that comes through a flat roof (a plumbing vent, a heat pump line, a skylight, a radon pipe) needs a flashing designed for that roof. Caulk alone isn’t flashing. In Vancouver, heat pump refrigerant lines came through both shingles and a flat membrane with only sealant around them. Skylights on low-slope roofs need a curb to lift the joint above the water, which the skylight post covers.

Edges and transitions get their own trouble. Where a flat roof meets a wall or a steeper roof, the membrane has to turn up and be flashed. The post on roof flashing problems covers those joints in general, and the drip edge post covers the roof edge.

Ponding, soft spots and what’s under the membrane

Ponding is water that stays on the roof after a rain. Roofers commonly call it ponding when water is still standing about 48 hours later. It wears the membrane faster, finds every weak seam, and its weight can make a low spot deeper.

Ponding usually traces to something under the roof. Sagging framing. Insulation board under the membrane that’s crushed or wet. A drain that’s clogged. The Portland garage roof with the soft depression was that kind of finding: the board under the membrane felt soft or missing in one spot. A Clackamas membrane had been laid on an uneven substrate, with the deck visible through it.

Drains and scuppers decide where the water goes. A Portland garage flat roof had its drain blocked with debris, organic matter and a crushed strainer. On another Portland house, the fascia and rafter tail behind a gutter scupper were stained and decayed, the kind of damage that happens when water overflows the scupper instead of going through it.

Aging membranes and roll roofing

Membrane and roll roofs show age differently than shingles. Torch-down cracks into a pattern of small blocks (alligatoring) as the surface dries out. Roll roofing loses its granules and goes bare. EPDM shrinks and pulls at the edges. Single-ply membranes come loose and “bridge” across low spots, which a Portland roof showed.

Russ found worn roll roofing on a Lake Oswego boathouse, and roll roofing used in roof valleys in West Linn and Gresham, where it’s generally less durable than metal flashing or a proper valley membrane. Those valley patches are usually repairs, and the post on what to look at on the roof before you make an offer covers what past repairs can tell you.

Decks over living space

Some flat roofs are also decks. A rooftop deck over a bedroom or a deck built over a finished room has a waterproof membrane under the decking that carries all the water off. In one Portland house, the rear deck sat directly over finished living space, and the membrane was hidden under the decking surface, so its condition couldn’t be seen. Those get noted for exactly that reason. The deck inspection post covers the rest of the deck.

What a home inspection can see

Russ looks at flat roofs from the roof surface when it’s safe to walk, and from the drone when it isn’t. He looks at the membrane surface, seams, edges, flashing, penetrations, drains and scuppers, and any ponding or soft spots. Inside, he looks at the ceilings below for stains, and in the attic or rafter space when there’s access.

He can’t see the deck or insulation under the membrane, check every seam bond, or know the age of the roof from the surface alone. When a low-slope roof has problems, the report recommends a licensed roofer, ideally one who works on low-slope systems.

To set up an inspection, you can book online.