Flat Roof Replacement Cost in the Seattle Area: TPO, Torch-Down and Metal

Almost every flat roof we replace in Snohomish and King County is one of four things: the low-slope roof on a mid-century house, the section over a 1970s addition, a carport, or a deck roof that was shingled when it never should have been. Searches for flat roof replacement are up sharply this fall, usually right after the first big rain finds the seam that failed in August.

Expect roughly $8 to $15 per square foot installed in 2026 for a residential membrane roof, including tear-off and new insulation board. A 400 square foot section typically lands at $4,000 to $8,000; a whole low-slope mid-century roof at $15,000 to $40,000.

The materials, priced

• TPO or PVC membrane ($9–$15 per sq ft). Single-ply sheets with heat-welded seams, fully adhered over tapered insulation. The default for most residential flat roofs here: 20 to 30 years, light-colored to stay cool, and seams that do not rely on adhesive.
• Torch-down modified bitumen ($8–$13 per sq ft). Asphalt-based rolls heat-fused in two layers. Tough, easy to patch, and common on older Seattle flat roofs. Fifteen to 25 years.
• Low-slope standing-seam metal ($14–$22 per sq ft). Mechanically seamed panels that work down to about a 1:12 pitch. The longest-lived option at 40 years and up, and the cleanest look on a modern house. Our metal vs asphalt guide covers metal in more depth.
• EPDM rubber ($7–$11 per sq ft). Cheaper and durable, but the glued seams are its weak point in a climate this wet. We use it less than the others.

What moves the price

• Tear-off and layers. Old tar-and-gravel roofs are heavy and slow to remove; two old layers cost more than one.
• Tapered insulation ($1.50–$4 per sq ft). A “flat” roof is supposed to slope a quarter inch per foot toward a drain or scupper. Many older ones do not, and tapered board is how a new roof fixes the ponding that killed the last one. It also brings the roof up to current energy code.
• Rotten decking ($90–$150 per sheet). Ponding water finds its way into the plywood; we price replacement per sheet on the bid.
• Walls, drains and transitions. Every place the membrane turns up a wall, meets a shingled slope, or wraps a drain or skylight is a detail that has to be flashed correctly. Most flat roof leaks start at one of these, not in the field.

Why flat roofs fail here

Ponding. Water that sits for more than 48 hours after rain ages the membrane fast and finds every weak seam. The fix is slope, not a better coating.

Shingles on a low slope. We see it constantly on carports and porch roofs tied into a split-level. Shingles are rated down to 2:12 and fail long before that in Western Washington rain. The Bothell and Everett split-level roofs we work on often need exactly this section redone in membrane.

Clogged drains and scuppers. Fir needles and moss block the one way out; our roof moss guide covers keeping a roof clear, and gutter replacement covers where the water goes after.

Repair or replace

A single failed seam or flashing on a roof under fifteen years old is a repair, often a few hundred to a couple of thousand dollars; our roof leak repair guide has the numbers. Widespread blistering, soft spots underfoot, standing water across the field, or a roof past its expected life is a replacement. If the flat section is part of a larger roof that is also near the end, replacing them together is cheaper and ties the transition in properly; see the full roof replacement guide.

Flat roofs from ENF

We replace low-slope roofs as part of our roofing work across Seattle, Everett, Edmonds and the rest of Snohomish and King County, and because we also build additions, the flat roof over a bump-out gets the same structure and slope as the rest of the house. The estimate names the membrane, the insulation, the slope plan and every flashing detail.

Get a flat roof estimate →