How much does a flat roof replacement cost?

First, is it a replacement?

A flat roof with one failed seam or one bad drain is a patch. A worn membrane over dry insulation can be coated. Replacement is the answer when the insulation is soaked, when seams are opening across the whole roof, or when the roof has been patched so many times the patches are the roof. Read patch, coat or replace for how we decide.

If the insulation is wet, there is no shortcut. Coating over it seals the water in and the next failure is bigger.

What is under the old roof moves the price most

The membrane you can see is one layer. Under it there may be insulation, and under that a deck of plywood or, on older rowhouses, wood planks. All of it has to be sound for the new roof to last. Wet insulation gets replaced. Soft or rotten decking gets replaced. Neither can be seen from on top, which is why the estimate states a per-sheet rate for decking and a rate for insulation before the tear-off starts.

How many layers are already up there matters as well. Older roofs were often covered over instead of stripped. Each layer is more labour to remove and more weight to dispose of, and it goes in the written price.

The edges and the penetrations

A flat roof almost never fails in the middle. It fails at the parapet, at the party wall, at the drain, at the pipe and at the skylight curb. Each one is a detail that has to be flashed and terminated properly, and each one is time. A plain roof with one drain and one vent is a day. The same square footage with two parapets, a party wall on each side, an HVAC curb and three pipes is a different job.

The estimate should say what happens at each edge, not just name the membrane. New coping or counter-flashing at the parapet, how the membrane ties into the neighbor on a shared roof, and whether the drains are being replaced.

What else moves the number

• Drainage. A roof that ponds will wear out its next membrane the same way. Tapered insulation or a new drain adds cost now and takes years off the maintenance later.
• Material. Modified bitumen, TPO and EPDM are priced differently and suit different roofs. The right one depends on the roof, the details and what is next door, and the estimate names which one and why.
• Access. A roof a truck can reach is cheaper than one at the back of a rowhouse with no alley, where everything goes up a ladder or through the house.
• Size. Small roofs cost more per square than large ones, because setting up takes the same time either way.
• Commercial roofs. Occupied buildings, after-hours work and tie-ins to rooftop equipment are all quoted per job. Short-notice, after-hours and weekend work is available for commercial clients.

What a fair quote lists

• Tear-off down to the deck, and disposal
• The per-sheet rate for decking and the rate for wet insulation, agreed before anyone goes up
• The membrane by type and thickness, not just a brand
• What happens at the parapet, the party wall, each drain and each penetration
• Whether drains, scuppers and coping are replaced or reused
• Clean-up, and photographs of the finished roof
• The warranty on the work. Ours is 60 to 90 days, and the term for your job is stated

The estimate is free and the price is in writing before the work starts. If the roof has leaked long enough to stain a ceiling, the inside is priced with the roof rather than turning up afterwards. See flat roofing and commercial roofing.