Flat roof replacement cost (2026):
prices by material and size
HOW MUCH DOES FLAT ROOF REPLACEMENT COST?
Replacing a flat roof typically costs $4,000 to $16,800, or $3 to $11 per square foot installed (HomeGuide). In Seattle, Angi puts the average at $9,088. Rolled asphalt is the cheapest material; PVC is at the top of the range.
A real example: two worn flat roofs
These photos show two low-slope roofs. One is covered in moss, leaves and twigs. The other has a white coating with dark patches and visible seams. Neither roof has been priced, so the figures below come from published guides.
For moss removal prices, see roof moss removal cost.
What a flat roof is made of
A simplified cross-section. Roofs differ, but most replacements involve these layers.
The membrane is the waterproof layer, and it's what the material names below refer to. A coating is a liquid layer painted on top of an existing roof. It can extend the life of a roof that's still sound, but it doesn't replace a failed membrane.
Flat roof cost by material
Installed price per square foot. A dash means the guide doesn't list it.
| Material | U.S. (HomeGuide) | Seattle (Angi) | Lifespan (HomeGuide) |
|---|---|---|---|
| Rolled asphalt | $2–$5 | — | — |
| Built-up (tar and gravel) | $3.50–$7.50 | $5–$12 | 20–30 years |
| EPDM (rubber) | $3.50–$10 | $6–$15 | 15–25 years |
| Modified bitumen | $4–$7.50 | $5–$10 | 10–20 years |
| TPO | $4–$10 | $5–$16 | 10–25 years |
| PVC | $5–$12 | — | 20–30 years |
A material that lasts longer can cost less over time. For example, at HomeGuide's prices a PVC roof can cost more up front than modified bitumen, but its listed lifespan is about ten years longer.
Flat roof cost by size
| Roof area | U.S. (HomeGuide) | Seattle (Angi) |
|---|---|---|
| 400 sq ft (20 × 20 ft) | $1,200–$4,400* | — |
| 600 sq ft | $1,800–$6,600* | — |
| 1,000 sq ft | $3,000–$11,000 | $8,000–$16,000 |
| 1,500 sq ft | $4,500–$16,500 | $12,000–$24,000 |
| 2,000 sq ft | $6,000–$22,000 | $16,000–$32,000 |
| 3,000 sq ft | $9,000–$33,000 | $24,000–$48,000 |
Ask for both the price per square foot and the total on every quote, so you can compare them.
Extra costs to ask about
| Item | Cost | Source |
|---|---|---|
| Tearing off the old roof | $1–$2 per sq ft | HomeGuide (Angi Seattle: $2–$5 for demolition and prep) |
| Insulation boards (polyiso) | $0.50–$3 per sq ft | HomeGuide |
| Spray foam insulation | $1.50–$5 per sq ft | HomeGuide |
| Inspection | $150–$400 | Angi Seattle |
| Permit | $250–$500 | Angi Seattle (see the permit note below) |
Permit in Seattle: the Seattle Department of Construction and Inspections says a permit is not required for re-roofing one- and two-family homes and townhouses "if no changes are made to the building envelope other than adding or replacing insulation." Commercial buildings and other cities have their own rules, so ask your roofer.
Repair, recoat or replace?
| Option | Cost (HomeGuide) | When it can make sense |
|---|---|---|
| Repair | $300–$1,100 on average ($2.50–$10 per sq ft) | A leak or damage in one spot on a roof that is otherwise sound |
| Recoat | $1–$4 per sq ft | The membrane is still sound, but the surface is worn |
| Replace | $3–$11 per sq ft | Leaks in several places, a wet deck or insulation, or a roof near the end of its lifespan |
Watch for standing water. Flat roofs are built with a slight slope so water drains. The National Roofing Contractors Association (NRCA) says proper slope is achieved "when there is no ponding water on the roof 48 hours after a rain during conditions conducive to drying," according to the Florida Roofing & Sheet Metal Contractors Association. The same article notes that manufacturer warranties often exclude membrane failures caused by poor drainage.
Sales tax and hiring in Washington
In Washington, a roofer working on a homeowner's property collects retail sales tax "on the gross contract price," labor included (Washington Department of Revenue). In Seattle the rate is 10.55% since January 1, 2026.
| Example | Amount |
|---|---|
| Seattle average flat roof replacement (Angi) | $9,088 |
| Sales tax at 10.55% | + $959 |
| Total if tax is not already included | $10,047 |
- Look the roofer up on L&I's Verify a Contractor tool: active license, bond, insurance and past violations.
- For jobs of $1,000 or more on a home, the contractor must give you a written disclosure statement before work starts (RCW 18.27.114).
- Ask what membrane they will install, whether the price includes tear-off and insulation, and how they will handle drainage.
Replacing a sloped roof instead? See roof replacement cost in Seattle or try the roof replacement cost calculator.
Frequently asked questions
How much does flat roof replacement cost?
HomeGuide puts the typical cost at $4,000 to $16,800, or $3 to $11 per square foot installed. In Seattle, Angi lists an average of $9,088 and a typical range of $5,309 to $13,593.
What is the cheapest flat roof material?
Rolled asphalt, at $2 to $5 per square foot installed according to HomeGuide. Built-up roofing and EPDM rubber start at about $3.50 per square foot.
How long does a flat roof last?
HomeGuide lists 10–20 years for modified bitumen, 10–25 years for TPO, 15–25 years for EPDM, and 20–30 years for PVC and built-up roofs.
Is it cheaper to recoat a flat roof than replace it?
Yes, if the roof is still sound. HomeGuide lists roof coatings at $1 to $4 per square foot, compared with $3 to $11 for a replacement. A coating doesn't fix a membrane that has already failed.
How long can water sit on a flat roof?
Not more than 48 hours after rain in drying conditions. That is the NRCA standard for proper slope, as cited by the Florida Roofing & Sheet Metal Contractors Association.
Do I need a permit to replace a flat roof in Seattle?
For a one- or two-family home or townhouse, usually not. SDCI doesn't require a permit when nothing changes in the building envelope other than adding or replacing insulation. Other buildings and other cities have their own rules.
Sources
- HomeGuide — Flat Roof Replacement Cost (edited Nov 5, 2025)
- Angi — How Much Does It Cost To Replace a Flat Roof in Seattle, WA? (updated Aug 1, 2026)
- Florida Roofing & Sheet Metal Contractors Association — Ponding Water and Manufacturer Warranties
- Seattle Department of Construction and Inspections — Re-Roof Permit
- Washington Department of Revenue — Custom construction
- Washington Department of Revenue — Local sales tax change, Seattle, effective January 1, 2026
- Washington L&I — Hiring a contractor
- Washington State Legislature — RCW 18.27.114, Disclosure statement
All sources accessed October 4, 2026.
These figures are estimates compiled from the published sources above. They are not quotes and can't account for your roof or your contractor. Tax and permit rules can change, so confirm them with your roofer and your city. Get written estimates from licensed, bonded and insured roofers before deciding. See our disclosure and disclaimer.