Flat roofs are priced by the square foot instead of by shingle bundles, and the material choice matters more than on a pitched roof. Here is how to estimate the cost and compare systems.
Flat roof materials compared
| System | What it is | Seams | Typical strengths | Watch out for |
|---|---|---|---|---|
| TPO | White single-ply membrane | Heat-welded | Reflects heat, strong seams | Quality varies by brand and thickness |
| EPDM | Black or white rubber sheet | Taped or glued | Proven, flexible in cold, simple repairs | Black absorbs heat; seams are the weak point |
| Modified bitumen | Asphalt-based rolls | Torched or self-adhered | Tough, good for foot traffic | Torch-down needs a skilled crew |
| Built-up (BUR) | Layers of felt and asphalt with gravel | Continuous | Very durable | Heavy, messy install |
| Roll roofing | Mineral-surfaced asphalt roll | Lapped | Cheapest, DIY friendly | Short life, only for sheds and garages |
How to estimate the cost
Cost = roof area × installed price per sq ft + extras
- Area: length × width including overhang. Slope adds almost nothing on a flat roof.
- Waste: about 5% for seams, laps and flashing.
- Price per sq ft: from local quotes for the system and thickness you want.
- Extras: tear-off of old layers, new insulation or tapered insulation, edge metal, drains and scuppers, and curb flashing around HVAC units.
Example price table
| Roof area | at $6 / sq ft | at $8 / sq ft | at $10 / sq ft | at $12 / sq ft |
|---|---|---|---|---|
| 300 sq ft | $1,800 | $2,400 | $3,000 | $3,600 |
| 500 sq ft | $3,000 | $4,000 | $5,000 | $6,000 |
| 1,000 sq ft | $6,000 | $8,000 | $10,000 | $12,000 |
| 2,000 sq ft | $12,000 | $16,000 | $20,000 | $24,000 |
These are example rates to show the math. Use your own quotes.
Worked example: a detached garage
A 20 × 20 ft garage with a 12 inch overhang and a flat roof.
- Area: 22 × 22 = 484 sq ft
- With 5% waste: 508 sq ft, about 5.1 squares
- At $8 per sq ft installed: 484 × 8 = $3,872
- Add tear-off and any new edge metal from the quote
The calculator handles this directly: open the garage example. The Flat roof type sets 5% waste, and the price toggle switches to per sq ft.
Drainage matters more than material
Most flat roof failures start with ponding water. Good practice:
- At least 1/4 inch of slope per foot toward drains or gutters
- Tapered insulation to fix low spots during a re-roof
- Clear drains, scuppers and gutters at least twice a year
- Proper flashing at walls, curbs and penetrations
Low slope vs flat
Between 2/12 and 4/12, shingles are allowed with double underlayment. Below 2/12, you need a low-slope system such as a membrane or roll roofing. See the minimums in standard roof pitch. For pitched roofs, use roof replacement cost instead.