Understanding your result
The headline is the rolls of membrane. The stats give the roof area, the number of strips, the seam length and the adhesive. The breakdown builds the sheet size with turn-ups, the net width of each strip after its seam overlap, the strips and rolls, the seams, the tape and adhesive and the edge metal.
The number to watch is the seams. Every foot of seam is a foot of joint to tape or weld, and seams are where flat roofs fail; if the result suggests a wider sheet would cover the roof in one piece, that is almost always worth the extra handling. The rolls are what you order; the seam tape and adhesive are what you forget.
For a steeper low-slope roof (2/12 to 4/12) the rolled roofing calculator covers mineral-surfaced roll roofing, and the cost of a membrane roof against other materials is in the roof replacement cost calculator.
How we calculate this
The strips run the long way so that seams are as few as possible, and each strip is cut from a roll: a 100-foot roll gives two 42-foot strips. When a strip is longer than a roll it needs an end splice, which the calculator counts. The turn-up is the membrane that runs up a parapet or over the edge to the fascia; it is part of the sheet, which is why the sheet is larger than the roof.
The flat roof membranes guide compares EPDM, TPO and PVC, and the roof area calculator gives the area for a roof with a slight pitch.
The assumptions behind the numbers
| Assumption | Default | Where it comes from |
|---|---|---|
| Roll size | 10 × 100 ft | Common EPDM, TPO and PVC rolls; EPDM also 20 ft wide |
| Seam overlap | 3 in | EPDM taped seams; TPO and PVC 2 to 3 in (manufacturers) |
| Turn-up | 12 in | Up a parapet (8 to 12 in) or over the edge to the fascia |
| Adhesive coverage | 60 sq ft of roof per gallon | Fully adhered EPDM, both surfaces (manufacturers’ data) |
| Seam tape | 100 ft rolls | Common packaging |
| Minimum fall | ¼ in per ft | IRC and manufacturers |
Assumptions last reviewed October 7, 2026.
The calculator does not count the insulation and cover board under the membrane, the pipe boots, curbs and drains, or the flashing at walls, and it assumes a rectangular roof. For an L-shaped roof, run each rectangle separately. The lifespan guide covers the expected life of each membrane.
Two worked examples
A 40 by 24 foot flat roof in 10-foot EPDM
10 × 100 foot rolls, 3-inch seams, 12-inch turn-ups, fully adhered.
- Sheet: 42 × 26 = 1,092 sq ft
- Net width: 10 ft − 3 in = 9.75 ft; strips: 3 of 42 ft
- Rolls: two strips per roll, so 2 rolls
- Seams: 2 × 42 = 84 ft, 1 roll of tape; adhesive: 1,092 ÷ 60 = 19 gallons; edge metal: 134 ft
Two seams of 42 feet. A 20 × 100 roll would make it one seam.
A garage roof in one 20-foot sheet
An 18 by 16 foot roof, a 20 × 50 foot EPDM roll, 12-inch turn-ups.
- Sheet: 20 × 18 = 360 sq ft, which fits the roll
- Strips: 1; rolls: 1; seams: none
- Adhesive: 360 ÷ 60 = 6 gallons; edge metal: 71 ft
No seam at all: the best flat roof is the one with nothing to tape.
Where to find your inputs
Length and width. Inside the parapets, or to the outside edge of the deck on a roof without parapets.
Turn-up. Up the parapet to the coping, or over the roof edge and down to the drip edge or fascia.
Roll size. From the supplier; choose the widest the crew can handle.
Adhesive coverage. On the adhesive’s data sheet; zero for mechanically attached or ballasted systems.
Common mistakes
- Ordering the roof area. The sheet includes the turn-ups. A 12-inch turn-up adds 13 percent on this roof.
- Narrow rolls on a wide roof. More seams, more leaks. Use the widest sheet available.
- No fall. Standing water shortens every membrane’s life and voids most warranties.
- Tape on a dirty seam. EPDM seams need primer on clean, dry rubber. Most seam failures are preparation failures.
- Forgetting the penetrations. Pipe boots and curb flashings are separate pieces from the same maker.
- Laying in the cold. Adhesives and tape have minimum temperatures; below them the bond fails.
Questions people ask
- How much EPDM do I need for a flat roof?
- The roof plus the turn-ups at every edge: a 40 by 24 foot roof with 12 inches turned up all round needs a 42 by 26 foot sheet, 1,092 square feet. In 10-foot rolls with 3-inch seams that is three strips 42 feet long, two 100-foot rolls; a 20-foot-wide roll would need two strips and one seam.
- What sizes does EPDM come in?
- EPDM is made in 10 and 20 foot widths in 50 and 100 foot lengths for residential roofs, and in larger sheets for commercial work, in 45 and 60 mil thickness. TPO and PVC come mainly in 10 and 12 foot widths, 100 feet long. The widest sheet you can handle minimises seams.
- How much should EPDM seams overlap?
- Taped EPDM seams overlap about 3 inches, with 3-inch seam tape and primer; heat-welded TPO and PVC seams overlap 2 to 3 inches with a 1½-inch weld. Follow the manufacturer: the overlap is part of the warranty.
- How much bonding adhesive for EPDM?
- A fully adhered roof coats both the membrane and the deck; one gallon covers about 60 square feet of finished roof that way (about 120 square feet of single surface). A 1,092 square foot sheet needs about 19 gallons. Mechanically attached and ballasted roofs need none in the field.
- How long does a flat roof membrane last?
- Industry surveys put EPDM at about 20 to 30 years, TPO at 15 to 25 and PVC at 20 to 30, depending on thickness, installation and sun exposure. Rolled roofing and built-up asphalt last less. The lifespan calculator compares cost per year.
- Does a flat roof need slope?
- Yes. A flat roof is never truly flat: it needs a fall of at least ¼ inch per foot to its drains or edges so water does not stand, which most codes require for new roofs and most membrane warranties exclude ponding damage. Tapered insulation boards or sloped framing provide it.