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A fully adhered EPDM rubber roof needs sheets that cover the deck plus everything turned up walls, curbs and parapets or over the edge, bonding adhesive for the whole area, and splice tape along every field seam. The EPDM Rubber Roof Calculator adds your turn-up to every side, lays out 10 ft and 20 ft wide sheets along the roof with a lap at each seam, and picks the layout that orders the least membrane. Adhesive is the covered area ÷ 60 sq ft per gallon, the finished-surface coverage on the Carlisle Sure-Seal 90-8-30A data sheet, in 5-gallon pails; seam tape is sold in 3 in × 100 ft rolls. EPDM needs a slope of at least 1/4 in per foot under the 2021 IRC (R905.12.1) and a thickness of at least 45 mil under ASTM D4637. A 30 × 20 ft garage roof with 6 in over every edge covers 651 sq ft: three 10 ft sheets 31 ft long, 11 gallons of adhesive and 62 ft of seam tape. Enter your supplier's price per square foot to total the membrane.
A fully adhered EPDM rubber roof is ordered as sheets that run the length of the roof, glued to the deck with bonding adhesive and joined at field seams with 3 in splice tape. The sheet must cover the deck plus whatever turns up walls, curbs and parapets or over the roof edge. Fewer, wider sheets mean less tape; narrower sheets can mean less offcut.
| Item | Figure | Source |
|---|---|---|
| Sheet widths | 10 ft and 20 ft; rolls to 100 ft long | Carlisle Sure-Seal and Holcim RubberGard EPDM data sheets |
| Bonding adhesive | ≈ 60 sq ft of finished surface per gallon | Carlisle Sure-Seal 90-8-30A Bonding Adhesive data sheet |
| Seam tape | 3 in wide, 100 ft rolls | Carlisle SecurTAPE, Holcim QuickSeam |
| Minimum slope | 1/4 in per ft (2%) | 2021 IRC R905.12.1 |
| Minimum thickness | 45 mil | ASTM D4637 |
| Roof | Cover | Sheets | Ordered | Adhesive | Seam tape |
|---|---|---|---|---|---|
| 12 × 10 ft | 143 sq ft | 1 × 20 ft | 260 sq ft | 2.4 gal (1 pails) | 0 ft |
| 20 × 12 ft | 273 sq ft | 1 × 20 ft | 420 sq ft | 4.5 gal (1 pails) | 0 ft |
| 24 × 24 ft | 625 sq ft | 3 × 10 ft | 750 sq ft | 10.4 gal (3 pails) | 50 ft |
| 30 × 20 ft | 651 sq ft | 3 × 10 ft | 930 sq ft | 10.8 gal (3 pails) | 62 ft |
| 40 × 25 ft | 1,066 sq ft | 3 × 10 ft | 1,230 sq ft | 17.8 gal (4 pails) | 82 ft |
The roof's length and width plus the membrane turned up walls or over the edge on every side, in sheets of a width you can buy. A 30 × 20 ft roof with 6 in turned over each edge needs a 31 × 21 ft sheet area — 651 sq ft — which you can cover with three 10 ft wide sheets 31 ft long (930 sq ft ordered) or two 20 ft wide sheets (1,240 sq ft ordered).
About 60 sq ft of finished roof per gallon when both the membrane and the deck are coated, per the Carlisle Sure-Seal 90-8-30A data sheet. 651 sq ft needs about 10.9 gallons, so three 5-gallon pails. Water-based and low-VOC adhesives cover differently — read the pail.
1/4 in per foot (2%) under the 2021 IRC section R905.12.1 for thermoset single-ply membranes. A dead-flat roof should be tapered with insulation so water does not pond.
45 mil is the minimum in ASTM D4637. 60 mil is the common residential choice and 90 mil is used where the roof sees foot traffic or hail. Thicker sheets are heavier to handle — a 20 × 50 ft roll of 60 mil is a two-person lift.
With seam tape: the 3 in splice tapes sold by Carlisle (SecurTAPE) and Holcim (QuickSeam) go between the overlapping sheets after the lap is primed. Every field seam runs the full length of the sheet, so fewer, wider sheets mean less tape and fewer places to leak.
Need the whole roof at once? The free roofing calculator works out roof area, squares, shingle bundles, underlayment, drip edge, ridge cap and installed cost from one set of measurements.