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Mineral-surfaced roll roofing comes in 36 in × 36 ft rolls. Laid single coverage with a 2 in top lap, each roll covers one square (100 sq ft); wide-selvage rolls laid double coverage expose only 17 in of every 36 and cover half a square, per CertainTeed and GAF product data. The 2021 IRC (R905.5.2) allows roll roofing down to a 1:12 slope, where shingles need 2:12, which makes it the usual covering for low sheds, porches and garages. Enter the building length and width, the pitch and the roof shape and the Rolled Roofing Calculator finds each slope's length with the pitch factor √(144 + rise²) ÷ 12, adds the overhangs and waste, and returns the roof area, the rolls to buy and the courses up each slope. A 20 × 12 ft shed roof at 2/12 with 12 in overhangs is 290 sq ft: 4 rolls single coverage or 7 rolls double coverage.
Mineral-surfaced roll roofing is asphalt-saturated mat with a granule surface, sold in 36 in × 36 ft rolls. Laid single coverage, each course laps the one below by 2 in and a roll covers one square (100 sq ft). Wide-selvage rolls laid double coverage bury a 19 in plain selvage under the next course and expose 17 in, so every point has two layers and a roll covers half a square. The 2021 IRC allows roll roofing down to a 1:12 slope (R905.5.2), where asphalt shingles need 2:12.
| Item | Figure | Source |
|---|---|---|
| Roll size | 36 in × 36 ft | CertainTeed Mineral Surfaced Roll Roofing, GAF Mineral Surfaced Roll data |
| Single coverage | 2 in top lap, 34 in exposure, 100 sq ft per roll | ASTM D6380 Class M product data |
| Double coverage | 19 in selvage, 17 in exposure, 50 sq ft per roll | CertainTeed Wide Selvage Asphalt Roll Roofing (ASTM D6380 Class WS) |
| Minimum slope | 1 in 12 | 2021 IRC R905.5.2 |
| Material standard | ASTM D3909 or D6380 | 2021 IRC R905.5.4 |
| Roof | Area | Single coverage | Double coverage |
|---|---|---|---|
| 8 × 6 ft shed (2/12) | 71 sq ft | 1 rolls | 2 rolls |
| 10 × 8 ft shed (2/12) | 109 sq ft | 2 rolls | 3 rolls |
| 12 × 10 ft shed (2/12) | 156 sq ft | 2 rolls | 4 rolls |
| 20 × 12 ft shed (2/12) | 290 sq ft | 4 rolls | 7 rolls |
| 24 × 24 ft gable (3/12) | 697 sq ft | 8 rolls | 16 rolls |
| 30 × 24 ft gable (3/12) | 858 sq ft | 10 rolls | 19 rolls |
A standard roll of mineral-surfaced roll roofing is 36 in wide and 36 ft long (108 sq ft) and covers 1 square — 100 sq ft — laid single coverage with a 2 in top lap, per CertainTeed and GAF product data. Wide-selvage (double coverage) roll roofing has a 19 in selvage and only 17 in exposed, so the same size roll covers half a square.
Work out the roof area from the footprint and the pitch, add waste, and divide by 100 sq ft per roll (50 for double coverage). A 20 × 12 ft shed roof at 2/12 with 12 in overhangs is 290 sq ft; with 10% waste that is 3.19 squares, so 4 rolls single coverage or 7 rolls double coverage.
1 in 12. The 2021 International Residential Code (R905.5.2) does not allow mineral-surfaced roll roofing below a 1:12 slope. Asphalt shingles need 2:12 (R905.2.2), so roll roofing is the usual choice on porch and shed roofs between 1/12 and 2/12. Below 1/12 use a low-slope membrane such as modified bitumen or EPDM.
Single coverage laps each course 2 in over the one below, so most of the roof has one layer. Double coverage uses wide-selvage rolls: the 19 in plain selvage is covered by the next course and only 17 in is exposed, so every point has two layers. It uses twice the rolls but is far more watertight on low slopes.
Manufacturers do not publish a long warranty for it — roll roofing is a thin single layer of asphalt and granules. It is chosen for cost and low slopes on sheds, garages and porches rather than for life span. Double coverage, a sound deck and sealed laps help it last longer.
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.