Understanding your result
The headline is the rolls to buy. The stats show the net coverage of one roll, the nails and the lap cement. The breakdown shows the top lap for the method, the net coverage, the area with waste and the division into rolls.
The number to remember is the net coverage per roll: about 100 square feet for exposed nail, 50 for double coverage. Ordering by the nominal square on the label is the usual mistake, and on double coverage it leaves you with half the rolls you need.
For a truly flat roof the membrane calculator covers EPDM, TPO and PVC, and for a shed roof in metal the corrugated shed roof calculator counts sheets and screws.
How we calculate this
The top lap is what sets the coverage. Exposed-nail roofing overlaps each course by 2 inches and nails through the lap; concealed-nail roofing overlaps 3 inches and nails under the next course; double coverage uses a roll whose upper 19 inches are an unsurfaced selvage, entirely covered by the next course, so every point has two layers. The same arithmetic as the underlayment calculator applies, with a heavier product.
The flat roof guide compares rolled roofing with the membranes on a low-slope roof.
The assumptions behind the numbers
| Assumption | Default | Where it comes from |
|---|---|---|
| Roll size | 36 in × 36 ft (1 square nominal) | ASTM D6380 mineral-surfaced roll roofing |
| Top lap | 2 in exposed, 3 in concealed, 19 in selvage | Manufacturers’ application instructions |
| End lap | 6 in | Manufacturers’ instructions |
| Minimum pitch | 2/12 exposed nail; 1/12 concealed and double | IRC R905.5 and manufacturers |
| Nails | 1 in galvanized at 3 in (exposed) or two rows at 4 in (concealed) | Manufacturers’ instructions |
| Waste | 10% | Estimating practice |
Assumptions last reviewed October 7, 2026.
The calculator does not count the underlayment, the drip edge or the starter strip some methods use at the eave, and it assumes one method for the whole roof. The roof area calculator gives the sloped area.
Two worked examples
A shed roof with exposed nails
400 square feet, exposed-nail method, 36 × 36 ft rolls, 6-inch end lap, 10% waste.
- Net per roll: 34 in × 35.5 ft = 101 sq ft
- With waste: 440 sq ft
- Rolls: 440 ÷ 101 = 4.37, so 5 rolls
- Nails: about 810; lap cement: 2 gallons
Five rolls for four squares: the laps and the waste take the fifth.
A low porch roof in double coverage
600 square feet at 1½/12, 19-inch selvage roll, 10% waste.
- Top lap: 19 in; net per roll: 17 in × 35.5 ft = 50 sq ft
- With waste: 660 sq ft
- Rolls: 660 ÷ 50 = 13.1, so 14 rolls
- Nails: about 3,000; lap cement: 11 gallons
Fourteen rolls for six squares, more than twice the exposed-nail count, because the roof is covered twice.
Where to find your inputs
Roof area. The sloped area from the roof area calculator.
Method. By pitch: exposed nail from 2/12, concealed or double coverage from 1/12.
Roll size. On the label; 36 × 36 is standard, selvage rolls are 36 inches wide with a 19-inch selvage.
Common mistakes
- One roll per square. Laps reduce coverage; double coverage halves it.
- Exposed nails on a low pitch. Below 2/12 use a concealed or double-coverage method.
- Laying it cold. Unroll it in the sun first; below about 45 °F it cracks.
- No cement on the laps. Concealed and double-coverage methods depend on it.
- Rolled roofing on a house. It is a short-lived roof for outbuildings.
- Skipping the drip edge. Edges need metal like any other roof.
Questions people ask
- How many rolls of rolled roofing per square?
- A standard 36-inch by 36-foot roll is one square nominal. Laid with exposed nails and a 2-inch lap and a 6-inch end lap it covers about 100 square feet; with concealed nails and a 3-inch lap about 98; as 19-inch selvage double coverage about 50. A 400 square foot shed roof needs 5 rolls with 10 percent waste by the exposed-nail method.
- What is double-coverage rolled roofing?
- Selvage roll roofing (19-inch selvage, 17-inch mineral surface) is laid so each course overlaps the one below by the 19-inch selvage, giving two layers everywhere. It covers half a square per roll and is allowed down to 1/12 pitch, where single-layer rolled roofing is not.
- What pitch does rolled roofing need?
- Exposed-nail rolled roofing needs about 2/12; concealed-nail and double-coverage methods go down to 1/12 because the nails are hidden and the laps cemented. Below 1/12 use a membrane. Asphalt shingles need 2/12 with double underlayment and 4/12 for standard installation.
- How long does rolled roofing last?
- About 5 to 10 years exposed-nail, a few years more concealed or double-coverage. It is the cheapest roof there is, and the shortest-lived, which is why it is used on sheds, barns and outbuildings rather than houses.
- How many nails for rolled roofing?
- Exposed-nail application puts a nail every 3 inches along the top lap and the ends, roughly 140 per roll plus edges; concealed-nail application uses two rows of nails at 4 inches under the next course. Use 1-inch galvanized roofing nails, or longer over an old roof.
- How much lap cement do I need?
- About a quarter of a gallon per roll for exposed-nail application (end laps and edges) and three quarters of a gallon for concealed-nail and double-coverage, where every lap is cemented. Use the cement the roll maker specifies.