Rolled Roofing Calculator
A rolled roofing calculator divides roof area by 100 sq ft per roll: a 30 × 20 ft roof (600 sq ft) needs 7 rolls with 10% waste, or $350 at $50 each.
How many rolls of rolled roofing you need
A rolled roofing calculator counts the rolls of mineral-surface roofing needed to cover a roof. Enter the length and the width of the roof surface in feet. The tool divides the area by 100 square feet per roll, adds 10 percent if you switch on the waste option, rounds up to a whole roll, and multiplies by your price.
Rolled roofing is the thick, rolled-up asphalt sheet used on sheds, porches, and low-slope additions. You unroll it across the roof and nail it down.
The formula in plain words
- Roof area = length × width.
- Rolls = area ÷ 100.
- With the waste option on, multiply by 1.10.
- Round up to a whole roll.
- Cost = rolls × price per roll.
One roll in the calculator is one roofing square, which is 100 sq ft.
Worked example: a workshop roof with waste
A low-slope roof measures 30 ft by 20 ft. Rolls cost $50, and you add 10% for cuts.
- Area: 30 × 20 = 600 sq ft
- Net rolls: 600 ÷ 100 = 6
- With 10% waste: 6 × 1.10 = 6.6
- Rounded up: 7 rolls
- Cost: 7 × $50 = $350
Without the waste option, the answer is 6 rolls for $300. The waste option is what adds the seventh roll.
Worked example: a small shed
A shed roof measures 14 ft by 10 ft.
- Area: 14 × 10 = 140 sq ft
- Net rolls: 140 ÷ 100 = 1.4, rounded up to 2
- With 10% waste: 1.54, rounded up to 2
- Cost: 2 × $50 = $100
Both settings land on 2 rolls, so the waste option does not matter here.
Rolls needed by roof area
| Roof area | Without waste | With 10% waste |
|---|---|---|
| 200 sq ft | 2 | 3 |
| 400 sq ft | 4 | 5 |
| 600 sq ft | 6 | 7 |
| 800 sq ft | 8 | 9 |
| 1,000 sq ft | 10 | 11 |
With the waste option on, every row here goes up by exactly one roll.
How to measure the roof
Measure the length and width of each roof plane along the surface, not across the floor plan. A flat or low-slope roof is close to the footprint, but if there is any pitch, the surface is a little bigger. Include the overhang.
A shed roof with one slope is a single plane. For a gable, measure each side and add them together. Use the roof area calculator if you only know the footprint and pitch.
Installing rolled roofing
Rolled roofing is laid in horizontal courses starting at the eave, with each course overlapping the one below it. The overlaps are sealed with roofing cement and nailed according to the manufacturer’s instructions. Because it is a single layer, it is thinner than a shingle roof and a lower-cost option for outbuildings.
Check the product’s minimum slope, the temperature range for installation, and the nailing pattern. Cold rolls crack when unrolled, so let them warm in the sun first. A roof with any structural concern should be looked at by a roofer before you cover it.
Common mistakes
- Using the floor plan size for a sloped roof. The surface is bigger.
- Forgetting the overhang. It is roofed too.
- Skipping waste on a cut-up roof. Dormers, vents, and valleys eat material.
- Ordering without checking slope limits. A roll that is not rated for your pitch can leak.
- Leaving off drip edge and flashing. Edges and penetrations are where leaks start.
Related calculators
For a layer underneath, the roofing felt calculator counts rolls of underlayment. To refresh an old roof with a coating, use the roof coating calculator. The roof area calculator turns a footprint and pitch into surface area.
Frequently asked questions
How many rolls of rolled roofing do I need?
Divide the roof area by 100 sq ft per roll and round up. A 600 sq ft roof needs 6 rolls, or 7 rolls with the 10% waste option turned on.
What does one roll cover?
The calculator uses 100 sq ft, or one roofing square, per roll. Real rolls are cut and overlapped, so net coverage is a bit less than the roll's total area. Check the label of the product you buy.
Can I use rolled roofing on a steep roof?
Rolled roofing is usually used on low-slope roofs such as sheds, porches, and garages. Check the manufacturer's minimum and maximum slope for the product, and the local building code.
Is the length and width the roof or the building?
Use the roof surface, not the building footprint. A pitched roof has more surface than the floor under it, so measure along the slope. The roof area calculator can convert from the footprint.
What else do I need besides the rolls?
Roofing cement, nails, drip edge, and sometimes underlayment. Those are not included in the count or in the cost.
Estimates only. Verify quantities with your supplier before purchasing.