To count underlayment rolls, run the roof area through this chain: footprint, times the pitch multiplier, times the overhang allowance, times the layer count, times one plus the waste factor. Divide by the roll's coverage and round up to whole rolls. These three examples show every number.
How do you work through an underlayment example?
Start from the building footprint, not the roof surface. Multiply by the pitch multiplier, which is the square root of one plus the square of rise over twelve. Add about 8 percent for eaves and rake overhangs. On low slopes from 2:12 to 4:12 under shingles, double the result for the second layer. Add waste: 10 percent for a simple gable, 15 percent for a cut-up or hip roof. Then divide by the roll coverage and round up.
Worked example: gable roof, 36 by 28 ft at 7/12
A 36 by 28 ft footprint is 1,008 sq ft. The 7/12 pitch multiplier is 1.1577, giving 1,167.3 sq ft of roof surface. Add 8 percent for overhangs: 1,260.7 sq ft. This is a simple gable, so apply a 10 percent waste factor: 1,386.4 sq ft to order.
In #15 felt (432 sq ft per roll): 1,386.4 / 432 = 3.21, so order 4 rolls. In #30 felt (216 sq ft per roll): 6.42, so order 7 rolls. In synthetic (1,000 sq ft per roll): 1.39, so order 2 rolls. If you had divided the bare footprint by the label coverage, you would have ordered 3 rolls of #15 felt and stopped short mid-roof.
Worked example: metric gable roof, 14 by 10 m at 5/12
A 14 by 10 m footprint is 140 sq m, which is 1,506.9 sq ft. The 5/12 multiplier is 1.0833, giving 1,632.5 sq ft, plus 8 percent for overhangs: 1,763.0 sq ft. With a 10 percent waste factor, order 1,939.4 sq ft.
That is 5 rolls of #15 felt (1,939.4 / 432 = 4.49, round up) or 2 rolls of synthetic (1.94, round up). Convert to square feet before dividing, because rolls are sold by their imperial coverage even when the roof was measured in meters.
Worked example: small hip roof, 30 by 26 ft at 4/12
A 30 by 26 ft footprint is 780 sq ft. The 4/12 multiplier is 1.0541, giving 822.2 sq ft, plus 8 percent for overhangs: 887.9 sq ft. A hip roof has more cuts, so apply a 15 percent waste factor: 1,021.2 sq ft to order.
That is 3 rolls of #15 felt (1,021.2 / 432 = 2.36, round up) or 2 rolls of synthetic (1.02, round up). The synthetic order lands just over one roll, which shows why the rounding rule matters: 1.02 is not 1 roll, it is 2.
| Material | Roll size | Theoretical coverage |
|---|---|---|
| #15 felt | 144 ft x 36 in | 432 sq ft |
| #30 felt | 72 ft x 36 in | 216 sq ft |
| Synthetic (typical) | 250 ft x 48 in | 1,000 sq ft |
Each course laps the one below, so installed coverage per roll is lower than the label number. Start from the gross roof area, add the waste factor, then round up.
Check your own numbers in five steps
- Measure the footprint, then multiply by the pitch multiplier from the pitch table. A 12/12 roof is 41 percent larger than its footprint.
- Add the overhang allowance, about 8 percent, unless you already measured the deck edges.
- Double the layers on slopes from 2:12 to 4:12 under shingles.
- Add the waste factor: 10 percent for a simple gable, 15 percent for a hip or cut-up roof.
- Divide by the roll coverage and always round up. An opened roll cannot be half-ordered.
Common mistakes in the worked math
- Dividing the footprint by the label coverage and skipping the pitch multiplier. Even a 4/12 roof is 5 percent larger than its footprint.
- Adding waste after dividing into rolls instead of before. Order area grows first, then the roll count follows.
- Rounding a 1.02 result down to 1 roll. Roll counts always round up.
- Converting meters to feet but dividing by square-meter coverage that the roll label does not state.
Limits of this planning math
Roll dimensions vary by manufacturer, so read the label on the exact product you will buy. Some synthetic rolls are 42 in wide rather than 48 in, and their theoretical coverage is lower. Valleys, ridge vents, and large penetrations add cuts that a single waste percentage only approximates. This article estimates material; it does not set code requirements or manufacturer installation rules. For measuring the roof, ordering the rolls, and choosing the type, see the measuring guide, the ordering guide, and the underlayment type guide.
Sources: TARCO roofing felts specifications; GAF FeltBuster synthetic underlayment product listing; Procore pitch multiplier table; Zuper roofing estimating guide; wutools.com roofing calculator. All accessed 2026-10-19.
