Order underlayment with this formula: measured roof area, times the pitch multiplier, times the layer count, times one plus the waste factor. Then divide by the coverage on the roll and round up to whole rolls.
How many rolls do you need?
Take the building footprint, multiply by the pitch multiplier, and add the overhang area. On low slopes from 2:12 to 4:12 under shingles, double the result for the second layer. Then add waste: 5 to 8 percent for a simple gable, 10 to 15 percent for a cut-up or steep roof, and up to 20 percent for multi-valley designs.
| Material | Roll size | Theoretical coverage |
|---|---|---|
| #15 felt | 144 ft x 36 in | 432 sq ft (4 squares) |
| #30 felt | 72 ft x 36 in | 216 sq ft (2 squares) |
| Synthetic (typical) | 250 ft x 48 in | 1,000 sq ft (10 squares) |
The table shows theoretical coverage. Each course laps the one below, so the installed coverage per roll is lower. Never divide by the label number and call it done. Start from the gross roof area, add the waste factor, and then round up, because one opened roll cannot be half-ordered.
Worked example: hip roof, 45 by 32 ft at 8/12
A 45 by 32 ft footprint is 1,440 sq ft. The 8/12 pitch multiplier is 1.202, so the roof area is 1,731 sq ft. Add 8 percent for overhangs: 1,869 sq ft. This is a cut-up roof, so apply a 15 percent waste factor: 2,149 sq ft to order.
In #15 felt: 2,149 / 432 = 4.97, so order 5 rolls. In synthetic: 2,149 / 1,000 = 2.15, so order 3 rolls. If you had skipped the waste factor and ordered by net area, the felt order would have been 4 rolls, and the job would have stopped mid-roof.
Worked example: gable roof, 12 by 9 m at 6/12
A 12 by 9 m footprint is 108 sq m, which is 1,162 sq ft. The 6/12 multiplier is 1.118, giving 1,300 sq ft, plus 8 percent for overhangs: 1,404 sq ft. With a 10 percent waste factor, order 1,544 sq ft. That is 4 rolls of #15 felt (1,544 / 432 = 3.57, round up) or 2 rolls of synthetic.
Worked example: low-slope roof needing a double layer
A 30 by 24 ft roof at 3/12 under shingles needs a double layer. The footprint is 720 sq ft, the 3/12 multiplier is 1.031, and 8 percent overhangs give 802 sq ft per layer, so 1,604 sq ft for two layers. A 10 percent waste factor brings the order to 1,763 sq ft: 5 rolls of #15 felt or 2 rolls of synthetic. Missing the double-layer requirement is the most expensive ordering mistake on low slopes, because it nearly doubles the material after the crew is already on the roof.
Ordering mistakes to avoid
- Ordering by footprint area with no pitch multiplier. A 12/12 roof is 41 percent larger than its footprint.
- Using the roll label coverage as installed coverage. Laps consume several percent of every roll.
- Rounding down to save a roll. Underlayment orders always round up to whole rolls.
- Ordering one type for the whole roof and forgetting the ice barrier at cold-climate eaves. It is a separate material on the order list.
- Mixing manufacturers or batches without checking the exposure ratings. Different synthetics allow different open-exposure windows, so keep the order consistent.
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. 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 how to measure the roof and how to pick the type, see the roofing underlayment measuring guide and the underlayment type guide.
Sources: TARCO roofing felts specifications; Home Depot GAF FeltBuster product listing (Store SKU 1000043180); HomeAdvisor roofing calculator; Zuper roofing estimating guide; FindPros roofing calculator; engineerfix synthetic roll coverage; Hunker roofing felt thickness. All accessed 2026-10-19.
