Estimate Underlayment With the Calculator
The roofing squares calculator estimates underlayment rolls using about 400 square feet per roll. Use that as a planning number, then confirm the exact roll coverage and overlap requirements for the product.
Basic Underlayment Formula
Use the estimated roofing area after pitch and waste, then divide by roll coverage. For example, 2,576 square feet divided by 400 square feet per roll equals 6.44 rolls, so the planning estimate rounds up to 7 rolls.
| Estimated roofing area | Roll coverage assumption | Planning rolls |
|---|---|---|
| 1,600 sq ft | 400 sq ft | 4 rolls |
| 2,400 sq ft | 400 sq ft | 6 rolls |
| 2,576 sq ft | 400 sq ft | 7 rolls |
| 3,300 sq ft | 400 sq ft | 9 rolls |
Why Actual Coverage Can Be Lower
Underlayment must overlap at seams and transitions. Valleys, hips, ridges, walls, chimneys, and penetrations can increase material use. Some synthetic underlayments cover more per roll than felt products, while some installation patterns reduce effective coverage.
Underlayment Is Not the Whole Water Barrier
Many roofs also need ice and water shield or other self-adhered membrane in specific areas, depending on climate, roof design, and local requirements. That material is usually calculated separately from standard underlayment rolls.