Roof Underlayment Calculator
Calculate how many rolls of underlayment you need for your roofing project with overlap factor applied.
Part of the Roofing & Structure calculators — see all tools in this category.
Current result
Use our Roof Area Calculator if you need to calculate this first
Standard 4" overlap on 36" roll
Need to calculate your roof area first? Use our Roof Area Calculator.
For eaves and valleys, consider ice & water shield membrane in addition to standard underlayment. Ice & water shield is not calculated here — consult local building codes for requirements.
Results
Synthetic Underlayment
Per roll after overlap
Slight overage from rounding up
Frequently Asked Questions
How it works and assumptions
- 1. Enter Roof Area
Enter the total surface area of your roof in square feet. If you don't know this, use our Roof Area Calculator to compute it from dimensions and pitch.
- 2. Select Underlayment Type
Synthetic underlayment is the most popular and covers 10 squares (1,000 sq ft) per roll. #15 felt covers 4 squares (400 sq ft) per roll. #30 felt covers 2 squares (200 sq ft) per roll.
- 3. Set Overlap
Standard 4" overlap on a 36" roll reduces effective coverage by approximately 11%. The calculator accounts for this reduction automatically. Increase overlap for steeper pitches or where local codes require it.
- 4. Get Roll Count
The calculator divides your roof area by the effective coverage per roll (after overlap reduction) and rounds up to whole rolls. Formula: Effective Coverage = Nominal Coverage × (1 − Overlap ÷ Roll Width).
Understanding Overlap Reduction
For a standard #15 felt roll with 4" overlap on a 36" wide roll: 400 × (1 − 4/36) = 355.6 sq ft effective coverage per roll. The overlap ensures waterproofing continuity but reduces the usable coverage of each roll.
Sources and revision
Page revision: 2026-10-12