Roof Underlayment Calculator
Roofing underlayment is installed between the roof deck and the final roofing material as a secondary water barrier. This calculator handles 15lb felt (400 sq ft/roll), 30lb felt (200 sq ft/roll), and synthetic underlayment (1,000 sq ft/roll). Accounts for row overlap and a 10% waste factor.
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Top PickGAF Felt Brite 15lb Underlayment — Home Depot~$27.48/roll* Affiliate links — we may earn a commission at no cost to you.
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How to Use This Calculator
- 1Enter your measurements in the fields above. Use the units shown next to each field (ft, in, etc.).
- 2Click Calculate to get your material estimate instantly.
- 3Review the highlighted result — it shows the key quantity you need to order.
- 4Use Copy Results to save your estimate, or bookmark the URL — your inputs are encoded in the link.
Pro tip: Always add 10% to your material order to account for waste, cuts, and defects.
Frequently Asked Questions
What is the difference between 15lb and 30lb roofing felt?
15lb felt is the standard choice for most residential projects — lighter, easier to handle, and covers twice as much area per roll. 30lb felt is heavier, more durable, and provides better tear resistance in high-wind areas or for temporary weatherproofing during a re-roof.
Is synthetic underlayment worth it?
Yes, for most applications. Synthetic underlayment costs more per roll but covers 1,000 sq ft vs. 400 sq ft for 15lb felt, so the cost difference narrows. It's also lighter, UV-resistant for longer exposure, and has better slip resistance for walking on the roof.