Roof Underlayment Calculator
Roofing underlayment is the secondary water barrier installed between the roof deck (OSB or plywood sheathing) and the final roofing material. If shingles blow off or water backs up under them, the underlayment keeps the structure dry. This calculator handles three product types: 15lb asphalt felt (400 sq ft/roll — the standard choice for most projects), 30lb felt (200 sq ft/roll — heavier and more wind-resistant), and synthetic underlayment (1,000 sq ft/roll — lighter and UV-stable for up to 6 months of exposure). All felt rolls are 36 inches wide. The calculator accounts for horizontal row overlap (6 inches standard, 12 inches in high-wind zones) and a 10% waste factor. Ice and water shield is required in many codes for the first 3 feet from the eave — enter that as a separate order.
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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.
Do I need ice and water shield?
Ice and water shield (self-adhering membrane) is required by most northern building codes along the eaves and valleys where ice dams can form. It's typically required for the first 24 inches inside the heated wall line, which often means the bottom 3 feet of the roof. In warm climates, it's optional but provides excellent leak protection at valleys.
How many squares of underlayment do I need?
For a 2,000 sq ft house with a standard 6/12 gable roof, the actual roof area is about 2,200 sq ft (accounting for pitch). With 6-inch overlaps and 10% waste, you need about 2,700 sq ft of effective coverage. At 400 sq ft per roll for 15lb felt: 7 rolls. At 1,000 sq ft per roll for synthetic: 3 rolls.