Underlayment Calculator
Calculate underlayment rolls needed for your roofing project. Accounts for overlap, roof pitch, and waste factor.
100% Free • No Sign-up
Calculator
Results
Underlayment Calculation
Adjusted Area = Roof Area × (1 + Overlap%)
Account for overlap waste
Rolls Needed = Adjusted Area ÷ Roll Coverage
Round up to whole rolls
Underlayment calculation must account for overlap between rows. Standard horizontal laps are 4 inches, but end laps and vertical surfaces need more. We recommend 10% extra for typical residential applications.
15# felt covers about 432 sq ft per roll. 30# felt is thicker and covers 216 sq ft. Synthetic underlayment covers up to 1,000 sq ft and is more durable.
Always add extra for valleys and penetrations where Ice & Water Shield is required by code in many areas.
Reference: International Residential Code (IRC) R905.2.7
Frequently Asked Questions
15# felt is standard for most residential roofs with pitches over 4/12. 30# felt is required for low-slope roofs (2/12 to 4/12) and recommended for very steep roofs or harsh climates. Some manufacturers require 30# for warranty coverage.
Synthetic underlayment is more durable, lighter, and covers more area per roll. It doesn't tear as easily and can be left exposed longer. However, it's more expensive. Felt is cheaper and has been used for decades. Both work when installed correctly.
Ice & Water Shield is required by code in most areas: (1) At eaves extending 24 inches inside exterior wall, (2) In valleys, (3) Around penetrations, (4) In areas with history of ice dams. Cold climate areas often require 6 feet at eaves. Check your local building code.
Related Tools
Shingle Calculator
Estimate materials needed for your roofing project
Valley Flashing Calculator
Calculate valley flashing requirements for roofing projects. Supports open, clos
Chimney & Skylight Flashing Calculator
Calculate flashing requirements for roof penetrations including chimneys, skylig
Metal Roofing Calculator
Metal roofing panel and material estimator