Roofing Underlayment Calculator
Calculate rolls of felt, synthetic underlayment and ice & water shield for any roof.
The roofing underlayment calculator returns rolls of felt or synthetic underlayment plus ice and water shield needed for any roof. Synthetic underlayment covers about 1000 sq ft per roll (huge productivity advantage over felt), 30lb felt covers 200 sq ft per roll, 15lb felt covers 400 sq ft. Ice and water shield is required by code in cold climates along the eaves - typically 6 ft up from the edge - and in valleys, around chimneys and around skylights everywhere. Plan on a 10% lap-overlap waste factor for any underlayment job since rolls overlap each other by 2-4 inches and run long at the gables.
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Frequently asked questions
With 10% lap overlap, you need 2,200 sq ft of underlayment. Synthetic underlayment (1000 sq ft per roll): 3 rolls. 30lb felt (200 sq ft per roll): 11 rolls. 15lb felt (400 sq ft per roll): 6 rolls. Synthetic is the modern standard - lighter, tougher and 5x faster to install than felt.
Most US codes require ice and water shield from the eave up to a line 24 inches inside the exterior wall (typically 6 linear feet up from the gutter line). It is also required in all valleys, around all roof penetrations (chimneys, skylights, vent pipes) and along rake edges in some jurisdictions. Warm-climate codes may not require it - check local code.
Synthetic for almost every job. It is 5x lighter, lays flat without curling, tears less easily and rolls cover 1000 sq ft vs 200-400 for felt. Felt remains common only for budget jobs and roofs that need certain warranty compliance. Synthetic adds about $200-400 to a roof but saves a full day of labor on a typical residential job.
Standard 36 inches wide, typical roll length 67 feet, total coverage 200 sq ft per roll. Larger 6 ft wide rolls exist for commercial work. The standard 3 ft width means you usually need 2 courses to get the code-required 6 ft up from the eave - so plan accordingly.
No - underlayment is required by every shingle manufacturer warranty and most building codes. It serves three jobs: (1) secondary water barrier if a shingle blows off, (2) keeps shingles from sticking to the deck during install, (3) provides a smooth substrate over rough or knotty plywood. Skipping underlayment voids the shingle warranty and almost always voids homeowner insurance for roof claims.