Roofing Underlayment Calculator
Enter your roof dimensions, pitch, and underlayment type to get an exact roll count — including overlap and waste. Works for felt, synthetic, and ice & water shield.
Underlayment coverage quick reference
| Type | Coverage/roll | Std overlap | Best for |
|---|---|---|---|
| 15 lb felt | 400 sq ft | 2 in | Budget jobs, mild climates |
| 30 lb felt | 200 sq ft | 4 in | High-wind, heavier duty |
| Synthetic | 1,000 sq ft | 4 in | Speed, weight savings, most installs |
| Ice & water shield | 225 sq ft | 3 in | Eaves (first 2 courses), valleys |
How underlayment coverage is calculated
Each roll has a rated coverage, but overlap reduces the effective area. A synthetic roll rated at 1,000 sq ft with a 4-inch overlap on a 36-inch wide roll actually covers about 967 sq ft. This calculator accounts for overlap automatically.
15 lb vs 30 lb felt vs synthetic
Synthetic underlayment has largely replaced felt on most residential jobs — it's lighter, installs faster, and won't wrinkle when wet. 30 lb felt is still required by code in some high-wind zones. Ice and water shield is code-required at eaves in cold climates.
Do I need ice and water shield?
Most building codes require ice and water shield on the first 24 inches past the exterior wall plate in climates where average January temperature is 25°F or below. It's also recommended in all valleys regardless of climate.
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