Roofing Underlayment Calculator

How It Works

Underlayment is the waterproof barrier between your roof deck and shingles. Synthetic underlayment has largely replaced traditional felt — it is lighter, stronger, and covers 1,000 sq ft per roll vs. 400 for #15 felt. Always install ice and water shield membrane at eaves in cold climates — most codes require it at least 24 inches past the interior wall line. This self-sealing membrane prevents damage from ice dams. For tile and slate roofs, two layers of underlayment are required. Account for 4-inch horizontal overlaps and 6-inch end laps, which reduce effective coverage by about 10%.