Roof Underlayment Calculator

Felt & Synthetic Rolls Required

The Underlayment Calculator returns the number of rolls of 15-lb felt, 30-lb felt, or synthetic roofing underlayment needed to cover your roof. A standard 4 sq roll of 15-lb felt covers about 400 sq ft after 2-inch overlaps; synthetic rolls cover 1,000 sq ft. Enter your roof area and select a material and the tool calculates roll count plus 10% waste. Underlayment goes on top of plywood and beneath shingles to provide secondary moisture protection — see our installation guide for fastening and overlap details.