How to Install Roof Underlayment

Felt & Synthetic Overlap and Fastening

This guide walks you through installing 15-lb felt, 30-lb felt, or synthetic roof underlayment over a clean roof deck. Topics include starting at the eave with ice & water shield (extend 24" past interior wall line), running the first felt course, proper 2-4 inch horizontal overlap and 6-inch end laps, fastener spacing (cap nails 12" o.c. on the field, 6" at edges), how to handle hips, ridges, and valleys, and integrating with drip edge and step flashing. Pairs with our Underlayment Calculator.