A layer of roofing felt or synthetic membrane placed between overlapping rows of steep-slope roofing units. Unlike standard underlayment which goes directly on the roof deck, interlayment is woven into the courses as they are installed. It acts as a secondary baffle to catch and drain away wind-driven rain or snow that penetrates the primary shingles.
Heard on the job
Code requires a 30-pound felt interlayment between every course of these cedar shakes.
Insider tip
Skipping the interlayment on wood shake roofs is a common rookie mistake that guarantees leaks during heavy, wind-driven rainstorms.