Underlayments, membranes and WRBs · Lesson 1 of 4

Underlayments, membranes and WRBs: principles and terminology

What each layer does, the code terms and their trade names, the shingle-fashion rule, where each layer meets the drip edge and step flashing, and the OSHA roof definitions.

You will be able to: Define underlayment, ice barrier, self-adhering membrane, water-resistive barrier, shingle fashion and head and end laps, place each layer against the metal, and state the fall-protection boundaries.

12 min · 10 slides · 10 questions

All lessons in this module

Slide 1 of 10

What underlayment does

Wilen (NRCA, 2013) lists three jobs for steep-slope underlayment: weather protection for a limited time until the covering is on, a secondary weatherproofing barrier, and separation between covering and substrate.

FEMA Fact Sheet 7.2 (12/10) ranks underlayment options by how well they resist weather after the roof covering is lost.

Sources

For your consideration (J. Wilen, NRCA), Professional Roofing, February 2013 - Professional Roofing (NRCA)
Home Builder's Guide to Coastal Construction, Technical Fact Sheet No. 7.2: Roof Underlayment for Asphalt Shingle Roofs (12/10) - FEMA (developed with the NAHB Research Center), hosted by the Town of Edisto Beach SC

Glossary

Underlayment: The sheet under a steep-slope roof covering. Wilen (NRCA) gives it three jobs: weather protection until the covering is on, a secondary weatherproofing barrier, and separation between covering and substrate.
Ice barrier: The 2018 IRC term (R905.1.2): at least two layers of underlayment cemented together, or a self-adhering polymer-modified bitumen sheet used in place of normal underlayment. Makers sell such sheets under ice and water names.
Water-resistive barrier (WRB): The code term (R703.2) for the layer over wall studs or sheathing, continuous behind the cladding: No. 15 felt, an ASTM E2556 sheet or another approved material. House wrap is a sheet WRB.
Shingle fashion: Laying layers so each upper layer laps over the one below, in the direction water runs, so water lands on the face of the next layer.
Head lap and end lap: Head (horizontal or side) lap: where one course of roll laps the course below. End (vertical) lap: where two roll ends meet within a course (ESR-4103 and Grace terms).
Vapor retarder class: R702.7 of the 2021 Seattle Residential Code sorts vapor retarders by perm rating into Class I, II and III. It is a separate decision from the WRB.
Self-adhering membrane: A self-adhering polymer modified bituminous sheet underlayment; ASTM D1970 covers this type for eaves and valleys against ice-dam leaks under shingle, tile and metal roofs. Makers sell it under ice and water names.
Steep roof (OSHA): Under 29 CFR 1926.500(b), a roof slope greater than 4 in 12. A slope of 4 in 12 or less is a low-slope roof.