Tech Today

Understanding underlayments


Proper underlayment is a critical component for steep-slope roof system performance. Building codes provide minimum requirements for underlayments, but some of these requirements may limit underlayment options.

Code requirements

Minimum requirements for underlayment products used as components for steep-slope roof systems are provided in the International Building Code, 2015 Edition (IBC 2015), Section 1507—Requirements for Roof Coverings. Separate requirements are provided for each steep-slope roof system type located in areas where the nominal design wind speed (Vasd) is less than 120 mph or 120 mph and greater.

Similarly, the International Residential Code, 2015 Edition (IRC 2015) provides product requirements for steep-slope underlayments in Table R905.1.1(1) Underlayment Types. Separate requirements are provided for each steep-slope roof system type located in areas where the ultimate design wind speed (Vult) is less than 140 mph or 140 mph and greater.