Some roofers install underlayment vertically over short steep roof decks to provide quicker protection from the weather, but not all building inspectors will approve this practice. In general, the IRC specifies horizontal underlayment installation.
Dave Molloy Some roofers install underlayment vertically over short steep roof decks to provide quicker protection from the weather, but not all building inspectors will approve this practice. In general, the IRC specifies horizontal underlayment installation.

Most roofers I know install roofing underlayment horizontally, but I’ve seen a few cases where the underlayment was pulled down vertically from the ridge. Does the direction of the underlayment matter?

A.Dave Molloy, a roofing contractor in Cincinnati, responds: The primary purpose of roofing underlayment is to provide temporary weather protection both before the finish roof installation and later (should the finish roof become damaged). According to code, underlayment should be “applied shingle fashion,” parallel to and starting from the...

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