Q. 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 eaves (2012 IRC, R905.2.7). In practice, though, there are times when it’s quicker to install the underlayment vertically, such as on a steeply pitched dormer roof (see photo). Over small areas like this, I don’t think it makes much of a difference whether the underlayment is installed vertically or horizontally.
Water moves quickly and predictably down a steep roof. I’d be more concerned about leaks if the rafter lengths were long or the roof pitch shallow, in which case the slow-moving water would have time to fan out horizontally beneath the edge of vertically run underlayment. Think of individual step flashings along a wall/roof junction compared with a single long base flashing: The step flashings are more effective because as water flows down the roof and starts seeping diagonally beneath a shingle tab, the flow is interrupted by the next flashing and redirected down the roof. In the example in the photo, the longest vertical lap appears to be less than 6 feet and the pitch is steep, so I doubt the roof will leak if the felt is temporarily exposed to rainfall.
Actually, I’ve seen countless shingled roofs with both shallow and steep pitches that have been installed without underlayment, and I’ve never seen the need to replace them to correct the violation. As a business owner, though, it’s really hard to argue against narrow code details. Lawsuits are based on these details, even if actual practice is based upon something less rigid. If your code official enforces the horizontally installed roofing underlayment requirement, I wouldn’t argue with him.