All asphalt shingle roofs with a pitch of 4/12 or more have an underlayment of 15-lb. asphalt felt paper that is tacked to the roof sheathing before the shingles are installed. Each sheet overlaps the one below it by 3”, called downlapping, and provides a backup layer of protection from water intrusion.
Roofs between 2/12 and 4/12 are required to have the underlayment more deeply lapped, so that there are effectively 2 layers of the 15-lb felts over the surface. Or, an alternative underlayment for low-slope roofs is a self-adhering bituminous membrane material is also self-sealing. The most popular brand of this underlayment is “Grace Ice and Water Shield.”
A roof slope of between 4/12 and 8/12 is considered the sensible range for most situations. While a higher pitch than 8/12 gives a home more dramatic curb appeal, there are several downsides. A high pitch increase the homes exposure to lateral wind loads, and the rapid flow of rainwater down the shingles can prematurely scour away the protective granules on the surface of the shingles. Valleys on steep roof are especially prone to premature granule loss, as shown in the photo below. Also, steep roofs are more difficult—and less safe—to maintain or replace. Workers may have to be wear a safety harness while on the roof, which slows them down.
While we hope you find this series of articles about home inspection helpful, they should not be considered an alternative to an actual home inspection by a local inspector. Also, construction standards vary in different parts of the country and it is possible that important issues related to your area may not be covered here.
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