How To Look At A House
McGarry and Madsen's home inspection blog for buyers of
site-built, mobile/manfuactured and modular homes
What is the minimum pitch/slope of an asphalt shingle roof?
Tuesday, September 11, 2018
Manufacturers of asphalt shingles recommend a pitch of 4/12 or more, but the absolute minimum pitch is 2/12. Pitch is measured in inches of vertical rise of the roof over one foot (12 inches) of horizontal run. So a 4/12 pitch roof rises 4 inches for every 12 inches of horizontal run.
Asphalt shingle roofing is designed to shed water that is flowing downward over its surface, with each shingle overlapping the one below. But shingle roofing is not watertight, in the sense that standing water—or wind-driven water on a low-slope roof—can work its way up between the laps, which is the reason for the minimum 2/12 slope.
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.
Roofs with a slope below 2/12, on the other hand, are prone to leakage from wind-blown rain or simply a heavy rain. The photos below show a roof just slightly below 2/12 pitch and without the required doubled-up underlayment, along with the area of wood rot at underside of sheathing due to leakage.
Also, see our blog posts What does a home inspector look for when examining a roof? and How can I tell if a roof has more than one layer of shingles?
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To learn more about roofs and attics, see these other blog posts:
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