The performance of all roof coverings is based in part on the slope of the roof surface. As the slope of the roof decreases, water drainage is slowed, and the potential for water intrusion (roof leaks) increases because of the greater potential of water traveling under the roofing.
The slope of a roof is expressed as a proportion of the vertical to the horizontal. The slope is the ratio of the vertical rise to the horizontal run, where the run is some portion of the entire span. This ratio is often expressed as inches per foot. The three types of roofs (flat, low, and conventional roof) and their corresponding slopes are illustrated above.
Slope Ratio
A roof that rises 4 inches for every 1 foot or 12 inches of the run is said to have a “4 in 12” slope. If the rise is 6 inches for every 12 inches of run, then the roof slope is “6 in 12.” The slope can be expressed numerically as a ratio. The slope ratio represents a certain amount of vertical rise for every 12 inches of horizontal run. For example, a “4 in 12” slope can be expressed as the ratio of 4:12. A “6 in 12” slope is expressed as 6:12. The slope is expressed as a ratio and in inches per foot. Why is all of this slope information important?
The slope of a roof affects the surface drainage of water and can determine the type of roof-covering materials that should be installed. The slope of a roof affects its ability to shed water and determines the limits for using asphalt shingles. Asphalt shingles should be used only on roof slopes 2:12 or greater. Asphalt-shingle roofs are designed to shed water and are not meant to serve as a waterproof barrier.
Most asphalt shingles may be used on roof slopes from 4:12 to 21:12, using standard application methods. Asphalt shingles may be used on slopes from 2:12 to 3.9:12 if special low-slope application procedures are followed. A home inspector will usually find roll roofing materials installed on slopes of less than 4:12.
The International Residential Code (IRC) specifies minimum requirements for roof coverings in relation to slope and mandates the use of the roof-covering manufacturer's installation instructions. The code is a good reference for home inspectors and contractors. The manufacturer's recommendations are also a good reference. But home inspectors are not required to inspect in relation to the code or the manufacturer's recommendations.