The short, simple answer is 2:12 has traditionally been considered the absolute minimum acceptable roof slope suitable for asphalt shingles.
Asphalt shingles continue to be the most widely installed roof covering option in North America. They provide excellent weather resistance, great value and, now more than ever, a tremendous range of beautiful designs and color blends.
And of course, their ease of installation and adaptability to just about any roof shape is another reason for their success. In recent decades, home designs have tended toward steeper roofs, but there are many instances where a house may have a lower roof slope, or a portion of the roof that is at a lower slope.
To help understand the issue better, let’s take a look at how roof slope is measured and other considerations.
Slope = Rise/Run (in inches)
4 in 12 and up
2 in 12 to 4 in 12
0 in 12 to 2 in 12
Roof slopes are most commonly defined by the rise-over-run ratio. If a roof slope “rises”, say, 4 inches when measured 12 inches along the bottom of a horizontal roof truss, the roof slope is said to be 4:12; i.e., 4 inches of rise per 12 inches (one foot) of run. (Note that roof ”pitch” is sometimes used in parallel with roof slope; however, roof pitch is technically measured and defined differently, and will not be discussed here). You can measure the roof slope on the bottom side of the roof deck at an easily accessible point such as the rake, using a tape measure and simple geometry. Alternatively, there are smart phone apps that can help, but shingle manufacturers distribute convenient pitch estimator cards which provide a quick way to ballpark the slope.
A handy roof slope estimator can help you determine if your roof slope is likely too low for shingles.
The majority of North American residential roofs have historically been constructed with a slope between about 4:12 and 9:12. Roofs at the lower end of the range (about 4:12 to 6:12) are relatively easy to walk and work on. (As always proper safety equipment and precautions must be followed when working on any roof). Steeper roofs “show” the shingles better, and are therefore better suited to premium designer styles. Shingles on a steeper roof also tend to weather at a slower rate due to their favorable angle to the sun’s rays.
The challenge with very low sloped roofs is simple – gravity. At a very low slope, any water on the roof, whether rain water or snow melt, drains very slowly and there is a risk of lateral water movement around and through the shingle overlaps. It is important always to confirm the specific slope requirements with the shingle manufacturer’s product installation instructions and in accordance with local building code requirements, as some shingles may have a minimum slope recommendation higher than 2:12. As well, a slope of 4:12 is most commonly considered the lowest slope for “standard shingle installations”. Most manufacturer and industry recommendations require, or at the very least recommend, special underlayment or other considerations on roofs between 2:12 and 4:12. (Note: some roof specifications may express slope in the typical geometric terms of degrees. For reference, 2:12 equates to about 9.5º and 4:12 to about 18.5º). At slopes below 2:12, a granule-surfaced asphalt membrane can be installed, and many are available in colors to complement the shingle colors.
If your existing roof has shingles on it and it’s time to re-roof, you can very likely use shingles again, but on questionable lower slopes it’s best to confirm the slope is above the 2:12 minimum just to be sure.
The video below explains what the minimum slope requirements are for a shingle roof, the difference between a sloped roof and a flat roof and how to calculate a roof’s slope.
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