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Minimum Slope For An Asphalt Roof

Shingles of all types need to have a slope to function properly. The minimum slope for an asphalt shingle roof pitch is 2:12, meaning that you need two units of vertical for every 12 units horizontal. This minimum slope requirement can be found in the roof installation instructions for almost every manufacturer and in many building codes. 

What Is Roof Slope?

Roof slopes are defined as a ratio of rise-over-run. If the rise of the roof slope is 2 inches when measured 12 inches along the bottom of a horizontal roof, the roof slope is then 2 inches of rise per 12 inches of run. 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 also apps you can get on your phone that can help, but shingle manufacturers distribute convenient pitch estimator cards that provide a quick way to estimate the slope.

A roof pitch estimator can help you determine if your roof slope is likely too low for shingles. The majority of the homes in the U.S have typically been constructed with a slope between about 4:12 and 9:12. Roofs at the lower end of the range between 2:12 – 6:12 are relatively easy to walk and work on. 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.

Low Sloped Asphalt Roof Shingle Installation

Asphalt shingles are an effective roof material for sloped roofs that can be used on slopes as low as 2:12. Concerning shingle application, roof slopes from 2:12 to 6:12 are called “low slope” roofs, while roof slopes of 6:12 or greater are considered to be standard slopes. Asphalt shingles should never be applied to roof slopes below 2:12, this is lower than the minimum slope of a metal roof so ensure to check the different minimums for each roofing material.

When installing asphalt shingles on a low slope roof, after the manufacturer’s application instruction has been completed, an extra underlayment with a specific method for application is required. As it is a lower slope, these styled roofs tend to allow more water to enter, primarily because of severe ice dams and harsh rain. The need for underlayment help to enhance the water-shedding element of the roof system.

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