When we talk about roof styles and shapes, there is one dimension that is common to them all: slope.
Also referred to as ‘angle’, ‘pitch’, or ‘incline’, the slope of a roof is just a measure of its degree of rise from a completely flat, horizontal line.
In this article, the roofing professionals at A-to-Z Roofing & Exteriors are going to explain just how small the slope can be for a pitched roof to properly function.
Defining Roof Slope
In geometry, angles are measured in degrees. And, while degrees can be used to describe the slope of a roof, it’s more common to use the following nomenclature:
X inches of rise / 12 inches of run
So, for example, a completely flat roof with no slope whatsoever would have an incline measurement of 0/12. This is incredibly rare, however, as a completely flat roof would not do a good job of allowing water to run off of it! Even commercial roofs that are termed ‘flat roofs’ have at least some angle to them.
If a roof were to rise three inches for every twelve inches of run, the measurement would be 3/12 (this can be simplified to ¼ for some applications). More commonly, pitched roofs that use asphalt shingles fall into two broad categories: Low-Slope and Conventional Slope.
A low-slope roof has a rise/run measurement of 2/12 to 4/12, generally speaking. These types of residential roofs aren’t as commonly found in Colorado, as they don’t do as good of a job at shedding snow and accumulated debris.
They do exist, though, and our team works on them fairly regularly.
Low-slope roofs require some extra attention when installing asphalt shingles onto them. Most low-slope roofs require a double layer of asphalt-saturated felt paper to be installed underneath the shingles. Alternatively, a single ice-and-water shield layer can be installed, instead.
Conventional Slope Roofs
Conventional slope roofs pick up where low-slope roofs leave off. That is, any roof with a slope of 4/12 and higher is classified as a conventional slope roof. This category of roof slope accounts for at least 75% of the residential roof types in Colorado.
Conventional slope roofs are ideal candidates for the use of asphalt shingles or wood shake. This is because the roof angle is sufficient enough to allow for speedy water runoff. A conventional slope roof is also better protected from shingle blow-off compared to a low-slope roof.
Minimum Slope for a Pitched Roof
Now that we’ve gone over the various slopes found in most residential roofs in Colorado, let’s answer the question: “What is the minimum slope of a pitched roof?”
While it is possible to use standard asphalt shingles on a low-slope roof, the minimum recommended slope prescribed by most roofing material manufacturers is 2/12. Some manufacturers even suggest 4/12 as a minimum, just to be safe.
Any slope lower than 2/12 will often require the use of roofing materials like single-ply membranes or built-up roofing (BUR) membranes.
But, why is this? Why is 2/12 the minimum slope for a pitched roof?
The short answer is gravity. When a roof isn’t pitched steep enough, asphalt shingles don’t do as good of a job at protecting the building. Shingles require slope to function properly, and without it, there’s a much higher chance that rainwater and snowmelt will move laterally underneath the shingles, which is a recipe for leaks!
How to Tell Which Roofing Material is Best for You
In Colorado, there are dozens of different roof styles, from Open Gable to Dormer and from Jerkinhead to Saltbox.
Every roof system should be designed to maximize on the strengths of the roof design itself. This means matching the most appropriate roofing material to the slope of the roof in question. This is something that should be done by a qualified roofing professional who knows how to take accurate measurements and prescribe the best possible roofing solution.
At A-to-Z Roofing & Exteriors, we’ve seen what can happen when mistakes are made by other roofers during the material selection process. Don’t risk the health and welfare of your home or business by working with a roofer that cuts corners!
We are happy to provide an on-site analysis of your unique roofing situation. After taking just a few basic measurements, we’ll be able to recommend the best roofing system for your home or business—a system that will provide decades of reliable protection from the challenging climate we have here in Colorado.
Contact us today to learn more!