Understanding how shingles work and their minimum roof pitch requirements are crucial to ensuring that a roof provides adequate protection for the living or storage area under it while also adhering to building code requirements.
While roofs with a 4/12 pitch and steeper are ideal, the minimum roof pitch for shingles is 2/12. A roof with a 2/12 pitch can safely use shingles by incorporating additional layers of underlayment around the eaves.
In this article, we’ll examine what roof pitch is, how to calculate a roof’s pitch, and what the minimum pitch requirements are for roofs with asphalt, architectural, and cedar shingles.
- What Is Roof Pitch?
- How to Calculate Roof Pitch
- Roof Pitch Calculator
- What Is the Minimum Roof Pitch For Shingles?
- Code Requirement for Asphalt Shingles Pitch
- Can You Put Shingles on a 2/12 Pitch Roof?
- Can You Shingle a 3/12 Pitch Roof?
- Can You Use Shingles on a Low Slope or Flat Roof?
- What Is the Minimum Roof Slope for Cedar Shingles?
What Is Roof Pitch?
You’ll often hear the words “pitch” and “slope” used interchangeably to describe the angle of a roof. Roof pitch is the angle of a roof given in a fraction while a roof’s slope is given in a ratio; however, both discuss the same thing.
The pitch of the roof is determined by its rise over its run. For roofs, the rise is always the variable, with the run always being constant at 12 feet. For example, a roof with a 5/12 slope will rise 5 inches of height for every 12 inches of length and a roof with an 8/12 slope will rise 8 inches for every 12 inches of length.
The greater the rise, the steeper the pitch of the roof. Whereas a roof with a 3/12 pitch has very little pitch to it, a 12/12 roof will have a very steep pitch.
How to Calculate Roof Pitch
To calculate roof pitch, one must employ a simple equation from high school algebra used to calculate the slope. Calculating roof pitch involves dividing the rise by the run and multiplying that number by 12: Rise/Run x 12. Placing the answer to this equation over 12 is the roof’s pitch.
A roof that rises two feet over a run of 4 feet would have the following equation: 2/4 x 12 = 6. Therefore, the pitch is 6/12.
While the concept of calculating slope may be simple, taking the measurements from an existing roof can be challenging. Attempting to take measurements from the rooftop won’t help you as you need to measure the run and the rise as opposed to the length of the roofline itself.
A better and safer way to do this is to take measurements from the attic. A roof’s pitch is created by the rafters and ceiling joists that create the roof’s shape. Take measurements by holding a level horizontal from one rafter.
Check to make sure the level is level, then make a mark with a pencil 1 foot in from the rafter. Keeping the level level, measure the distance between the mark and the rafter, keeping the tape measure perpendicular to the level.
The measurement you take is the rise of your roof, with the total pitch being that number over 12. So, if your vertical measurement is 5 inches, the pitch is 5/12. If it’s 10 inches, the pitch is 10/12.
Roof Pitch Calculator
If you’re looking for a simpler way than taking measurements, don’t have an extra set of hands to help you, or don’t have access to the attic space, then consider using a SmartPhone app. Android and iOS apps such as ImageMeter, Ruler, and iLevel allow you to quickly identify the roof pitch by holding the smartphone against the roof or a rafter.
If math isn’t your thing, the tool below will make the calculations for you by entering the rise and run in inches.
What Is the Minimum Roof Pitch For Shingles?
Shingles require a certain amount of pitch to work properly. This has to do with the way shingles work. Shingles overlap each other, beginning at the eaves of the roof up to the peak, where a roof cap overlaps the final layer of shingles.
This overlap protects the roof by using gravity. Once rain hits the roof, it runs down the shingles over each overlap, guiding rain down before falling off the eaves and into the gutter. For the shingles to work properly, there needs to be a steep enough pitch to cause the water to flow over each overlapping shingle in a downward direction.
If there isn’t enough pitch, the water can pool or encounter an obstruction, such as leaves or other debris and run in the opposite direction, finding its way under the overlaps in shingles and to the vulnerable roof sheathing, eventually resulting in a leaky roof.
Lowest Pitch Roof for 3-Tab Shingles
Standard 3-tab asphalt shingles work best on roofs with pitches of 4:12 or steeper; however, it is possible to get away with a roof pitch as low as 2:12 by adding additional protection (more on that later).
Minimum Roof Slope for Architectural Shingles
Although architectural shingles offer certain structural advantages, such as durability that allows them to last up to 50 percent longer than standard asphalt shingles, they work the same as a standard 3-tab shingle when it comes to channeling water and thus demand the same roof pitch requirements. Architectural shingles, like standard 3-tab shingles, require a pitch of at least 2:12.
Code Requirement for Asphalt Shingles Pitch
Roof pitch standards for asphalt shingles aren’t just recommended; they are typically a requirement for a roof to pass local building codes. According to International Building Code, asphalt shingles should only be used on roof slopes of 2:12 or greater.
The building code also stipulates that roof slopes from 2:12 up to 4:12 must use double underlayment to ensure that no water infiltrates the roofing material to reach the roof underlayment, where it can cause rot and leaking.
This double layer begins with a minimum 19-inch wide strip starting at and running parallel with the eaves of the roof. This strip is then covered by a second 36-inch wide sheet that also starts at and runs parallel to the eaves. This sheathing should be attached securely and flat enough to allow for the proper installation of the shingles.
This overlapping provides extra protection in the event water backflows into the shingles near the eves, such as when a gutter is overwhelmed during a downpour and overflows, sending water back up the roof.
Can You Put Shingles on a 2/12 Pitch Roof?
As discussed above, it is possible to put shingles on a roof pitch as low as 2/12. It’s also important to note that the International Building Code supports this. That said, it’s a good idea to check local building codes to make sure your municipality allows this.
Remember, putting shingles on a roof with a pitch of 2/12 up to 4/12 requires a double layer of underlayment in the section of the roof along the eaves as an extra water barrier if the shallower roof pitch doesn’t carry off rainfall fast enough, causing some water to backflow underneath the shingles. Some roofers even suggest adding additional stopgaps such as asphalt cement and ice and water shield to the seams to stop water that may make it past the shingles.
Keep in mind that although building codes allow for shingles to be installed on a 2/12 roof, it may not be the best idea. Shingles on a 2/12 roof are more susceptible to failure than on roofs that are 4/12 and greater. The general rule of thumb is that shingles on steeper roofs last longer than those on shallower roofs.
This is because the steeper roof angle not only channels water away more quickly than shallower roof pitches but it also better sheds debris and snow that can create dams that allow water to pool on a roof.
In comparison, a 2/12 roof is much more likely to collect debris and will take longer to shed snow. This buildup of debris can cause water to pool and backflow between the shingles, especially around the eaves, where debris generally collects. In areas that see considerable snowfall, a shallower roof is more likely to hold snow, which can melt and refreeze, creating snow dams that cause water to pool.
Shingles also age over time, which can cause them to curl. While a slight curl may not cause problems on a higher-pitched roof, a slight curl might be enough to push water back up into the shingles on a flatter 2/12 roof.
With these elements in mind, it may make sense to consider other roofing options, especially if your home or shed is under trees that constantly shed debris or you live in an area with a significant amount of annual snowfall.
If you do plan to shingle a 2/12 roof, make sure the shingle manufacturer’s warranty covers roofs with this flat of a roof pitch. Most do but some do not, so it’s worth checking beforehand.
Can You Shingle a 3/12 Pitch Roof?
A 3/12 pitch roof is a little steeper than a 2/12 pitch roof, so it’s eligible for shingles. Since it’s a bit steeper, it will do a better job of shedding rainwater than a 2/12 pitch roof. Remember, since it falls below the 4/12 pitch, you must use the additional layers of roofing underlayment described above for the roof to meet the building code.
Can You Use Shingles on a Low Slope or Flat Roof?
Shingles won’t work on a flat roof or a low slope roof with a pitch of less than 2:12. As we discussed above, shingles need gravity to work. They overlap each other from roof peak to eaves, with gravity carrying the water over each overlap until it reaches the eaves of the home and falls either into a gutter or onto the ground below.
Without a substantial pitch in the roof, water will instead collect on the roof instead of running off it, seeping under the shingles and ultimately causing roof leaks. Even though a roof with a slight pitch will cause water to run in one direction, this pitch will not carry water away fast enough during a downpour or even a steady rain to prevent water from flowing back into the seams between each overlap.
Snowfall and any debris from nearby trees that may collect on a flat or low slope roof only exacerbate this problem by creating dams that cause water to pool on the roof.
If you have a low slope or flat roof, consider other roofing options, including metal roofing, PVC, rubber, or tar and gravel roofing.
What Is the Minimum Roof Slope for Cedar Shingles?
If you’re planning on putting a cedar roof on your home or shed, you’ll need a minimum roof pitch of 5/12. This is because wood shingles tend to leak more than asphalt shingles. They don’t fit as tightly together and don’t lay as flat as asphalt shingles. When installed, cedar shingles sit slightly upward instead of lying flat, countering some of the pitch of the roof framing.
A steeper roof pitch is necessary to make up for the structural differences between asphalt and cedar to ensure that water can run quickly off wood shingles and not pool and make its way into gaps between the shingles.
Knowing the slope of the roof of a home or shed is crucial to choosing the right roofing material, be that asphalt shingles or some other option. The minimum required roof slope for asphalt shingles is 2/12, with additional layers of roofing paper needed for roofs with pitches from 2/12 up to 4/12.
Those installing a cedar roof need a minimum roof pitch of 5/12 to account for the structural differences that make wood roofing more susceptible to leaks than asphalt shingles. Whether installing shingles on a 2/12 roof or an 8/12 roof, always make sure to use the proper underlayment and proper installation procedures to prevent the roof from leaking.