Asphalt shingles are a common roofing material that is widely used today. They come in various colors and styles to best suit the building type and aesthetic desired by the person installing them. Here are the critical aspects that you should know regarding the minimum slope for asphalt shingle roof:
What Is the Minimum Slope for an Asphalt Roofing Shingle?
The standard slope of an asphalt shingle roof is 2 inches in 12 inches, with a 1-inch overhang on all sides. This slight slope makes asphalt shingles such a popular and attractive roofing material to install.
An asphalt shingle roof can have any slope between 2 to 16 inches in 12, the standard being 2 inches in 12. A residential roof under 10 feet wide and has a pitch of at least 1:12 will require at least a two-inch slope per foot of roof length. A commercial roof under 16 feet wide and has a pitch of at least 1:12 will need at least a four-inch slope per foot of roof length. The pitch is determined by the number of units in 12 inches (or units per foot).
How to calculate the minimum slope for an asphalt roofing shingle
To calculate a minimum slope for an asphalt shingle roof, follow these steps:
- Determine the length of the shortest side of your roof (L) from one corner to another point on the same level surface (A). That will be shown in inches or fractions of an inch. The shorter side is referred to as A and the longer side as B. For instance, if your house is 40 feet long and the shortest side is 20 feet, then A = 20ft and B = 40ft. Be sure to use all dimensions in inches or fractions of an inch whenever possible, and do not convert to a decimal.
- Measure the height of your roof from the highest point on your roof (A) to the lowest point on your roof (B). Again, do this using all measures in inches or fractions of an inch. If you have features such as dormers or other structures that interrupt the 1st story height measurements from corner to corner, you need to measure these separately and add them individually into equation B (on top roof level).
- Divide the distance between A and B on your house’s roof by the length of B (B). That will result in an approximate number of inches per foot (such as 12 inches). If you use fractions, convert them to whole inches before dividing by the length of B: 48/21 = 2.4 or 2.2013265922; inches.
- Divide the height from A to B measurement by inches per foot. That is the “slope” percentage and is usually expressed as a decimal like 0.78 or 0.18 percent.
- Multiply the slope percentage by 0.5 (0.5 is half of 1). That will tell you the minimum slope that your roof must be built at to stay functional. Your roof must be at least this level, or the shingles will not withstand strong winds and rain. For example, if your roof has a built-in 10 percent slope, it must have a minimum of 8 percent. For a 25 percent slope, it would have to have 18 percent.
It is important to remember that different types of shingles and roofs will require different slopes for optimum results. For as seen in the Technonicol website:
A steeper pitch (steep roof) requires more material and usually requires that it be installed in 2 separate passes (half the shingles face up, then the other half face up). It is easier to install large amounts of shingles on a steep slope, but it also risks covering your lower level if too much water accumulates. Therefore, when calculating your minimum required slope, you should consider how high your roof will be overall.
A flat roof has less surface area to cover than a steep roof does, so have less insulation and is often not as strong in high wind conditions due to the minimal slope. Also, you will likely find that this kind of roof is more insulated and will therefore require less time and materials to install.
Lateral pressure: The roof’s pitch must be considered in the design phase because if it is too steep, there could be enough weight from the sides of the structure, which could cause your shingles to crack or blow off. That can cause water damage to other parts of your house as well. Therefore, it is always recommended that you consult with your local building inspector before selecting a roofing product for your home or business.
Factors that affect an asphalt roof’s slope
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The weight of the roof
The steeper the pitch, the heavier it is, and the more it presses down on the deck or framing. The weight will pile up at your ridge board, which can cause sagging at the peak and either wind damage or water leaks if you are not careful. There are many ways to prevent this; be sure to consult a professional before attempting any home improvements.
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Confined space
The pitch of your asphalt roofing shingle may also determine how much room you have between your roof and your dormer walls, chimney, and skylights. If your roof is too steep, the shingles could easily slip off. If the pitch is too shallow, the shingles could blow right off. Most asphalt shingle manufacturers will have a recommendation for each of these situations to ensure that your roof has an adequate slope and features enough ventilation.
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The slope of roof ridges
Some roof valleys are designed with a negative pitch so that water will flow down into them instead of outwards, which it could cause damage to the siding. That is done to redirect or prevent water from pooling at ridges and potentially causing damage to framing members or getting into the structure of your house.
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Climate and Rainfall
Most local building codes require that roofs slope between 10 and 20 percent, but you may want to choose a steeper pitch if your area has higher annual rainfall or seasonal rains. Even if you do not require additional drainage from having a steep roof, it is essential to consider how much rain is falling in your city to know how slippery the surface of your shingles might be during high-wind conditions or floods. For instance, in the northern states such as New Hampshire and Maine, homeowners expect a more significant amount of snow to fall during the winter. They wish to have a more enormous, steeper roof that will manage the weight of ice and snow and keep precipitation out. Areas that do not seem as much snowfall may have smaller roof pitches with an architectural style.
Benefits of a roof pitch/slope
The pitch and slope of any roof are critical in maintaining the roof’s integrity. That includes everything from preventing ice dams to leaks caused by wind-driven rain and even structural damage. Asphalt shingles prevent leaking through the gradual accumulation of water due to a slight incline, allowing gravity to pull the water down the roof consistently. If there is not enough slope, water can build up within an area, causing damage before draining off.
A roof’s slope and pitch are critical for snow loading. Snow will be more likely to pack down over a roof with a slope and pitch that enable it to drain at a consistent rate. Snow will also cover an area more quickly if the roof is flat, meaning that the snow will not be able to drain out as quickly. That can cause problems with ice dams and property damage caused by wind-driven rain and snow.
An asphalt shingle roof can also act as insulation in certain climates where it is prone to freeze. A flat roof will only insulate at around 30-35 percent efficiency, while an asphalt shingle roof can insulate up to 70 percent of thermal mass, which can help a home stay warmer in colder months when temperatures are below freezing.
The minimum slope can vary by region and depending on the type of shingles being used and if any special materials are required for a short recovery time between installation and weather loading. Check local regulations to verify what your area needs before any construction begins