Know your roof from every angle
Whether you're building a new home or addition, remodeling, or reroofing, knowing how to calculate roof pitch is an essential tool. Calculating roof pitch is necessary for designing and constructing, estimating roof replacement costs, or calculating roof truss costs. It can even help when installing various types of insulation or battling the climate in your location. Here's a look at determining your home's roof pitch and why it's so crucial.
Roof pitch is one of the most commonly used terms in roofing. It expresses the roof's steepness as a ratio of the vertical rise to the horizontal span. The higher the rise in the ratio, the steeper the roof.
For example, a roof that rises 6 inches in the span of 12 inches would have a 6:12 pitch. It means that for every 12 inches of the span, the rise will equal 6 inches.
A flat roof will seemingly have no—or a very slight—pitch. Gable roofs will have a higher pitch ratio, typically between 4:12 and 9:12. For roofs with a steeper slope, the pitch could be up to 18:12 or higher.
Keep in mind that when roof pitch is represented as a ratio, such as 4:12, it measures the slope, or incline, of a single roof surface. In the rare occurrence in which pitch is represented as a fraction, like 1/3, the measurement relates to the steepness of the entire roof rather than a single portion.
While some homeowners, home center retailers, and home builders refer to roof pitch and slope as identical, the assumption is technically incorrect. In reality, and according to the International Association of Certified Home Inspectors, pitch and slope are different measurements for two different roof attributes.
Roof pitch equals a ratio of the roof height as it relates to the horizontal span of the roof, not the rafter lengths, which are stated as a fraction. The roof slope is the slant at which the rafters of a single roof plane rest, stated in a ratio of height to rafter run.
However, the distinction is most important for architects and designers to understand. For our purposes and those of your material supplier, we can use the terms interchangeably, which is acceptable in most cases.
While several roof styles are possible on a home, including gable, hip, or mansard, and others with crazy names like saltbox, skillion, or even jerkinhead, they all have one thing in common, even the flat ones—roof pitch. Your roof's pitch determines its steepness in all those roofing styles.
Surprisingly, flat roofs are rarely what their name would have us believe. Even the flattest roofs have some degree of intentional slope. Roofs considered flat typically have a pitch of half an inch up to a couple of inches of height for every horizontal foot. Large flat roofs may have several low-angle slopes in various directions that divert water to the roof's edges or draining locations.
Low-pitched roofs have a more accurate name than flat roofs. Roofs with a low pitch are visibly slanted but easy to walk on and have a pitch of between 2:12 and 3:12. These roofs are common for dormers that extend from steeper roofs, single-room additions on a home, or contemporary homes that don't have flat roofs.
A roof with a medium pitch is steeper but walkable with caution or while wearing fall protection gear. The shallowest of these rooflines has a 4:12 roof pitch, meaning they rise 4 inches for every 12 horizontal inches, while the steepest ones can measure 9 inches in 12.
You may have already guessed that high-pitched roofs are the steepest and have pitches greater than nine vertical inches in 12 horizontal ones. High-pitched roofs aren't walkable and require scaffolding or other physical aid and wearable fall protection to navigate. For safety, it’s best to let a pro inspect a high-pitched roof or perform maintenance.
Whether you're determining how much roofing material your reroofing project requires or planning to build roof trusses for a new addition, calculating roof pitch starts with understanding some roofing lingo.
Roof span: When talking about roof pitch, the roof span equals the distance from the outer edge of the exterior walls that support the roof's edge at the bottom of its slopes.
Rafter run: The rafter run is a horizontal measurement from one edge of the roof span to a point directly below the highest exterior location of an individual roof plane or rafter. The rafter run on a gable roof equals half of the roof span.
Roof rise: The roof rise measures the vertical distance between the highest exterior point of a rafter and the top of the ceiling joist below on the same plane as the roof span measurement.
Roof slope: The roof slope is the measured angle of the rafters in a single roof plane. The roof slope equals a ratio of the number of inches of height over 12 horizontal inches of span. For instance, a roof that rises 6 inches for every 12 inches has a slope of 6:12.
Roof pitch: For our purposes in this article, roof pitch equals the roof slope. However, differentiating between pitch and slope is crucial when building roof trusses for homes with particular roof types or making other calculations.
Determining this measurement is possible in various ways with just a few roof pitch calculations.
Always use extreme caution if you venture onto your roof. Working from a ladder and the height of the roof always presents the risk of serious injury. However, if you're up there, you can determine its pitch using a level and a tape measure.
Make a temporary mark 12 inches from one end of your level.
Rest that end of the level on the roofing material and hold it perfectly horizontal or level.
Measure from the 12-inch mark on the tool straight down to the roof surface.
The measurement from the bottom of the level to the roof is the first number in the ratio that ends with “:12.” If you measure 4 inches, your roof's slope equals 4:12.
To calculate your home's roof pitch from inside the attic, simply invert the method you'd use from the other side of the roof deck.
Make a temporary mark 12 inches from one end of a level.
Hold that end of the level on the underside of the roof sheeting material and hold it perfectly horizontal or level (holding the level against a rafter is helpful for support).
Measure from the 12-inch mark on the tool straight up to the roof deck.
Your inch measurement is the first number of your x:12 ratio that signifies your roof's slope.
While interchanging roof slope and pitch is acceptable in most situations, it's important to know the difference when calculating roof slope with just a tape measure and some math.
The roof pitch equals the ratio of height to the total width of the roof.
First, measure the distance between the outside of the exterior walls that support the roof.
Then, measure the distance from the top of the ceiling joist to the roof's ridge. The ratio will be the height to span. (In reality, a roof pitch is a fraction that equals the rise divided by the span.)
To convert to roof slope on a gable roof:
Divide the total span by two to find the rafter run. For example, a roof that's 8 feet tall and 24 feet wide will have an 8:12 slope.
For roofs with rafters of different lengths:
Use the Pythagorean Theorem (C = A squared + B squared) to determine the roof slope, with A equal to the roof plane's height and C equal to the sloping plane.
While calculating your roof pitch is pretty straightforward, keep a couple of things in mind to ensure accuracy, and remember that going onto any roof is dangerous.
For safety, measuring from inside the attic is the best option possible.
Remember to add the material thickness above the inside of the ridge when measuring the total roof height from inside the attic.
Measuring the overall roof span and dividing it by two to determine the rafter run on gable roofs is often more accurate than guessing the center point of the roof from inside the attic. Or make a mark on the ceiling joist at the halfway point between exterior walls and measure from there to the ridge.
If you're hiring a local roofing company to replace your roofing materials or work on your new home or addition, there may be little reason for you to determine your roof's pitch, as your roofing pro can do it for you. However, knowing how to calculate roof pitch can be helpful when talking with your roofer or remodeling designer about your project and estimating costs for the project.
Marwa Hasan contributed to this piece.
While there's no standard pitch for a roof, there are several guidelines that architects, designers, and builders use when deciding how steep a roof needs to be to perform best. Homes in locations that must endure snowy weather or heavy rainfall are often steeper than those in dryer climates. However, there are numerous exceptions to the rules, and you're just as likely to find roofs of all different slopes in any location.
Using a roof pitch multiplier is an accurate way to determine the total surface area of a roof plane. It’s handy for purchasing the correct amount of materials. To determine your roof pitch multiplier, you'll need to calculate the roof slope, square it, and add it to 144. Now find the square root of the total and divide it by 12. The answer is your roof pitch multiplier.
Once you know the roof pitch multiplier, multiply it by the total square feet of ground covered by the roof to determine the total roof surface area.
The golden ratio for roof pitch is 1:1.62. It’s simply a standard aspect ratio that home and building designers and architects use when deciding how steep a roof should be to maintain visual appeal. The golden ratio can apply to any home or structure with a visible roof that isn't considered a flat roof.
