How to Find and Calculate the Pitch of a Roof

Whether you're building a new home or addition, installing a new roof, or remodeling your home, knowing how to calculate roof pitch is an essential skill. 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 in your attic. Here's a look at how to calculate roof pitch and why getting accurate measurements is so crucial.  

What Is Roof Pitch?

Roof pitch expresses the roof's steepness as a ratio of the vertical rise to the horizontal run. 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.

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.

Types of Roof Pitches 

While several roof styles are possible on a home, including gable, hip, or mansard, and others like saltbox, skillion, or even jerkinhead, they all have one thing in common: roof pitch. Your roof's pitch determines its steepness regardless of the roofing style. 

• Flat roofs: Flat roofs aren’t actually flat and instead have a slight pitch to direct water toward roof drains. The average pitch of a flat roof is 1/4:12, but it can go as low as 1:80. Flat roofs are most common on commercial buildings and multi-family residential homes.

• Low-pitched 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. 

• Medium-pitched roofs: A roof with a medium pitch is steeper but walkable with caution or while wearing fall protection gear. The pitch ranges from 4:12 to 9:12. These are the most common roof pitches in the U.S. for residential homes.

• High-pitched roofs: High-pitched roofs are the steepest and have pitches greater than 9:12. These are dangerous to walk on without scaffolding and fall protection. They’re most common in areas that see heavy snowfall, as they shed snow and ice well.

Terms to Know When Calculating Roof Pitch 

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: The roof span is the distance from the outer edge of the roof on one side of the building to the outer edge on the opposite side. 

• 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, called the ridge. The rafter run on a simple gable roof equals half of the roof span. 

• Roof rise: The roof rise measures the vertical distance between the highest exterior point (the ridge) and the top of the ceiling joist below.

• Roof slope: The roof slope is the measured angle of the rafters on a single roof plane. The roof slope equals a ratio of the number of inches of vertical rise 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: Roof pitch is usually interchangeable with roof slope and measures the same thing. However, differentiating between pitch and slope is crucial when building roof trusses for homes with particular roof types or making other calculations.

How to Calculate Roof Pitch

Determining your roof pitch is possible in various ways with just a few roof pitch calculations. 

Calculate Roof Pitch From the Existing Roof Surface

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. 

1. Make a temporary mark 12 inches from one end of your level. This will set the run of the pitch to 12 inches for an easy calculation.

2. Rest that end of the level on the roofing material and hold it perfectly horizontal or level.

3. Measure from the 12-inch mark on the level straight down to the roof surface. This will determine the rise of your roof over the 12-inch run.

4. Your roof pitch for that slope will be the rise as compared to the 12-inch run. For example, if your vertical measurement is 4 inches, your roof pitch is 4:12.

Calculate Roof Pitch From Inside an Attic

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. 

1. Make a temporary mark 12 inches from one end of a level. 

2. 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).

3. Measure from the 12-inch mark on the tool straight up to the roof deck.

4. Your roof pitch is the rise over the run, so if your vertical measurement is 4 inches, your roof pitch is 4:12.

Calculate Roof Slope From Pitch 

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.

1. First, measure the distance between the outside of the exterior walls that support the roof, from the end of one eave to the other.

2. 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.)

3. 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.

4. 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. 

Roof Pitch vs. Roof Slope

While some homeowners, home center retailers, and home builders use the terms “roof pitch” and “roof slope” interchangeably, this 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.

Tips for Calculating Your Roof Pitch Properly 

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. 

• 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. You can also make a mark on the ceiling joist at the halfway point between exterior walls and measure from there to the ridge.

DIY vs. Hiring a Pro to Determine the Pitch of Your Roof

If you're hiring a local roofing company to replace your roofing materials or work on your new home or addition, the only reason for you to determine your roof's pitch beforehand is to estimate the cost. When you hire a roofer, they’ll always take measurements and calculate roof pitch. Since most pros also offer free estimates, you’ll only need to calculate roof pitch if you’re planning on DIYing your roof replacement.

Marwa Hasan contributed to this piece.