Roof pitch describes how steep a roof is by comparing its vertical rise with its horizontal run. In the United States, it is commonly written as a ratio such as 6:12, meaning the roof rises 6 inches for every 12 inches of horizontal distance. With the rise and run, anyone can calculate roof pitch, convert it to an angle, and estimate rafter length. The key is measuring the horizontal run—not the sloped roof surface—and using consistent units.
What Rise, Run, and Roof Pitch Mean
Rise is the vertical distance from the roof’s low point to its high point over a measured section. On a typical gable roof, it is the height difference between the top of the exterior wall and the roof ridge.
Run is the horizontal distance covered by that rise. For a simple gable roof measured from an exterior wall to the ridge, the run is usually half the building’s span, assuming the ridge is centered and both roof sides have the same pitch. Run is not the length of the sloped rafter.
Roof pitch is the ratio of rise to run. A 6:12 pitch means a rise of 6 units for every 12 horizontal units. The units can be inches, feet, or another consistent measure; what matters is that rise and run use the same units.
Roof Pitch Formula
To express pitch in the common U.S. format, divide the rise by the run and scale the result to a 12-inch run:
Roof pitch (rise per 12 inches) = (rise ÷ run) × 12
For example, if a roof rises 5 inches over a horizontal run of 12 inches, its pitch is 5:12. If it rises 10 inches over a run of 24 inches, the calculation is (10 ÷ 24) × 12 = 5, so the pitch is also 5:12.
The ratio can also be stated as a decimal or percentage. Divide rise by run for the decimal slope, then multiply by 100 for percent slope. A 5:12 roof has a slope of 0.417, or about 41.7%. That percentage is not the same as the roof angle in degrees.
How to Calculate Roof Pitch Step by Step
- Identify the measurement points. Choose a horizontal run and the vertical rise over that exact distance. For a roof framing calculation, measurements are commonly taken from the top of the wall plate toward the ridge.
- Measure the horizontal run. Use a level, tape measure, or reliable plans. Do not measure along the roof surface; that gives the sloped distance, not the run.
- Measure the vertical rise. Record the height change between the endpoints of the run. Keep the measurement units consistent.
- Apply the formula. Divide rise by run, then multiply by 12 to get rise per 12 inches of run.
- Write the result as a ratio. A result of 4 means the roof rises 4 inches per 12 inches of horizontal run, or 4:12.
For a quick example, a section of roof rises 18 inches over a 36-inch run. The calculation is (18 ÷ 36) × 12 = 6. The roof pitch is 6:12.
Examples of Roof Pitch Calculations
| Rise | Run | Calculation | Roof Pitch |
|---|---|---|---|
| 4 inches | 12 inches | (4 ÷ 12) × 12 | 4:12 |
| 8 inches | 24 inches | (8 ÷ 24) × 12 | 4:12 |
| 9 inches | 18 inches | (9 ÷ 18) × 12 | 6:12 |
| 2 feet | 8 feet | (2 ÷ 8) × 12 | 3:12 |
These examples show why measuring the actual horizontal run matters: different measurements can describe the same pitch. The 4-inch rise over 12 inches and the 8-inch rise over 24 inches both produce a 4:12 roof pitch.
How to Convert Roof Pitch to Degrees
To find the roof angle, use the inverse tangent function:
Roof angle = arctan(rise ÷ run)
Make sure the calculator is set to degrees. For a 6:12 roof, arctan(6 ÷ 12) is approximately 26.6 degrees. For a 4:12 roof, arctan(4 ÷ 12) is approximately 18.4 degrees.
Pitch ratios and angles describe the same steepness in different ways. A 12:12 roof rises 12 inches over 12 inches of run and has a 45-degree angle. A 3:12 roof is about 14 degrees. The angle does not equal the ratio; a 6:12 pitch is not a 6-degree roof.
How to Find Rafter Length From Rise and Run
Once rise and run are known, the length of the sloped section can be estimated with the Pythagorean theorem:
Rafter length = √(rise² + run²)
For a 6:12 roof with a 12-foot horizontal run, the rise is 6 feet. The straight-line length from the wall point to the ridge is √(12² + 6²), or approximately 13.42 feet. This is the theoretical length before accounting for cuts, overhangs, ridge details, and other framing adjustments.
For a symmetrical gable roof, the run from the exterior wall to the ridge is generally half the building span. A 24-foot-wide building therefore has a 12-foot run on each side if the ridge is centered. That assumption does not apply to every roof design, and the eave overhang may extend beyond the wall.
Rafter sizing and construction require more than pitch and length. Loads, lumber species and grade, spacing, connections, roof covering, and local requirements all matter. A qualified building professional should verify structural plans rather than relying on a pitch calculation alone.
Ways to Measure Existing Roof Pitch
Measure From Inside an Attic
When the roof framing is safely accessible, a level and tape measure can establish rise and run without walking on the roof. Hold a level horizontally from a rafter, mark a 12-inch distance, and measure the vertical gap from the level to the rafter at that point. The measured rise gives the pitch per 12 inches of run. A longer horizontal measurement can improve accuracy, provided the rise is measured over the same distance.
Use a Level and Tape Measure Outdoors
A level can be placed horizontally against a roof surface near an accessible edge, with the vertical distance from the level to the roof measured over a known horizontal distance. This method can be useful, but it should never require standing or climbing on a steep, wet, icy, or otherwise unsafe roof. Falls from roofs can cause severe injuries.
Read Plans or Use a Pitch Tool
Construction drawings may list roof pitch directly, often as a ratio such as 5:12. A framing square or digital angle gauge can also help determine pitch, but the reading method varies by tool. If a device reports degrees, convert the angle to a ratio using pitch per 12 inches = 12 × tan(angle). For example, 30 degrees corresponds to about 6.9:12.
Common Roof Pitch Measurement Mistakes
- Using sloped length as run: Run is horizontal. The sloped measurement is the rafter length, which is longer than the run.
- Mixing units: Convert measurements first if one is in feet and the other is in inches. For instance, 2 feet of rise over 24 inches of run is 24 inches over 24 inches, or 12:12.
- Measuring different sections: Rise and run must correspond to the same endpoints and roof plane.
- Assuming every roof is symmetrical: Hip, shed, gambrel, and other roof designs may have different geometry on different sides.
- Confusing pitch with percentage or degrees: A 6:12 pitch equals a 50% slope and an angle of about 26.6 degrees; these are three different expressions.
- Ignoring roof layers or framing details: Finished surfaces, fascia, ridge boards, and overhangs can affect where a measurement begins or ends.
Why Roof Pitch Matters
Roof pitch affects drainage, material selection, installation details, attic space, and the appearance of a home. A steeper roof generally sheds water more quickly than a low-slope roof, but performance also depends on roof design, flashing, underlayment, installation, and maintenance. Snow, wind, and rain conditions vary across the United States, so pitch alone does not determine whether a roof is suitable for a location.
Roofing products have manufacturer-specific installation requirements, including minimum slope limits. Some products require additional underlayment or other measures at lower slopes. Before choosing or replacing roofing, homeowners and contractors should check the product instructions and applicable local building code rather than assume one pitch works for every material.
Building codes and permitting rules are adopted and enforced by local jurisdictions, and requirements can differ. The International Residential Code is a widely used model code, but a locality may adopt a particular edition with amendments. The local building department and the current product documentation are the appropriate sources for project-specific requirements.
Quick Roof Pitch Reference
| Pitch | Approximate Angle | Percent Slope |
|---|---|---|
| 2:12 | 9.5° | 16.7% |
| 3:12 | 14.0° | 25% |
| 4:12 | 18.4° | 33.3% |
| 6:12 | 26.6° | 50% |
| 8:12 | 33.7° | 66.7% |
| 12:12 | 45° | 100% |
Values are approximate. The angle is calculated as arctan(rise ÷ run), and percentage slope is (rise ÷ run) × 100.
Reliable Sources and Further Reading
For roof-specific product limits, manufacturer installation instructions should take priority over general rules of thumb. For code questions, check the locally adopted building code and contact the local building department. The International Code Council’s International Residential Code resources provide information about the model residential code, while the Occupational Safety and Health Administration’s fall-protection guidance explains important precautions for work at height.