Roof pitch and roof slope describe how steep a roof is, but they are not expressed in exactly the same way. Roof pitch is commonly written as a ratio such as 6:12, while roof slope is often expressed as a ratio, decimal, percentage, or angle.
For most roofing calculations, both terms are based on the same basic measurements: vertical rise and horizontal run.
If you know the rise and run of a roof, you can calculate its pitch or slope and then use that information when working out roof area, rafter length, drainage and roofing material requirements.
Roof Pitch vs Roof Slope at a Glance
| Term | Common expression | What it describes |
|---|---|---|
| Roof pitch | 6:12, 4:12, 8:12 | The vertical rise compared with a standard 12-unit horizontal run. |
| Roof slope | 0.50, 50%, 1:2 | The rise compared with the horizontal run, often shown as a ratio or percentage. |
| Roof angle | 26.57 degrees | The angle formed between the roof surface and the horizontal plane. |
The important point is that these measurements are closely related. A roof with a 6:12 pitch has a slope of 0.5, or 50%, and an angle of about 26.57 degrees.
What Is Roof Pitch?
Roof pitch describes the rise of a roof over a standard horizontal distance. In residential roofing, pitch is commonly written as a ratio using 12 as the second number.
For example, a 6:12 roof pitch rises 6 units vertically for every 12 units of horizontal run.
When the result is converted to the common roofing format, the ratio is scaled to a 12-unit run.
Example: A roof rises 3 feet over a horizontal run of 6 feet.
3 / 6 = 0.5
0.5 x 12 = 6
The roof pitch is therefore 6:12.
What Is Roof Slope?
Roof slope compares the vertical rise with the horizontal run without necessarily converting the result to a 12-unit roofing ratio.
The same roof that has a 6:12 pitch has a slope of 0.5.
That value can also be expressed as a percentage:
For a roof with a rise of 3 feet and a run of 6 feet:
3 / 6 = 0.5
0.5 x 100 = 50%
So a 6:12 roof pitch corresponds to a 50% slope.
How Roof Pitch, Slope and Angle Relate
Pitch, slope and angle describe the same roof geometry from different measurement conventions.
| Roof pitch | Slope | Slope percentage | Approximate angle |
|---|---|---|---|
| 2:12 | 0.167 | 16.7% | 9.46 degrees |
| 3:12 | 0.250 | 25.0% | 14.04 degrees |
| 4:12 | 0.333 | 33.3% | 18.43 degrees |
| 5:12 | 0.417 | 41.7% | 22.62 degrees |
| 6:12 | 0.500 | 50.0% | 26.57 degrees |
| 8:12 | 0.667 | 66.7% | 33.69 degrees |
| 10:12 | 0.833 | 83.3% | 39.81 degrees |
| 12:12 | 1.000 | 100.0% | 45.00 degrees |
How to Calculate Roof Pitch From Rise and Run
To calculate roof pitch, first measure the vertical rise and horizontal run. The measurements must use the same unit.
Step 1: Measure the rise
The rise is the vertical distance from the top of the roof slope to the horizontal reference point.
Step 2: Measure the run
The run is the horizontal distance covered by the roof section. For a symmetrical gable roof, the run is usually half of the building's horizontal span, before considering any other roof geometry.
Step 3: Divide rise by run
Divide the rise by the run to obtain the slope as a decimal.
Step 4: Convert to the format you need
Multiply the decimal slope by 12 to express it as a common roofing pitch ratio.
Example: Suppose a roof has a 2.5 m rise and a 5 m run.
2.5 / 5 = 0.5
0.5 x 12 = 6
The roof has a 6:12 pitch.
The same roof has a 0.5 slope or 50% slope.
For a faster calculation, use the Roof Pitch Calculator.
How to Calculate Roof Slope as a Percentage
Percentage slope is useful when a project specification, drainage calculation or construction drawing gives the slope as a percentage rather than a pitch ratio.
A roof rises 1.8 metres over a 3.6 metre run.
1.8 / 3.6 = 0.5
0.5 x 100 = 50%
The roof slope is 50%, which corresponds to a 6:12 pitch.
You can also calculate slope from known horizontal and vertical dimensions with the Slope Calculator.
How to Calculate Roof Angle
Roof angle is different from pitch and slope because it is measured in degrees from the horizontal.
The angle can be calculated from the rise and run using the inverse tangent function.
For a roof with a rise of 3 feet and a run of 6 feet:
3 / 6 = 0.5
arctangent(0.5) = approximately 26.57 degrees
This means a 6:12 roof has an angle of approximately 26.57 degrees.
Why Roof Pitch Matters When Calculating Roof Area
Roof pitch is important because the actual surface of a sloped roof is larger than its horizontal footprint.
A roof area calculation that ignores slope can underestimate the amount of roofing material needed.
For a simple sloped roof section, the sloping length can be found using the rise and run. That sloping length can then be used with the roof length to calculate the actual surface area.
For example, if the rise is 2 m and the run is 3 m:
Sloping length = square root of (2 x 2 + 3 x 3)
Sloping length = square root of 13
Sloping length = approximately 3.606 m
If that roof plane is 10 m long, its approximate surface area is:
A two-plane gable roof with identical sides would have approximately 72.12 square meters of sloped surface before accounting for overhangs, valleys, hips or other roof details.
For a broader explanation, see How to Calculate Roof Area.
Roof Pitch and Roofing Materials
Roof pitch can affect more than the geometric area of a roof. It can also influence how roofing materials are measured, installed and estimated.
A steeper roof has a longer sloping surface for the same horizontal footprint. Roofing products may also have minimum or recommended slope requirements, depending on the material and installation method.
Important: A pitch calculation tells you the roof geometry. It does not determine whether a particular roofing material is suitable for that roof. Always follow the manufacturer's installation requirements and applicable building requirements.
Roof Pitch vs Roof Slope: Which Term Should You Use?
Both terms are useful, but the preferred format depends on the task.
| Situation | Useful measurement |
|---|---|
| Discussing residential roof steepness | Roof pitch, such as 6:12 |
| Working with mathematical calculations | Slope as a decimal or ratio |
| Drainage or engineering calculations | Slope percentage or ratio |
| Working with angles | Degrees |
| Calculating sloping roof surface area | Rise, run and sloping length |
Common Roof Pitch and Slope Mistakes
Using the full building width as the run
For a symmetrical gable roof, the horizontal run of one roof plane is commonly half of the overall span. Using the full span can produce an incorrect pitch calculation.
Mixing measurement units
Do not divide a rise measured in metres by a run measured in feet without converting them first. Both measurements need to use the same unit.
Confusing slope percentage with degrees
A 50% slope is not a 50-degree roof angle. A 50% slope corresponds to a roof angle of about 26.57 degrees.
Assuming pitch gives the complete roof area
Pitch describes steepness. Roof area also requires the roof's dimensions and geometry. Hips, valleys, dormers, overhangs and multiple roof sections can change the final area.
Ignoring the roof design
Simple formulas work well for basic roof planes. More complicated roofs should be divided into measurable sections before the areas are added together.
Roof Pitch, Roof Slope and Roof Angle Conversion
The following relationships are useful when converting between common formats:
Pitch to slope: divide the pitch rise by 12.
Slope to percentage: multiply the decimal slope by 100.
Slope to pitch: multiply the decimal slope by 12.
Slope to angle: use the inverse tangent of the slope.
Quick example: 8:12 pitch means 8 / 12 = 0.667 slope. That is about 66.7% slope and approximately 33.69 degrees.
Roof Pitch and Slope Calculators
You can calculate these measurements manually, but a calculator is useful when you need several conversions or want to reduce arithmetic mistakes.
- Roof Pitch Calculator for rise, run, pitch and roof angle calculations.
- Slope Calculator for slope, percentage and angle calculations.
- Roofing Calculator for roofing-related material estimates.
- How to Calculate Roof Area for roof surface area calculations.
Frequently Asked Questions
Is roof pitch the same as roof slope?
They are closely related, but the terms are commonly expressed differently. Roof pitch is often written as a ratio such as 6:12, while roof slope may be written as a ratio, decimal or percentage.
What is the slope of a 6:12 roof?
A 6:12 roof has a slope of 0.5, or 50%. Its roof angle is approximately 26.57 degrees.
What is the angle of a 4:12 roof?
A 4:12 roof has a slope of about 0.333 and an angle of approximately 18.43 degrees.
How do I convert roof pitch to percentage?
Divide the first number of the pitch by the second number and multiply the result by 100. For example, 6:12 gives 6 / 12 x 100 = 50%.
Does a steeper roof have a larger roof area?
For the same horizontal footprint, increasing the roof slope increases the length of the sloping surface. This can increase the actual roof surface area.
Can roof pitch be used to calculate roofing materials?
Yes. Roof pitch can help determine the sloping surface area, which is useful when estimating roofing materials. The final estimate also needs the roof dimensions, design details and an appropriate allowance for cutting and waste.
What is the difference between roof slope and roof angle?
Roof slope is commonly expressed as rise divided by run, while roof angle is expressed in degrees. A 6:12 roof has a 50% slope and an angle of about 26.57 degrees.
Related BuildersMeasures Tools
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