Roof Slope Calculator
Reviewed by the My Roof Pitch editorial team · Updated · Free, no sign-up, works offline after first load
Roof slope is rise divided by run, stated as a percentage. A roof rising 6 units over 12 units of run has a 50% slope, equal to 26.57° or a 6:12 pitch. Multiply rise ÷ run by 100 for percentage, or take the arctangent for degrees.
Your measurements
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Height gained from eave to ridge on one roof plane.
Half the building span for a symmetrical gable, measured on plan.
Analysis
Live results from your measurements.
Adequate slope for shedding water with conventional roofing.
Show the working
- Ratio:
5 ÷ 12= 0.4167 - Percentage:
0.4167 × 100= 41.67% - Degrees:
arctan(0.4167)= 22.62° - Slope length:
√(5² + 12²)= 13
What this calculator does
Roof slope is the same measurement as roof pitch, expressed as a percentage instead of a ratio. It tells you how many units of height the roof gains per hundred units of horizontal distance. Civil engineers, drainage designers and most non-US builders prefer percentage or degrees because they scale cleanly and drop the fixed 12-unit convention. A 2% slope is the usual minimum fall for a flat roof; 8.33% is the 1:12 boundary where membrane roofing gives way to shingle and panel systems.
How to use it
- Measure the vertical rise of one roof plane from the wall plate to the ridge.
- Measure the horizontal run that rise happens over, on plan, not along the roof surface.
- Divide the rise by the run.
- Multiply the result by 100 to state the slope as a percentage.
- Take the arctangent of the same ratio if you need the angle in degrees.
The formula
Slope percentage
slope % = (rise / run) × 100
The direct grade expression used in drainage and civil work.
Slope angle
angle = arctan(rise / run)
Convert the radian result to degrees by multiplying by 180 ÷ π.
Slope length
slope length = sqrt(rise^2 + run^2)
The true distance along the roof surface — the rafter line.
Worked example
A garage roof rises 2 ft over a 24 ft run.
- Ratio: 2 ÷ 24 = 0.0833
- Percentage: 0.0833 × 100 = 8.33%
- Angle: arctan(0.0833) = 4.76°
- Pitch: 0.0833 × 12 = 1:12
The garage has an 8.33% slope — a 1:12 pitch at 4.76°, which is membrane roofing territory.
Reference chart
| Slope (%) | Pitch (rise:12) | Angle (degrees) |
|---|---|---|
| 1 | 0.12:12 | 0.57 |
| 2 | 0.24:12 | 1.15 |
| 4 | 0.48:12 | 2.29 |
| 8.33 | 1:12 | 4.76 |
| 16.67 | 2:12 | 9.46 |
| 25 | 3:12 | 14.04 |
| 33.33 | 4:12 | 18.43 |
| 50 | 6:12 | 26.57 |
| 66.67 | 8:12 | 33.69 |
| 75 | 9:12 | 36.87 |
| 100 | 12:12 | 45 |
Minimum slope by roof covering type
| Covering | Minimum slope (%) | Equivalent pitch |
|---|---|---|
| Fully adhered single-ply membrane | 2.08 | 0.25:12 |
| Modified bitumen | 2.08 | 0.25:12 |
| Standing seam metal | 2.08 | 0.25:12 |
| Clay or concrete tile | 20.8 | 2.5:12 |
| Asphalt shingles (double underlayment) | 16.7 | 2:12 |
| Asphalt shingles (standard) | 33.3 | 4:12 |
Common mistakes
- Measuring run along the roof surface instead of horizontally. That gives the slope length, not the run, and understates the slope.
- Confusing a 1:12 pitch with a 1% slope. 1:12 is 8.33%.
- Applying a single slope figure to a roof with a change in plane, such as a gambrel.
- Designing a flat roof at exactly the code minimum with no allowance for deflection or ponding.
- Treating gutter fall and roof slope as the same specification — gutters run at roughly 0.4%, far shallower.
Assumptions and limits
- Rise and run are taken on a single, straight roof plane.
- Run is a true horizontal distance, measured on plan.
- Minimum slope figures reflect the 2021 IRC; local amendments and manufacturer warranties may be stricter.
Frequently asked questions
What is a 2% roof slope in pitch?
A 2% slope is roughly a 0.24:12 pitch, about 1.15°. It is the practical minimum fall used on flat membrane roofs so water drains rather than ponds.
Is slope percentage the same as pitch?
They describe the same steepness in different units. Divide the percentage by 100 and multiply by 12 to get the X:12 pitch. A 50% slope is a 6:12 pitch.
What is the minimum slope for a flat roof?
The IRC requires at least 1/4 inch per foot, which is a 2.08% slope. Many designers specify 1/2 inch per foot so the roof still drains after the structure deflects under load.
How do I convert slope percentage to degrees?
Divide the percentage by 100, then take the arctangent. A 33.33% slope is arctan(0.3333) = 18.43 degrees, the same as a 4:12 pitch.
Why does slope matter for material choice?
Water moves slower on shallow roofs and can be driven back under laps by wind. Every covering has a tested minimum slope below which the manufacturer will not warrant it against leaks.
Can a roof be too steep?
Structurally no, but above about 60% slope shingle installation needs extra fasteners and adhesive, and above 45° most manufacturers require hand-sealing of every tab.
Related guides
- How to Calculate Roof Pitch: 6 Methods That Actually WorkSix field-tested ways to find the pitch of a roof — from the ladder, from the attic, from a photo — plus the arithmetic that turns any measurement into X:12, degrees and percent.
- Roof Pitch to Degrees: Full Conversion Chart and FormulaThe complete pitch-to-degree conversion table, the trigonometry behind it, and why 6:12 is 26.57° rather than the 30° people expect.
Last updated 2026-08-05. Results are estimates for planning and are not a substitute for a licensed engineer or local code review.