Two-Slope Comparison
| Results | Side A · left | Side B · right |
|---|---|---|
| Angle (°) | — | — |
| Slope (%) | — | — |
| US pitch (X/12) | — | — |
| Rafter length | — | — |
| Rise | — | — |
| Run | — | — |
| Complementary angle | — | — |
| Grade ratio (rise : run) | — | — |
| 1 in X (run per rise) | — | — |
Two-slope roof pitch calculator
RiseRunRafterAngle (°)Slope (%)US pitch (X/12)Vertical height.Horizontal distance, not full span.Sloping length, excluding overhang.From 0° to less than 90°.Rise per 100 horizontal units.Rise per 12 horizontal units; enter X only.Check the selected measurements and enter numeric values.Choose a supported calculator mode.Choose one of the listed pairs of known values.Use only the selected known values.Enter a number with a decimal point, such as 2.5. Commas and fractions are not supported.This value is outside the supported numeric range. Enter a smaller finite number.Enter zero or a positive number. Rise cannot be negative.Run must be greater than zero. A vertical slope has no horizontal run.Geometric rafter length must be greater than zero.Enter an angle from 0° up to (but not including) 90°.Choose a supported length unit.The geometric rafter cannot be shorter than the rise or run. Check both measurements.These measurements describe different slopes. Check both known values.These values are too extreme to represent reliably. Use less extreme measurements.Choose between 0 and 4 decimal places.RiseRunRafter lengthAngle (°)Slope (%)US pitch (X/12)Grade ratio (rise : run)Complementary angle1 in X (run per rise)PitchLengthsConvertFrom rise and run.Any two values — find the missing length.Degrees, percent and X/12, side by side.CalculatedProportions onlyAwaiting measurementsCheck your inputsDone. Lengths describe one triangle.Proportions only — no lengths yet. Add a length in Lengths to get dimensions.Enter both values, or try an example below.These values don’t determine a unique size. Pick a pair with a run or rafter length.Flat (no rise)1 in {value}Unavailable in {unit}Unknown (a length is required){field} unite.g. {value}Unit unchanged.Some source units stayed unchanged. Correct incomplete or invalid values before converting them.Results copied.Copy unavailable. Select and copy the visible results manually.CalcSlope — roof pitch & slopeUS pitch = rise per 12 horizontal run. Rafter excludes overhang; no structural sizing.Rise {rise}; run {run}; rafter {rafter}; angle {angleDeg}.Proportions only — dimensions unknown. Angle {angleDeg}; slope {slopePercent}; US pitch {pitchX12}.Enter values to draw your slope. Rise is vertical, run is horizontal, rafter is the sloping side.Proportions only · dimensions unknownTo scale · values on the labelsIllustration onlyAngle {value}{status}. Angle {angleDeg}, slope {slopePercent}, US pitch {pitchX12}.PinPinnedPin {field} valueUnpin {field} valueMeasurement pinned on this device.Measurement unpinned.Enter a valid measurement before pinning.Invalid measurement unpinned; its saved value was removed.Device storage is unavailable. Pins work on this page only; use browser site-data settings to remove any earlier saved values.Saved measurements cleared. Current inputs remain.This example would change a pinned measurement. Unpin it or choose another example.
Enter valid lengths to print a calculation report.
How to use the two-slope roof calculator
Treat the roof as two right triangles meeting at one ridge. Enter the rise and horizontal run for each side to compare their pitch and rafter lengths.
Measure each side separately
- For Side A, enter the vertical rise from its eave to the ridge and the horizontal run from that eave to the point directly below the ridge.
- Repeat for Side B. Use its own eave as the height reference. Do not assume half the building span unless the two horizontal runs are equal.
- Choose a unit for each measurement. Select the result unit and decimal precision, then read the two sets of angles, pitches and rafter lengths.
- Use Enlarge to inspect the roof at a shared scale, or Copy results to save both sides together. Pin measurements to keep them on this device; unpinned values reset when you reload or leave.
One ridge, two independent slopes
- Horizontal run
- The two runs add up to the horizontal eave-to-eave span. A ridge away from the centre produces unequal runs.
- Vertical rise
- Equal rises mean level eaves at the common ridge. Different rises mean different eave heights; the diagram preserves that difference.
- Rafter length
- The straight distance from each eave to the ridge. Each side has its own length, excluding overhangs and construction allowances.
Calculate A and B independently
For each side, use its own rise h and run d in the same unit. Never average the two pitches before calculating the rafters.
Slope (%) = 100 × h ÷ d
US pitch X/12: X = 12 × h ÷ d
θ = arctan(h ÷ d) × 180 ÷ π
l² = h² + d²
Example: a ridge away from the centre
Both sides rise 3 m. Side A runs 4 m and Side B runs 6 m, giving a 10 m horizontal span. The eaves are level, but the pitches and rafter lengths differ.
| Results | Side A · left | Side B · right |
|---|---|---|
| Rise | 3.00 m | 3.00 m |
| Run | 4.00 m | 6.00 m |
| Rafter length | 5.00 m | 6.71 m |
| Angle (°) | 36.87° | 26.57° |
| Slope (%) | 75.00% | 50.00% |
| US pitch (X/12) | 9.00/12 | 6.00/12 |
Two-slope roof questions
Can both sides have the same pitch?
Yes. Equal rise-to-run ratios produce the same pitch. Their rafter lengths can still differ if the triangles have different sizes.
Does the ridge have to be centred?
No. Enter each run independently. The diagram places the ridge according to those runs and draws both sides using one scale.
Does this calculate roof area or structural sizes?
No. These results describe a roof cross-section. Roof area needs additional dimensions along the building; structural sizing and material suitability are outside this calculator.