Lean-To Roof Calculator
Reviewed by the My Roof Pitch editorial team · Updated · Free, no sign-up, works offline after first load
A lean-to roof's pitch comes from the rise (high wall minus low wall) divided by the horizontal projection. For a 9 ft high wall, 7 ft low wall and 10 ft projection, the rise is 2 ft, giving roughly a 2.4:12 pitch, a 11.7° angle, and a rafter length near 10.2 ft before adding the overhang.
Key takeaways
- Pitch is the rise (high wall minus low wall) divided by the horizontal projection.
- Rafter length always includes the overhang, not just the projection.
- A lean-to roof has no ridge — every rafter is cut identically.
- Roof area is rafter length times building width.
Your measurements
Enter the values you know — results update live.
Height of the taller wall the rafters sit on, typically against the host structure.
Height of the shorter, outer wall supporting the low end of the rafters.
Horizontal distance (run) the roof extends from the high wall to the low wall.
Width of the structure along the walls, used to find total roof area.
Analysis
Live results from your measurements.
Show the working
- Rise:
9 − 7= 2 ft - Pitch:
(2 ÷ 10) × 12= 2.4:12 - Angle:
atan(2 ÷ 10)= 11.31° - Rafter length:
√(run² + rise-adjusted²) with overhang= 11.22 ft - Roof area:
11.22 × 12= 134.6 sq ft
What this calculator does
A lean-to roof, also called a skillion or shed roof, is a single sloped surface that runs from a tall wall down to a shorter wall, with no ridge. It is the simplest roof framing shape — common on porches, additions, sheds and modern single-slope homes — because every rafter is identical and the pitch is set entirely by the height difference between the two walls over the horizontal projection. This calculator converts a high wall, low wall and projection into the pitch, cut angle and actual rafter length you need to order lumber and set the saw.
How to use it
- Measure the high wall height where the rafters land against the host structure or ridge beam.
- Measure the low wall height at the outer edge of the addition or lean-to.
- Measure the horizontal projection — the flat distance between the two walls, not the sloped distance.
- Enter the building width to get total roof area.
- Add any eave overhang you want past the low wall.
- Read off the pitch, angle and rafter length, then order lumber a size longer to allow for trimming.
The formula
Pitch ratio
pitch (X:12) = (high wall − low wall) / projection × 12
Rise over run expressed per 12 inches of horizontal run, the standard US framing convention.
Rafter length
rafter length = √(run² + rise²), where run includes any overhang
Pythagorean theorem applied to the full run (projection plus overhang) and its proportional rise.
Worked example
High wall 9 ft, low wall 7 ft, projection 10 ft, width 12 ft, 12 in overhang.
- Rise: 9 − 7 = 2 ft
- Pitch: (2 ÷ 10) × 12 = 2.4:12
- Angle: atan(2 ÷ 10) = 11.31°
- Run with overhang: 10 + 1 = 11 ft
- Rafter length: √(11² + (2.2)²) ≈ 11.22 ft
- Roof area: 11.22 × 12 = 134.6 sq ft
This lean-to is roughly a 2.4:12 pitch with an 11.3° slope, needing about 11.2 ft rafters and covering about 135 sq ft.
Lean-to pitch by rise and a 10 ft projection
| Rise (high − low wall) | Pitch | Angle |
|---|---|---|
| 1 | 1.2:12 | 5.71° |
| 2 | 2.4:12 | 11.31° |
| 3 | 3.6:12 | 16.70° |
| 4 | 4.8:12 | 21.80° |
| 5 | 6.0:12 | 26.57° |
Common mistakes
- Measuring the sloped roof surface instead of the flat horizontal projection for the run.
- Forgetting the overhang when ordering rafter stock, leaving no material for the eave.
- Setting a pitch below the minimum for the chosen roofing material, risking water infiltration.
- Ignoring the birdsmouth cut depth, which effectively shortens usable rafter length at both ends.
- Not accounting for ledger board thickness at the high wall, which shifts the effective rise.
Assumptions and limits
- The roof is a single flat plane with no ridge, hips or valleys.
- Rafter length is measured along the top of the rafter before birdsmouth cuts.
- Both walls are plumb and the projection is measured level.
- Width is the full roof length along the wall, with rafters evenly spaced across it.
Frequently asked questions
What is a lean-to roof?
A lean-to (skillion or shed) roof is a single sloped surface spanning between a tall wall and a short wall, with no ridge, typically used for additions, porches and sheds.
What is the minimum pitch for a lean-to roof?
Most asphalt shingle lean-tos need at least a 2:12 to 4:12 pitch depending on underlayment; metal roofing can go as low as 1/4:12 to 3:12 depending on the panel system.
How do I find the rafter length for a lean-to?
Take the square root of the horizontal run squared plus the rise squared, where the run includes any eave overhang you plan to add.
Does the lean-to pitch depend on the building width?
No. Pitch depends only on the rise (height difference between the two walls) and the horizontal projection; width only affects total roof area.
Can a lean-to roof pitch be too steep?
Yes. A pitch much steeper than needed wastes headroom on the low side and adds unnecessary rafter length and material cost without a drainage benefit.
How much overhang should a lean-to roof have?
Most lean-tos use a 6 to 18 inch overhang on the low wall for weather protection; larger overhangs may need extra bracing or a fascia support.
Is a lean-to roof the same as a shed roof?
Yes, these terms are used interchangeably for a single-slope roof with no ridge.
How do I cut the rafters for a lean-to roof?
Cut both ends at the roof angle (plumb cuts) found from the pitch, and cut a birdsmouth notch where each rafter crosses the wall top plate.
Last updated 2026-10-08. Results are estimates for planning and are not a substitute for a licensed engineer or local code review.