Free materials calculator Lean-To Roof Pitch Calculator
A lean-to roof attaches to an existing wall on its high side and slopes down to a lower outer wall - a single slope, like a shed. Enter the width and either a slope or the two wall heights to get the rafter length, the angle in degrees and the slope factor, with the minimum pitch your covering needs.
How this calculator works
A lean-to roof is a single slope (a mono-pitch) that attaches to an existing building on its high side and falls to a lower outer wall or row of posts. The rafter spans the full width from the house wall to the outer wall, so the run equals that width:
run = width, house wall to outer wall
rise = ledger height − outer wall height
slope ratio = rise ÷ run
rafter = sqrt(run² + rise²) = run × sqrt(1 + slope ratio²)
Pick Shed / lean-to (single slope) as the roof type, enter the width, and set the slope the way that suits you. If you already know how high the ledger on the house sits above the outer wall, choose Rise over the run and enter that height difference as the rise; otherwise enter a x:12 pitch or an angle directly.
Minimum lean-to roof pitch by covering
| Covering | Minimum pitch | Angle | Notes |
|---|
| EPDM / TPO / felt membrane | ~0.5 : 12 | ~2.4° | Best for shallow lean-tos under a window |
| Standing-seam metal | 1 : 12 - 2 : 12 | 4.8° - 9.5° | Low-slope panel option |
| Corrugated / ribbed metal | 3 : 12 | 14° | Common on carports and stores |
| Glass / polycarbonate (conservatory) | 2.5 : 12 | ~12° | Typical minimum to shed water and dirt |
Lean-tos are often kept shallow to fit beneath a first-floor window, which pushes you toward a membrane or low-slope metal rather than shingles.
Worked example
A 3 m (10 ft) deep lean-to where the house ledger sits 0.75 m above a 2.4 m outer wall, with a 0.3 m overhang:
- Run = full width = 3 m (house wall to outer wall).
- Rise = ledger − outer wall = 0.75 m, so slope ratio = 0.75 ÷ 3 = 0.25 = a 3:12 pitch.
- Angle: atan(0.25) = 14.0°; slope factor = √(1 + 0.25²) = 1.031.
- Rafter: 3 × 1.031 = 3.09 m, plus a 0.31 m overhang tail = 3.40 m to cut.
Tips
- Decide the ledger height from the pitch your covering needs and the headroom you want at the outer wall - then check it clears any window above.
- The run is the full width; a lean-to is not halved like a gable.
- Bolt the ledger into solid masonry or framing and flash the top junction properly against the house wall.
- For a freestanding version use the shed roof pitch calculator; for a two-slope roof or to measure an existing pitch, use the main roof pitch calculator.
