Edit the pitch or the outer wall — the other one updates to match.
Heights are to the top of the wall framing; the finished roof sits a little higher once rafters and sheathing stack on the plate. Confirm minimums with your local building department and the roofing manufacturer.
What This Calculator Does
This lean-to roof pitch calculator turns the one trade-off every attached single-slope roof comes down to into a live answer: set the height where the roof meets the house and the span out to the front, then flip between the roof pitch and the outer wall height. It returns the angle, slope percent, the total height difference from high side to low side, and the rafter length.
A lean-to (also called a shed or mono-pitch roof) is one slope leaning against a taller wall. Pick the pitch and the outer wall height falls out of it; pick the outer wall height and the pitch falls out instead. For a standard gable roof use the main roof pitch calculator; for rafter cuts alone use the rafter length calculator. Roof Pitch Calculator · Rafter Length Calculator
How to Use It
- Enter the attached (high) wall height and the span — the horizontal run from the house out to the front wall.
- Type a target pitch, or type the outer wall height you want. The other value updates to match.
- Read the angle, slope, height difference, and rafter length, then check the drainage verdict and the roofing your slope allows.
Minimum Pitch for a Lean-to Roof by Material
The flattest slope each covering can sensibly take on an attached single-slope roof. A 2:12 pitch drops 2 inches for every foot the roof runs out from the wall.
| Material | Minimum Pitch | Percent | Degrees | Why |
|---|---|---|---|---|
| EPDM / TPO membrane | 0.25:12 | 2.08% | 1.19° | Single-ply membrane; the lowest slope that still drains. Best for near-flat additions. |
| Mineral-surface roll roofing | 1:12 | 8.33% | 4.76° | Budget roll goods work from 1:12 — a common cheap lean-to choice. |
| Standing-seam metal | 1:12 | 8.33% | 4.76° | Locked seams ride above the water line; about the lowest a DIY lean-to should go. |
| Corrugated / exposed-fastener metal | 2:12 | 16.67% | 9.46° | Screw-down panels need more slope so water clears the fastener washers. |
| Asphalt shingles | 2:12 | 16.67% | 9.46° | 2:12 is the absolute floor with doubled underlayment; 4:12 is the trouble-free minimum. |
| Wood shakes / shingles | 3:12 | 25.00% | 14.04° | Wood needs slope to dry out between rains. |
| Slate / clay tile | 4:12 | 33.33% | 18.43° | Heavy lapped materials need 4:12+ to stay watertight. |
1:12 is the practical floor for any lean-to you want to keep dry. Below it, only a single-ply membrane drains, and even then any low spot or sag holds water.
These blend the model IRC R905 code minimums with DIY shed-building practice, which runs a little more conservative than bare code. Always confirm against your local code.
Good Lean-to Pitch for Snow and Rain
A lean-to has a single slope, so the pitch you pick decides how fast water and snow clear it. Match the slope to your climate.
Rain-only climates
1:12 to 2:12 is fine for sheds, carports, and shade structures where snow never piles up. Keep the covering to metal or membrane at the low end.
Snow country
Start at 4:12. A single slope this steep sheds most snow on its own and gives meltwater a fast, straight path off the roof.
Heavy snow load
6:12 or steeper sheds snow most reliably and cuts the load that builds on the rafters. The steeper the lean-to, the less snow sits long enough to ice up.
Why a shallow lean-to struggles with snow
With only one slope, snow that does not slide has nowhere to go. Shallow pitches let it sit, melt against the warmer roof, and refreeze at the cold eave — the classic ice-dam setup. More pitch keeps snow moving and water draining, which is why snow-country lean-tos start steeper than rain-country ones.
How Lean-to Pitch and Wall Height Connect
A lean-to is one right triangle. Fix any two of the three — attached height, span, and pitch — and the rest follow.
Pitch from the two walls
pitch = (attached − outer) × 12 ÷ span
Outer wall from a pitch
outer = attached − (pitch × span ÷ 12)
Height difference
rise (in) = pitch × span (ft)
Rafter length
rafter = √(rise² + run²)
Worked example: a 10 ft wall over a 12 ft span
Take the calculator's starting numbers: the lean-to attaches to the house at 10 ft and runs 12 ft out to the front wall. At a 4:12 pitch the roof drops 4 × 12 = 48 inches, so the outer wall stands at 10 ft − 4 ft = 6 ft. Flatten it to 2:12 and the drop is only 24 inches, so the outer wall rises to 8 ft. That trade — more pitch for a shorter outer wall — is the whole job of this calculator.
One thing to measure for real
These heights are to the top of the wall framing. The finished roof sits a little higher because the rafters and sheathing stack on the plate, and a birdsmouth cut drops it back a touch. Frame to your real plate heights and treat the calculator as your design target.
When to Use This Calculator
Use it for
Planning an attached lean-to, shed-roof addition, carport, or patio cover — sizing the outer wall, picking a pitch that drains, and checking which roofing your slope allows before you frame.
Sources
Minimum slopes: 2015/2021 IRC R905 (via InterNACHI's roof slope guide). Lean-to spans, snow practice, and the 1:12 cautionary notes draw on builder and DIY roofing community guidance. Figures checked 2026-06-12.
Lean-to Roof Pitch FAQ
The questions DIY builders and homeowners actually ask before framing an attached single-slope roof.