Lean-To Roof Pitch Calculator
Adding a lean-to against a wall or building? Enter how far it projects and the heights at the wall and the outer edge — get the pitch (X/12), the angle, the rafter length, and whether your slope suits shingles or needs metal or membrane.
Projection is the horizontal distance from the wall to the outer posts.
Planning estimates only — verify against local building codes and roofing-manufacturer requirements before you build.
How the lean-to roof pitch calculator works
A lean-to is a single slope anchored to an existing wall: the run is how far the roof projects from the wall, and the rise is the drop from the ledger board to the outer beam. Divide rise by projection and scale to 12 inches of run for the standard X/12 pitch, then take the arctangent for the angle.
Pitch choice drives the roofing material. Lean-tos often end up shallow because the wall height is fixed — if your numbers land below 2/12, plan on standing-seam metal or a membrane roof rather than shingles, and flash the wall junction carefully since that joint sees the most water.
rise = wall − outer · pitch = rise/projection × 12 · angle = atan(rise/projection) · rafter = √(projection²+rise²)
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Lean-to roof pitch calculator FAQ
What pitch should a lean-to roof be?
1/12 to 4/12 is typical. The wall you attach to usually caps the high side, so many lean-tos land at 2/12–3/12 — workable for shingles with proper underlayment, or metal if shallower.
Can you put shingles on a lean-to roof?
Yes, at 2/12 or steeper with most manufacturers, though below 4/12 you should use ice-and-water shield instead of standard felt. Under 2/12, switch to standing-seam metal or membrane.
How do you calculate lean-to roof pitch?
Subtract the outer-edge height from the wall-attachment height to get the rise, divide by the projection from the wall, and multiply by 12. Example: 1 ft of drop over a 6 ft projection = (1 ÷ 6) × 12 = 2/12.
How much slope does a lean-to need for drainage?
At least 1/12 (about 4.8°) so water actually runs off; 2/12+ is safer in wind-driven rain. Dead-flat lean-tos pond water and leak at the wall flashing first.
How is a lean-to different from a shed roof?
A lean-to attaches to an existing wall (one side is already framed), while a freestanding shed roof spans two walls you build. The math is identical — only the high-side support differs.