Calculate a lean-to roof pitch from two wall heights, or enter a target pitch to find the outer wall height. The diagram updates with the span, drop, angle, and roof-line length.
Lean-to planning
What do you want to find?
Find outer wall height selected.
Imperial results round to the nearest 1/8 in. Metric results round to the nearest millimetre.
Roof-line length excludes overhangs, birdsmouth, and end cuts. It is not a finished rafter cut length.
Planning geometry only. This calculator does not size framing or connections, check loads, approve roofing products, or replace local code and professional review.
Both modes describe the same lean-to triangle. Pick the mode that treats your three known measurements as inputs.
Choose this mode when you know the high wall height, low wall height, and horizontal span. The calculator subtracts the wall heights to find the vertical drop, then reports the roof pitch and its equivalent angle and slope percent.
Choose this mode when you know the high wall height, horizontal span, and target pitch. The calculator finds the vertical drop across that span and subtracts it from the high wall to give the required outer wall height.
Pitch, angle, and slope percent describe one slope in different formats. Vertical drop, outer wall height, and roof-line length are dimensions of your current example.
With the high-wall point and span fixed, a steeper pitch creates more vertical drop and lowers the outer wall. A shallower pitch preserves more outer-wall height and headroom. There is no universal ideal pitch: the usable choice also depends on the roofing assembly, drainage details, loads, local code, and the clearance your project needs.
Measure the high wall at the point where the roof line meets the existing building. Measure the low wall at the outer roof edge. The span is the horizontal distance between those two wall reference points, not the slope along the roof.
Use the same top-of-wall reference for both heights. Do not measure one wall to its framing plate and the other to finished roofing. If the walls sit on different foundations or plates, include that level difference in the heights you enter.
Measure span horizontally from the high-wall reference to the low-wall reference. A tape laid along the roof follows the longer slope line and will produce the wrong pitch if you enter it as span.
The calculator turns the two wall heights and horizontal span into a right triangle. It does not include wall thickness, roof layers, flashing, overhangs, or framing cuts.
Small field errors carry into the answer. In Find roof pitch, incorrect wall heights or span change the calculated slope. In Find outer wall height, an incorrect span changes the drop, outer wall height, and roof-line length. Recheck measurements before layout.
For an existing roof where the wall heights are not available, use the roof pitch measurement guide.
These right-triangle equations show both calculation directions.
pitch = (attached − outer) × 12 ÷ span
outer = attached − (pitch × span ÷ 12)
Suppose the high wall is 10 ft, the low wall is 7 ft, and the horizontal span is 12 ft. The wall-height difference is 3 ft. Divide 3 by 12, then multiply by 12 to get a 3:12 pitch. The same slope is 14.04 degrees or 25 percent.
Start with a 10 ft high wall, a 12 ft horizontal span, and a 4:12 pitch. The roof drops 4 ft across the span, so the outer wall reference is 6 ft high. The straight roof line between those points is about 12.65 ft before any overhang or framing cut is added.
Most wrong answers come from describing a different triangle than the one shown in the profile.
Enter the higher attachment wall as the high wall and the lower outside wall as the low wall. If the outside wall is higher, this calculator's downward lean-to direction does not match the project.
Use the level horizontal distance between walls. If you measured along the roof, take a horizontal measurement instead before calculating.
Measure both heights to matching structural points. Correct for different slab, foundation, or plate levels before entering the values.
Use one measurement system for all three dimensions. In Imperial mode, put whole feet and remaining inches in their separate fields. Switch to Metric before entering metres.
Use it only as the triangle's sloping side. A finished rafter may need overhang, bearing, birdsmouth, end cuts, and connection details that this page does not calculate.
Use this result to plan geometry. Check the construction decisions separately before ordering materials or framing.
A roofing assembly's minimum slope depends on its exact product, laps, seams, underlayment, and installation instructions. The International Residential Code lists requirements by assembly. Confirm the product instructions and the local code that applies to your project.
Pitch alone cannot predict snow movement or structural safety. Snow, wind, drifting, roof loads, framing, and connections all matter. FEMA and ASCE provide load guidance, but a project still needs a local code check and professional review when required.
Roof-line length is not a finished rafter cut length. For cuts and layout, use the rafter length calculator.
Sources checked: July 23, 2026.
Four quick checks for the geometry this calculator can and cannot answer.