Use the lean-to roof pitch calculator to find the slope from two wall heights, or set a pitch to get the outer wall height.
Lean-to planning
What do you want to find?
Find outer wall height selected.
Fraction precision
rounding for Imperial results
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Pitch, angle, and slope percent describe one slope in different formats. The three dimensions of your current example are vertical drop, outer wall height, and roof-line length.
Pitch
4:12
Angle
18.43°
Slope percent
33.33%
The same slope in three formats, using the 4:12 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. The material table below shows the minimum slopes to weigh before you pick a pitch.
Reference minimums for every material in the lean-to roof pitch calculator, with the code or manufacturer source for each. The manufacturer's installation instructions and your local code take precedence.
| Material | Minimum slope | Source |
|---|---|---|
| Metal panels | 3:12(14°) | ICC 2024 IRC R905.10.2 |
| Asphalt shingles | 2:12(9.5°) | ICC 2024 IRC R905.2.2 |
| Corrugated metal | 3:12(14°) | ICC 2024 IRC R905.10.2 |
| Polycarbonate panels | 1.5:12(7.1°) | Palram Suntuf installation guide |
| EPDM/TPO membrane | 1/4:12(1.2°) | ICC 2024 IRC R905.12.1 |
Reference values only. Confirm each product's installation instructions and the applicable local code before purchase.
Use the lean-to roof pitch calculator to plan one slope. Pick the mode that matches the three numbers you already know, and you get pitch, angle, slope percent, drop, and roof-line length in one pass.
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.
Use this modeChoose 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.
Use this modeChoose this mode when you know the low-wall clearance, horizontal span, and target pitch. The calculator adds the rise across the span to the clearance and returns the attached wall height that keeps it.
Use this modeA lean-to roof is one slope, so it is also called a shed roof, skillion roof, or mono-pitch roof. Measure three numbers and the calculator does the rest.
Measure at the point where the roof line meets the existing building.
Measure at the outer roof edge.
Measure horizontally between the two wall reference points, not along the roof.
A tape laid along the roof follows the longer slope line. Enter the level distance instead, or the calculated pitch will be wrong.
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, add that level difference to the heights you enter.
Wall thickness, roof layers, flashing, overhangs, and framing cuts.
In Find roof pitch, a wrong wall height or span changes the slope. An incorrect span in Find outer wall height changes the drop, outer wall height, and roof-line length. Recheck the measurements before layout.
For an existing roof where the wall heights are not available, use the roof pitch measurement guide.
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.
Once the numbers line up, check what the result can and cannot cover in Before you build.
Two right-triangle equations cover all three modes.
pitch = (high − low) × 12 ÷ span
low = high − (pitch × span ÷ 12)
high = low + (pitch × span ÷ 12)
Suppose the high wall is 10 ft, the low wall is 7 ft, and the span is 12 ft. The drop is 3 ft, which is a 3:12 pitch, or 14.04 degrees and 25 percent.
Start with a 10 ft high wall, a 12 ft span, and a 4:12 pitch. The roof drops 4 ft across the span, so the outer wall reference is 6 ft high.
For a 6 ft clearance at the low wall, a 12 ft span, and a 4:12 pitch, the roof rises 4 ft. The attached wall reference is 10 ft high.
These numbers give you the slope and the geometry, not the structure. Framing, loads, drainage, and approvals each need their own check.
The minimum slope depends on the exact product, laps, seams, underlayment, and installation instructions. ICC 2024 Chapter 9 lists requirements by assembly, and the manufacturer's instructions take precedence.
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.
The ledger that attaches the lean-to to the existing building must be flashed and anchored into the wall framing. Rafter sizes, bearing, and connection details follow the local code.
Water flows to the low side, so plan the gutter and downspout there. Keep at least the material's minimum slope so water moves instead of pooling.
Most areas require a permit for a roof addition. Check with the local building department before ordering materials.
Roof-line length is not a finished rafter cut length. For cuts and layout, use the rafter length calculator.
Sources checked: August 10, 2026. Last reviewed: August 10, 2026.
Straight answers from the lean-to roof pitch calculator on geometry, materials, and what still needs a permit or a professional.
Subtract the low wall height from the high wall height to get the vertical drop. Divide that drop by the horizontal span, then multiply by 12. A 10 ft high wall and 6 ft low wall over a 12 ft span give a 4 ft drop, which is a 4:12 pitch.
Subtract the vertical drop from the high wall height. For a 10 ft high wall, a 12 ft horizontal span, and a 4:12 pitch, the drop is 4 ft, so the outer wall height is 6 ft. This is planning geometry, not a framing plan.
It depends on the roofing material. Asphalt shingles need at least 2:12, metal panels 3:12 without sealed laps, polycarbonate panels about 1.5:12, and membrane roofs 1/4:12. Check the product installation instructions, because the manufacturer's requirement takes precedence.
The common method is a ledger board bolted to the existing wall, with the rafters resting on it. The ledger must be flashed and anchored into the wall framing, and the connection details follow the local code. This page covers geometry, not the connection design.
Often yes. Most areas require a permit for a roof addition because it changes structure and drainage and can affect zoning. Ask the local building department before you start, since requirements vary by area and project size.
You choose it. The clearance is the lowest usable height at the outer wall, for example headroom at a door or under a shelf. Enter it in the Find attached wall height mode, and the calculator returns the high wall height that keeps it.
The roof line is the hypotenuse of the lean-to triangle: span times the square root of 1 plus the pitch over 12 squared. A 12 ft span at 4:12 gives about 12.65 ft. That is the geometric line, not the finished rafter cut length.
Membrane roofs such as EPDM or TPO are designed for low slopes and can run at 1/4:12. Polycarbonate panels suit about 1.5:12 and above. Asphalt shingles need at least 2:12, and metal panels 3:12 unless the laps are sealed. The manufacturer's instructions and the local code decide the final answer.
More roof slope calculators and references for the rest of your lean-to or addition project.