Lean-to Roof Calculator

Lean-to Diagnostic Dashboard

Type your dimensions and the dashboard matches each input against five framing root causes — pitch too low, birdsmouth too deep, rafter undersized, slope too steep, overhang too long — then returns a quick fix for any problem it finds.

Lean-to vs Shed vs Skillion: Three Names, One Slope

A single-slope roof goes by three names depending on who built it. A lean-to is a single-pitch roof that attaches along its high side to an existing wall or ledger — picture a shed addition leaning against the house. A shed roof is the same single-slope geometry but freestanding — both walls are new, and there is no existing structure to tie into. A skillion is the Australian and New Zealand term for any single-slope roof, whether attached or standalone. The math is identical for all three: one rafter spans from a low bearing point to a high bearing point, with a plumb cut at the top and a birdsmouth seat at the bottom. What changes is the high-side connection — a lean-to bolts to a ledger on the existing wall, a shed roof nails to a tall stud wall, and a skillion does whichever the framing calls for.

The distinction matters for permits and flashing, not for rafter math. If you are framing a skillion lean to roof, use the same plumb cut (90° − pitch angle), seat cut (pitch angle), and birdsmouth layout you would use on a North American shed. The dashboard above does not care which name you use — enter the run and the rise or pitch, and it returns the same cut list.

Lean-to Rafter Length Calculation

The fundamental lean to rafters formula is the Pythagorean theorem applied to a right triangle: L = √(Run² + Rise²). Because Rise = Run × (pitch ÷ 12), you can also write it as L = Run ÷ cos(pitch angle), where the pitch angle is atan(X ÷ 12) × 180 ÷ π. The two expressions are mathematically identical — use whichever is more convenient.

The rafter length formula, worked

Take a 10'-0" horizontal run at 4/12 with a 12" overhang. Rise = 10 × (4 ÷ 12) = 3.333'. Line length = √(10² + 3.333²) = √111.11 = 10.541' ≈ 10' 6-1/2". Overhang along the rafter = 12" ÷ cos(18.43°) = 12 ÷ 0.9487 = 12.65" = 1' 0-5/8". Total board length = 10' 6-1/2" + 1' 0-5/8" = 11' 0-1/8". Cut two plumb cuts (one at the high wall, one at the tail) and one birdsmouth seat at the low wall.

Plumb cut and seat cut angles

The plumb cut at the high wall is the complement of the pitch angle: plumb cut = 90° − pitch angle. At 4/12 (18.43°), the plumb cut is 71.57° — that is the angle between the rafter's bottom edge and a vertical line. The seat cut at the low wall birdsmouth is the pitch angle itself: seat cut = pitch angle, so 18.43° at 4/12. Set your speed square or bevel gauge to these two angles and you can lay out every rafter on the lean-to without re-measuring.

Lean-to Roof Pitch and Slope

Pitch, slope, and angle are three notations for the same ratio. Pitch is X/12 — the rise in inches for every 12" of horizontal run. Slope is the same ratio as a percentage: (rise ÷ run) × 100. Angle is the arctangent of the ratio: atan(X ÷ 12) × 180 ÷ π. A 4/12 pitch is a 33.33% slope and an 18.43° angle. The lean to roof slope matters because it controls which roofing materials are permitted: below 2/12, only continuous membranes or standing-seam metal are allowed; 2/12 to 3/12 needs full-coverage peel-and-stick underlayment; above 3/12, standard asphalt shingles work.

Lean to roof slope in percent and degrees

Pitches below 2/12 cause roughly 90% of lean-to leak callbacks — water ponds on the flat plane, and every shingle lap becomes an entry point for wind-driven rain. A 3/12 lean-to (14.04°, 25% slope) is the practical minimum for asphalt shingles with a peel-and-stick underlayment. A 4/12 (18.43°, 33.33%) is the sweet spot for residential lean-tos — it sheds rain, accepts standard shingles, and stays walkable. In snow zones, design for at least 6/12 (26.57°, 50%) so the roof sheds snow instead of accumulating dead load.

Use it as a lean to roof pitch calculator and shed roof slope calculator

Toggle the input to Enter Rise and the dashboard becomes a lean to roof pitch calculator: type the height difference between your low wall and the existing high wall, and it solves pitch = (rise ÷ run) × 12. Because a shed roof and a lean-to share identical geometry, the same panel doubles as a shed roof slope calculator — enter the run and the rise (or pitch) and read slope in degrees, percent, and X/12 from one input. For a 6'-0" run with a 24" rise, both modes return 4.00/12, 18.43°, and 33.33% slope.

Rafter Size Selection

Sizing lean to rafters by span

Lean-to rafters span the full building width — there is no ridge board to halve the run the way a gable rafter does. That means a lean-to rafter on a 12' span resists roughly twice the bending moment of a gable rafter on the same 12' building. Size accordingly. The reference table below lists #2 Spruce-Pine-Fir spans at 16" o.c. under a 20 psf roof live load — for heavier snow loads or wider spacing, step up one size and verify against your local building code's rafter span table. For a 10'-0" run, a 2x6 is adequate; for a 12'-0" run, step to a 2x8; for a 16'-0" run, use a 2x10; over 18'-0", a 2x12 or an engineered I-joist is the safer call.

Connection Details to the Main Roof

Ledger board and bolt spacing

The high side of a lean-to bolts to a ledger board — a 2x6 or 2x8 nailed across the face of the existing wall studs at the rafter bearing height. Use 1/2" lag bolts or through-bolts every 16" o.c. into the studs (not just the sheathing), and slip self-sealing flashing tape behind the ledger before bolting it down so the bolt holes do not become leak paths. Set the ledger height so the rise from the low wall plate to the ledger top equals run × (pitch ÷ 12) — for a 10'-0" run at 4/12, that is 3'-4". Plumb-cut the top of each rafter against the ledger's face and toenail or use hurricane ties to tie the rafter to the ledger; never rely on toenails alone in wind zones. At the low wall, birdsmouth the rafter onto the doubled top plate and strap the rafter down with an RF-301-style rafter tie.

Waterproofing and Flashing

Kick-out flashing — the hidden leak point

The single most missed detail on a lean-to is the kick-out flashing at the bottom of the ledger-to-wall junction. Water runs down the lean-to roof, hits the wall, and without a kick-out it travels behind the siding and into the house. Install a prefabricated kick-out at the lower end of the step flashing so water is kicked out onto the roof plane and into the gutter. Run peel-and-stick membrane up the wall 4" minimum behind the ledger, lace the step flashing into every shingle course, and cap the top of the ledger with drip-edge. In ice-dam zones, run the membrane full-coverage from the eave to at least 24" inside the heated wall. These three details — kick-out, step flashing, and membrane — prevent the leaks that cause the majority of lean-to insurance claims.

Common Span & Load Reference Table

Educational reference only — #2 SPF, 16" o.c., 20 psf roof live load (no snow). These spans are the horizontal run, not the rafter length along the slope. Verify against your local building code for your species, spacing, and ground snow load before ordering lumber.

Skillion Lean-to Roof Framing Notes

A skillion lean to roof follows the same framing rules as a North American lean-to — the term is regional, not geometric. Skillion roofs in Australia and New Zealand commonly run 5/12 to 9/12 (22.62° to 36.87°), often with exposed rafters and a thinner roof plane (no cathedral ceiling cavity). When framing a skillion, the same birdsmouth HAP rule applies: notch no deeper than 1/3 of the rafter's actual depth. Because skillion roofs often expose the rafter tails as a design feature, the plumb cut at the tail must be clean and consistent — cut a pattern rafter first and use it as a template for the rest. The dashboard above and every formula on this page apply directly to skillion framing; only the connection detail changes if the skillion is freestanding (shed) rather than attached (lean-to).

Frequently Asked Questions

Planning estimates only — verify all spans, loads, and connections with your local building code and a licensed builder before construction.