Shed and Lean-To Roof Pitch: How to Choose and Calculate It

Why shed roof geometry is different

A gable roof has two planes meeting at a ridge, so each plane spans half the building and the run is half the span. A shed roof — also called a mono-pitch, skillion or, when attached to another structure, a lean-to — is one plane running from the high wall to the low wall. Its run is the entire depth of the building. That single difference invalidates most gable rules of thumb, and it is why people who apply half-span logic to a shed end up with a roof twice as steep as intended.

Because the run is large, small pitches produce large height differences. A 12-foot deep shed at 4:12 needs 4 feet of height difference between the two walls — often more than a stock shed kit allows. This is the practical reason most sheds are built at 2:12 or 3:12 rather than the residential norm of 6:12.

Working from wall heights

In the field you rarely start with a pitch. You start with two wall heights that were set by headroom, a door, or the point where the roof has to tuck under a house's siding. The pitch falls out of them.

Worked example: a garden office 10 feet deep with a 9-foot front wall and a 7-foot 6-inch rear wall. Rise = 9 − 7.5 = 1.5 feet = 18 inches. Pitch = (18 ÷ 120) × 12 = 1.8:12, which is 8.53 degrees. That is below the 2:12 shingle minimum, so either the front wall goes up or the covering changes. Raising the front wall to 9 feet 3 inches gives a rise of 21 inches and a pitch of 2.1:12 — just legal for shingles with double underlayment.

Reverse example: you want a 3:12 pitch on a 14-foot deep carport with an 8-foot low side. Rise = (3 ÷ 12) × 14 = 3.5 feet. The high wall must be 11 feet 6 inches. Check that against any planning height limit and against the fascia line of the house if the carport is attached.

Choosing a pitch for a shed

1. Set the covering first. Asphalt shingles push you to 3:12 or more; corrugated metal and polycarbonate work from 3:12; EPDM works from 1/4:12.
2. Check headroom at the low wall. A 6-foot 6-inch low eave is the minimum comfortable working height for a workshop.
3. Check snow. In snow country every extra step helps, and 4:12 or more keeps a shed roof from building a deep load on a lightly framed structure.
4. Check the neighbouring structure. A lean-to must terminate under a flashing, below a window sill and above the ground by enough to keep the wall dry.
5. Check the fall direction. Point the low side away from doorways and paths; a shed roof discharges its entire volume of water along one edge.

Lean-to roofs attached to a house

An attached lean-to has one constraint the free-standing shed does not: the high edge must be flashed into an existing wall. That flashing detail dictates the maximum height, and therefore the maximum pitch for a given depth.

• The ledger board must land on solid structure — a rim joist or studs — and be flashed with a continuous head flashing that tucks behind the siding or into a reglet in masonry.
• Keep the top of the roof covering at least 4 inches below any window sill or door threshold above it.
• On a house with lap siding, plan the ledger height so the flashing lands at a lap line rather than mid-board.
• The rafters bear on the ledger at one end and the low wall plate at the other, so both need a bearing cut at the roof angle — a plumb cut at the ledger and a birdsmouth at the plate.
• Water shed by the main roof lands on the lean-to. Size the lean-to gutter for the combined catchment area, not just its own.

Worked example: a lean-to 8 feet deep at 3:12 needs a 24-inch rise. If the low eave sits at 8 feet, the ledger lands at 10 feet, plus the rafter depth and any furring — call it 10 feet 8 inches to the top of the flashing. Confirm no window on that wall starts below 11 feet before ordering material.

Rafters, area and material for a single slope

Every quantity on a shed roof derives from the same triangle. For a 12 × 16 foot shed, 12 feet deep, at 3:12 with 12-inch overhangs on all four sides:

1. Rise over the 12-foot depth = (3 ÷ 12) × 12 = 3 feet.
2. Rafter line = √(3² + 12²) = √153 = 12.37 feet, plus 1 foot of overhang at each end measured along the slope: order 14-foot stock.
3. Roof footprint including overhangs = 14 × 18 = 252 sq ft.
4. Multiplier at 3:12 = 1.0308, so roof area = 252 × 1.0308 = 260 sq ft = 2.6 squares.
5. With 10 percent waste, order 3 squares — 9 bundles of architectural shingle.
6. Rafters at 16-inch centres over an 18-foot length: (18 × 12 ÷ 16) + 1 = 15 rafters.
7. Sheathing = 260 ÷ 32 = 8.1, order 9 sheets of 4 × 8.

One detail specific to single-slope roofs: because all the water leaves along one edge, the gutter and downpipe on that edge carry the whole roof. For 260 square feet in a 4 inch-per-hour rainfall region, a 5-inch K-style gutter with one downpipe is ample, but a 30 × 20 foot skillion on a workshop can easily need two.

Drainage, snow and ventilation on a single slope

A shed roof concentrates everything on one edge. All of the rainfall, all of the shed snow and all of the ice that forms at the eave arrives along a single line, which is why single-slope roofs fail at the low eave far more often than gables fail anywhere.

• Size the gutter for the whole roof, not half of it. A 200 square foot shed roof in a 4 inch-per-hour region sheds about 500 gallons an hour in a downpour, which a single 5-inch K-style gutter handles only with a properly sized downpipe.
• Extend the downpipe discharge away from the structure. Water dumped at the base of a shed wall undermines the slab or rots the bottom plate within a few seasons.
• In snow country, treat the low eave as a shedding zone. Do not put a door, a path or a window under it, or fit snow guards if you must.
• Ventilate high and low. A shed roof has no ridge, so intake goes at the low eave and exhaust at the high eave, with a continuous free path between the rafters — which means baffles if the roof is insulated.
• Insulated single-slope roofs are prone to condensation because the air path is short and steep. Either keep a vented 2-inch gap above the insulation or build it as an unvented assembly with rigid foam above the deck.

Pitch helps with all of it. Going from 2:12 to 4:12 nearly doubles the drainage velocity, halves the dwell time at every lap, and makes light snow slide rather than sit. If the height budget allows the extra rise, the extra pitch is almost always worth the small increase in material.

Frequently asked questions