A lean-to shed has a single-pitched roof attached to an existing wall — the back of a garage, the side of a house, or a fence line. The roof slopes away from the existing wall toward the open side.
The right slope is constrained by two things: the available headroom under the existing eave (you can't set the ledger above the existing roof), and the depth of the shed (the wider the shed, the steeper the slope drops the front wall). Most lean-tos work well at 2/12 to 4/12.
Geometric constraints
Lean-to slope is determined by ledger height and shed depth. If the ledger sits at 9 feet (just below an existing 9-foot eave) and the shed is 8 feet deep, a 4/12 pitch drops the front wall to 9 - (8 × 4/12) = 6.33 feet — workable but low for entry.
Wider shed depths force lower pitches if you want usable front-wall height. For a 12-foot-deep lean-to, a 3/12 pitch drops the front wall by 3 feet; a 6/12 pitch drops it by 6 feet.
Recommended slopes
For utility lean-tos (firewood storage, garden tools, bikes): 2/12 to 3/12. Low enough to keep front-wall height reasonable, steep enough for asphalt shingles with double underlayment.
For shop or workshop lean-tos with people working inside: 3/12 to 4/12. Better drainage and walkability, with a slightly higher front wall for window or door clearance.
For climates with significant snow: 4/12 minimum. Lean-to roofs are vulnerable to snow accumulation because they cannot shed both directions, so a steeper pitch helps.
Ledger detailing
The ledger is the horizontal lumber bolted to the existing wall that the rafters bear on. It must be flashed properly so water cannot run down the existing wall and behind the ledger — a common source of leaks.
Use a piece of step flashing tucked behind the existing siding, lapped over the ledger, and counter-flashed where the lean-to roofing meets the existing wall. Self-adhered membrane behind the ledger is a belt-and-suspenders detail in wet climates.
Need to run the numbers?Use the free roof pitch calculator on the home page to convert pitch to angle, calculate rafter length, or estimate roof area in any unit.