The simplest roof in all of construction is also one of the easiest to get wrong at exactly one spot — the line where it meets the house. A lean to roof is a single-slope (skillion) roof that leans against an existing structure: one high edge attached to the building via a ledger board, one low edge carried on posts or a wall, and straight rafters running between them. No ridge, no hips, no valleys. That simplicity makes it the default choice for porch covers, patio roofs, carports, side-yard storage, and firewood shelters — and it’s why a competent DIYer can frame one in a weekend. This guide covers designing the slope, sizing the frame, attaching it to the house correctly, and the flashing detail that decides whether it lasts thirty years or leaks in three.
Where a Lean-To Makes Sense
Because one side borrows the house’s structure, a lean-to costs 30 to 40 percent less than a freestanding gable of the same footprint and gains rigidity from the building it’s anchored to. Classic applications: covered patios and outdoor kitchens, carports along a garage wall, wood sheds and garden storage against a fence-side wall, and greenhouse leans glazed with polycarbonate. The same skillion geometry also works on freestanding sheds — a standalone building with one tall wall and one short wall — but that version has its own bracing and wind considerations; here we focus on the attached build, which is what most homeowners mean by lean-to.
One task before any lumber: check your frost line and permit requirements. Nearly every attached roof structure requires a permit, and inspectors care most about the two things this guide spends the most time on — ledger attachment and footings.
Designing the Slope
A lean-to’s pitch is set by two heights: where the ledger lands on the house and how tall the outer posts are. Practical rules:
- Minimum 1/4 inch per foot (about 2 percent) for membrane or properly seamed metal — but don’t design this shallow if you can avoid it.
- 3/12 is the comfortable floor for corrugated metal and polycarbonate panels with sealed laps; many panel manufacturers allow 2/12 with butyl tape at every lap, but 3/12 sheds debris and forgives imperfect installation.
- 4/12 or steeper if you want asphalt shingles to match the house.
Work the geometry backward from headroom. Example: a 12-foot-deep patio roof at 3/12 drops 36 inches across its run. If the ledger sits at 10 feet (just under the house’s second-floor windows), the low edge lands at 7 feet — enough for comfortable standing, with the beam a couple of inches higher than head height. Snow country changes the calculus: a shallow lean-to below a main roof catches everything that slides off the upper slope, so either steepen it, beef up the framing for drift loads, or add snow guards on the roof above.
Sizing Rafters, Beam, and Posts
For typical 30 to 40 psf snow-plus-dead loads, these conservative spans keep you inside most span tables — verify against your local snow load:
- 2×6 rafters at 16 inches on center: spans to about 9 feet
- 2×8 at 16 inches: to about 12 feet
- 2×10 at 16 inches: to about 15 feet
- Doubled 2×10 beam: posts at 8 feet apart or less; step to doubled 2×12 or a 4×12 for 10-foot post spacing
- Posts: 6×6 pressure-treated, on concrete footings below frost depth with metal post bases — never buried wood in dirt for a structure attached to your house
Use joist hangers or a birdsmouth-plus-hurricane-tie at the ledger, and Simpson H2.5 ties where rafters cross the beam. Uplift, not gravity, is what removes patio roofs in storms.
The Ledger: Do This Part Like a Pro
The ledger board transfers half the roof’s load into the house, and it must connect to framing — the rim joist, wall studs, or blocking — never to siding, brick veneer, or sheathing alone. The sequence that passes inspection: snap a level line, remove or cut back the siding to expose sheathing, apply self-adhering flashing membrane over the exposure, bolt a pressure-treated 2×8 or 2×10 ledger with 1/2-inch structural screws (LedgerLOK or Simpson SDWS) or through-bolts at 16 inches on center staggered top and bottom, hitting framing every time.
Then the detail that decides everything: metal step or continuous Z-flashing must lap behind the house’s weather barrier above the ledger and over the lean-to roofing below. Water running down the wall has to be handed off onto the new roof surface with no caulk-dependent joints. Screwing a ledger over vinyl siding and running a bead of sealant along the top is the signature move of a roof that leaks at year two and rots the rim joist by year ten.
Framing and Roofing Sequence
- Set footings and posts; brace plumb and cut to the calculated height.
- Install the beam atop the posts with post caps; crown up.
- Hang rafters from ledger to beam at 16 or 24 inches on center, with a 12 to 18 inch overhang past the beam for drip control.
- Add blocking between rafters over the beam, and fascia across the tails.
- Deck and roof: for metal or polycarbonate, run 2×4 purlins flat across the rafters at 24 inches and screw panels through with gasketed fasteners; for shingles, sheath with 1/2-inch plywood, underlayment, drip edge, then shingles.
- Flash the wall joint (step flashing up the slope on a side-attach; continuous flashing on a top-attach), then gutter the low edge — a 12 x 20 foot lean-to sheds over 140 gallons in a 1-inch rain, and that water lands somewhere.
Roofing Material Choices for a Lean-To
Corrugated metal owns this category for good reason: at $2 to $4 per sq ft it handles pitches down to 3/12 (lower with sealed laps), installs directly over purlins with no sheathing, and a 29-gauge painted panel lasts 30-plus years. Classic ribbed panels from Fabral, Union, or the local metal supplier come cut to length, so a 12-foot rafter run needs zero panel seams top to bottom. Polycarbonate panels cost slightly more but bring daylight through — the standard choice over patios where you want cover without a cave. Asphalt shingles need 4/12 and full sheathing but let an attached roof visually match the house, which matters on front-facing additions and for some HOAs. Whatever the surface, one detail is universal: install the panels or shingles after the wall flashing is in place, so the flashing laps over the roofing in shingle fashion.
Think about the underside too. An open-rafter patio lean-to looks fine in cedar or stained fir but shabby in bare treated lumber; budget either better-grade beam stock or a simple tongue-and-groove or soffit-panel ceiling if the space is living area. Screened porch conversions later are far easier if the posts land at clean 8-foot modules now.
Costs and Common Mistakes
DIY materials for a 12 x 16 foot attached patio roof run roughly $2,000 to $3,500 — treated framing around $900 to $1,400, metal roofing $700 to $1,200, footings, hardware, and flashing making up the rest. Contractor-built, the same structure prices between $6,000 and $12,000 depending on region and finish level. Either way, the failure points are consistent: ledgers fastened to siding instead of framing, pitch under 2/12 with lapped panels (capillary action pulls water uphill through laps), posts on grade-level pavers that heave with frost, no uplift hardware, and missing wall flashing. None of those mistakes costs much to avoid on day one; all of them are expensive to fix after the drywall inside the family room shows the stain.
Built with a bolted ledger, honest pitch, and real flashing, a lean-to is the rare project where the simplest possible design is also the right one — a single sloped plane doing quiet work for decades.
