Free Lean-To Roof Calculator — Rafters & Area (2026)

How to Use This Calculator

1. 1Enter the roof length (the dimension along the supporting wall).
2. 2Enter the roof run (how far the roof extends from the wall).
3. 3Enter the pitch (rise per 12 inches of horizontal run).
4. 4Select rafter spacing and click Calculate for rafter count, length, area, and rise height.

About This Material

A lean-to roof (also called a shed roof, skillion, or mono-pitch roof) is the simplest roof form — a single sloped plane that leans against an existing structure or spans between walls of different heights. Lean-to roofs are the standard choice for additions, covered porches, carports, storage sheds, and firewood shelters. The high side is typically attached to an existing building wall using a ledger board bolted to the studs, while the low side rests on a beam supported by posts. Because there is only one slope, lean-to roofs are the most economical to frame — they require approximately half the rafters of a gable roof of the same footprint and have no ridge board, no hip or valley rafters, and no compound cuts. Typical lean-to pitches range from 2/12 to 6/12, with 3/12 to 4/12 being the most common for attached structures. The minimum pitch depends on the roofing material: 1/4:12 for membrane (TPO, EPDM), 2/12 for metal panels with sealant tape at laps, and 4/12 for standard asphalt shingles. Rafter sizes follow the same span tables as gable roofs — 2x6 for runs up to 10 ft, 2x8 for up to 13 ft, and 2x10 for up to 16 ft at 16-inch on-center spacing. Common materials for the ledger connection include a 2x8 or 2x10 ledger board fastened with 1/2-inch lag bolts every 16 inches, with Z-flashing above to prevent water infiltration. A 12x20 ft lean-to costs $800 to $2,000 in framing lumber and $300 to $800 for roofing materials, with professional labor adding $1,500 to $3,500.

Installation Tips

• •Attach the ledger board to the existing wall studs or rim joist with 1/2" lag bolts — never anchor to siding alone.
• •Install Z-flashing or step flashing above the ledger to prevent water from entering the wall.
• •Use a string line from the ledger to the beam to verify consistent rafter slope before cutting.
• •Slope the roof away from the existing building to direct water runoff to the low side.
• •For runs over 12 feet, consider using an intermediate support beam to reduce rafter size requirements.

Common Mistakes to Avoid

• ⚠Attaching the ledger to siding instead of framing — the ledger must be lag-bolted to studs or the rim joist.
• ⚠Forgetting flashing above the ledger — this is the most common source of water damage in lean-to additions.
• ⚠Pitching the roof toward the existing building instead of away — water must drain to the low (outer) side.
• ⚠Using too shallow a pitch for the chosen roofing material — asphalt shingles need at least 4/12 for standard installation.

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