Shed Roof Rafter Calculator

Shed roof rafter length uses the same Pythagorean formula as common rafters: L = √(Run² + Rise²), where Run is the horizontal span from wall to wall and Rise = Run × (Pitch/12). For a shed roof, there's typically no ridge board, so the run is the full span (not half). Add eave overhang divided by cos(pitch) for the total board length.

A shed roof (also called a pent roof or mono-pitch) is a single-slope roof spanning between two walls of different heights — one wall acts as the low side and one as the high side. A lean-to is a specific type of shed roof that attaches to an existing structure (like a house wall) on the high side. The rafter calculation is identical for both — only the support structure differs.

Typically no. A shed roof rafter spans from the low wall plate (eave side) to the high wall plate (ridge side). The high wall itself acts as the ridge, so no separate ridge board is needed. The rafter plumb-cuts directly against the high wall's top plate or fascia. However, for wider sheds (over 12 ft), a nominal ridge board can be added to stiffen the high-side connection.

A shed roof rafter has three cuts: (1) Top plumb cut — at the high wall, angle = 90° − Pitch, bearing against the high wall plate; (2) Bottom seat cut (birdsmouth) — at the low wall, angle = Pitch, forming the horizontal seat on the wall plate; (3) Tail cut — the eave end, typically plumb or square. No ridge deduction is needed since there's no ridge board.

For a 10' wide shed (span) at 6/12 pitch: Run = 10' (no ridge board deduction needed). Rise = 10 × 0.5 = 5'. Rafter length = √(10² + 5²) = √125 = 11.18' = 11' 2-1/8". Add 12" overhang: 12" ÷ cos(26.57°) = 13.42". Total rafter length = 11' 2-1/8" + 1'-1-7/16" = 12' 3-9/16".

Shed rafters span the full width of the shed, so they typically need to be slightly larger than gable rafters for the same clear span. Common sizes: 2x6 for spans up to 10' at 16" OC, 2x8 for spans 10–14', 2x10 for spans 14–18'. Check your local building code's rafter span table for your species and spacing.

Lean-to rafter calculation follows the same formula as shed roof: L = √(Run² + Rise²). The run is the horizontal distance from the lean-to's low wall to the existing high wall. The rise is the height difference between the two walls. Add overhang and account for the birdsmouth seat cut depth on the low wall side.

Maximum shed rafter span depends on lumber size, species, spacing, and load (snow, dead load). For #2 Douglas Fir-Larch at 16" OC: 2x6 = 12'-5", 2x8 = 16'-3", 2x10 = 20'-9". These are reference values — verify with your local building code and structural engineer, especially in snow load zones.

Yes. The birdsmouth cut (seat cut + plumb heel cut) is made at the low wall side of the shed rafter. The seat creates a horizontal bearing surface on the wall plate, and the heel cut provides structural engagement. The HAP (Height Above Plate) rule still applies — keep at least two-thirds of the rafter depth above the cut.

After calculating the rafter length in decimal feet (e.g., 11.18'), separate the whole number (11') from the decimal (0.18'). Multiply the decimal by 12 to get inches: 0.18 × 12 = 2.16". So 11.18' = 11' 2.16". Then convert the fractional part to 16ths: 0.16 × 16 = 2.56 ≈ 3/16". Final: 11' 2-3/16".

To make a lean to roof: (1) Determine the height difference between the existing wall (high side) and the new low wall — this is your rise. (2) Measure the horizontal span between the two walls — this is your run. (3) Calculate the effective pitch: Pitch = arctan(Rise ÷ Run) converted to X/12 format. (4) Calculate rafter length using L = √(Run² + Rise²) or the pitch coefficient. (5) Add eave overhang: Overhang ÷ cos(Pitch Angle). (6) Cut rafters: plumb cut at high wall (90° − Pitch), birdsmouth at low wall (seat = Pitch angle, plumb heel = 90° − Pitch). (7) Install rafters and add sheathing and roofing.

A lean to roof diagram illustrates the single-slope roof geometry attached to an existing structure. It typically shows: (1) The existing wall as the high side; (2) A new low wall or ledger board; (3) The rafter spanning between them at the calculated pitch angle; (4) Key dimensions: span (horizontal run), rise (vertical difference), rafter length (hypotenuse); (5) The birdsmouth cut at the low wall bearing point; (6) The eave overhang beyond the low wall. Our shed roof illustration on this page provides the geometric reference for all shed and lean-to calculations.

The proper lean to roof pitch depends on your climate and roofing material. For the lean-to attached to an existing structure, you can choose a pitch based on your needs: (1) 2/12–3/12: Dry climates, minimal rain/snow (requires low-pitch roofing materials like metal or EPDM); (2) 4/12–5/12: Most common for residential lean-tos, works with asphalt shingles; (3) 6/12+: Heavy snow zones, steeper shed roof pitch helps shed snow quickly. However, for a lean-to the effective pitch is determined by the actual height difference between walls — if your desired pitch doesn't match the height difference, adjust the low wall height to achieve it.

To frame a lean-to roof on an existing house: (1) Install a ledger board (2x6 or 2x8) on the existing wall at the desired rafter bearing height; (2) Set the low wall height to achieve your target pitch (Rise = Run × Pitch/12); (3) Cut and install rafters between the ledger and the low wall plate; (4) Add collar ties if the span exceeds code limits; (5) Install sheathing (OSB or plywood) and roofing underlayment; (6) Apply your roofing material (shingles, metal, etc.); (7) Flash the connection between the lean-to roof and the existing wall to prevent water intrusion. Always verify local building code requirements and obtain necessary permits.

Technically, a lean-to roof IS a shed roof — the geometry and rafter calculations are identical. The difference is structural: (1) A shed roof is a standalone single-slope structure (e.g., a detached shed or addition); (2) A lean-to (also called a 'lean-to addition') attaches to an existing building on its high side, using the existing wall as one of the two supports. The rafter length formula, cut angles, and birdsmouth layout are exactly the same for both — only the connection detail to the existing structure differs. Use the same shed roof rafter calculator on this page for both types.

Yes. The calculator above doubles as a shed roof slope calculator — enter the span (run) and pitch (X/12) and it returns the roof slope as a percentage (slope % = (Rise ÷ Run) × 100), as an angle in degrees, and as the X/12 pitch ratio, alongside the rafter length. For a 10' span at 6/12 pitch, the slope is 50% (6 ÷ 12 × 100) or 26.57°. Use the same inputs for a lean to roof slope: because the single-slope geometry is identical, the slope, angle, and rafter length all update together in one calculation.

Yes. Because a lean-to roof is a single-slope roof, the same tool serves as a lean to roof pitch calculator: enter the horizontal run from the low wall to the existing high wall and either the pitch (X/12) or the height difference (rise), and the calculator solves for the missing value, the pitch angle in degrees, and the rafter length. To convert a measured rise and run into a pitch ratio, use Pitch (X/12) = (Rise ÷ Run) × 12; to convert that pitch straight to degrees and slope percent, use our roof pitch calculator.

A skillion roof (often called a skillion lean to roof) is the Australian and New Zealand term for a single-slope, single-plane roof — the same geometry as a North American shed roof or lean-to. A skillion lean to roof has one sloped surface spanning between a high wall (or ridge) and a low eave wall, with no ridge board and a pitch typically between 3/12 and 9/12. The rafter length, plumb cut, seat cut, and birdsmouth calculations are identical to a shed or lean-to roof, so the calculator and formulas on this page apply directly to skillion framing.

Yes. The embedded calculator above works as a shed roof slope calculator: enter the horizontal run (from the low eave wall to the high bearing wall) and either the pitch (X/12) or the rise (height difference between the low and high walls), and the tool returns the slope in degrees, the slope percentage, and the rafter length along the slope. For a 6 ft run with a 24" height difference, the shed roof slope calculator returns a 4/12 pitch (18.43°, 33.33% slope). To convert that slope to other pitch notations or cross-reference against a pitch chart, use the Roof Pitch Calculator.

Lean to rafters are sized the same way as shed rafters: take the horizontal run (the distance from the low eave wall to the high bearing wall), pick a pitch (typically 3/12–6/12 for a lean-to), and look up the required rafter size on the Rafter Span and Size Guide using the 20 psf roof live load. For a 10 ft lean-to run at 4/12 pitch, a #2 SPF 2x6 at 16" o.c. comfortably covers the span (11'-0" reference span). For the full rafter length, plumb cut, seat cut, and birdsmouth layout at that size, use the embedded calculator on this page; for the lumber size lookup, use the Rafter Lumber Size Calculator.

Choose lean-to roof rafters (stick framing) for runs under about 12 feet — they cost less, keep the bay open for storage, and use one size of 2x lumber with no engineering. Choose a lean-to truss beyond that span, for carports over 20 feet, or when building many identical bays: trusses arrive as rigid units, erect faster, and clear longer spans with engineered chords, but their webs block storage space and the fabrication cost only pays back at scale. The rafter math on this page covers the stick-framed route end to end.

Lean-to rafter spacing is 24" on center as the default for light shed roofing (asphalt shingles, corrugated metal, rolled roofing) on 2x4 or 2x6 rafters, and 16" on center when the roof carries snow load, tile, or storage loads, or when the run approaches the 24" span-table limit. 19.2" o.c. works as a cost compromise only where local code accepts it. Because lean-to rafters are usually shallow 2x4s or 2x6s, spacing affects capacity more than it does on deep house rafters — when in doubt, tighten to 16" o.c.

One slope roof framing (mono-pitch or single-slope framing) is a roof built from a single inclined plane with no ridge: a ledger on the high wall, identical rafters at uniform spacing, a bearing plate on the low wall, and sheathing over the top. Because there is no ridge board, hip, or valley, every rafter is the same length and every cut repeats — it is the simplest roof to frame. The trade-off is wall height: the high wall absorbs the entire rise (Run × Pitch/12), so set the low wall height first and let the pitch follow.

A shed pitch roof works best at 3/12 as an all-around default: it clears asphalt shingles (3/12 minimum), sheds rain decisively, and keeps the high wall height manageable. Go to 2/12 or flatter only with metal, EPDM, or TPO coverings in dry or rain-only climates, and go to 4/12–6/12 in snow country so the plane sheds its load. Remember the covering sets the floor: asphalt shingle needs ≥ 3/12, corrugated metal 1/12–2/12 depending on profile, and membrane roofing can go nearly flat.