🪚 Rafter Length Calculator

Rafter vs Truss: Which Roof Framing Are You Calculating?

Rafters and trusses both get a roof from the walls to the ridge, but they are different animals at the lumberyard. A rafter is a single sloping board cut on site: you notch a birdsmouth over the wall plate, cut a plumb cut at the ridge board, let the tail run past the wall as the overhang, and repeat down both side walls, bracing the pairs with ceiling joists and collar ties. A truss is an engineered triangle — top chords, bottom chord and webs — built in a factory, delivered whole, and set by crane in an afternoon. The truss carries its load through its web pattern; the rafter carries it by bending along a single deep board, which is exactly why the birdsmouth notch is policed so tightly.

This page is the stick-framing side of that divide. It computes the length of each rafter from wall plate to ridge, adds the overhang tail, gives the ridge height, and hands you the two cut angles — plumb and seat — plus a pass/fail check on the birdsmouth. If your plan calls for engineered trusses, the numbers you want are on the dedicated truss pages instead: the roof truss span calculator solves truss length, height and angle, while the roof truss design calculator goes deeper into member layout and joint forces.

One vocabulary note, because search results mix the terms: queries for “rafter trusses” almost always mean trusses that replace rafters — prefabricated common trusses doing the job stick framing once did, which is most modern production housing. If that is your project, the truss pages above are your destination; everything below is for the builder holding a saw and a framing square.

Rafter Length Calculator with Overhang: Tails and Fascia

The overhang tail is the part of the rafter that flies past the wall, and it changes the order length: total rafter length = (run + overhang ÷ 12) × factor. The overhang figure is horizontal projection from the wall face, so it goes into the run before the slope factor stretches it along the board. Run the same 24 ft, 6/12 building with 16 in tails: total run = 12 + (16 ÷ 12) = 13.333 ft, and the rafter is 13.333 × 1.1180 = 14.907 ft ≈ 14′ 10⅞″ from fascia to ridge. Along the slope, that 16 in projection is 16 × 1.1180 = 17.89 in of actual lumber past the plumb line at the plate.

The fascia board follows the footprint, not the slope: fascia length = building width + 2 × overhang. On this roof that is 24 + (2 × 1.333) = 26.667 ft — about 26′ 8″ of board along each eave, measured before any scarf joints. The gable (rake) overhang is a separate animal: it runs up the slope on the gable ends, uses the same slope factor as the rafters, and is framed from lookouts rather than from the fascia line — order its trim by slope length, not by building width.

This is also where a rafter cut calculator and a rafter layout calculator earn their keep. Every rafter cut calculator on the market returns the same three numbers — length, plumb angle, seat depth — and the layout step is purely mechanical: hook your tape on the long point of the plumb cut, pull the plate-to-ridge length along the top edge, mark the plumb line for the birdsmouth heel, then step the square 12-and-pitch to level the seat and set the tail. Do that on the first rafter, test fit it, and it becomes the pattern board for the whole roof.

Shed and Lean-To Rafters: Single-Slope Math

Shed roofs and lean-tos use exactly the same slope-factor arithmetic with one twist: there is no ridge, so the full building width is the run — never halve it. Everything else (factor, cut angles, birdsmouth) behaves identically, which makes these the easiest roofs to lay out by hand.

Shed Rafter Length Calculator Example

Take a 12 ft shed at 3/12. The run is the whole 12 ft, and the factor is √(1 + (3 ÷ 12)²) = √(1 + 0.0625) = 1.0308, so each rafter is 12 × 1.0308 = 12.369 ft ≈ 12′ 4⅜″. The high wall stands 12 × 3 ÷ 12 = 3.0 ft taller than the low wall — build that height difference into the wall framing and the rafters land on both plates without shims. At 3/12 the birdsmouth is barely a nick: the seat over a flat 2×4 plate needs only 3.5 × 0.25 = 0.88 in of depth, so even a 2×4 rafter passes the one-third test with a wide margin.

Layout detail worth knowing: shed rafters often get a birdsmouth at both ends — one over the low wall plate, one over a ledger or high-wall plate — or a beveled top cut where the rafter lands against the high wall. Mark both from the same 12-and-pitch square setting so the rafter keeps a single, consistent slope line from end to end.

Lean-To Roof Rafter Span Calculator Notes

A 14 ft lean-to at 2.5/12 (about 11.77°) shows the shallow-slope pattern: factor = √(1 + (2.5 ÷ 12)²) = √(1 + 0.04340) = 1.0215, so the rafter is 14 × 1.0215 = 14.301 ft ≈ 14′ 3⅝″, and the wall plates differ in height by 14 × 2.5 ÷ 12 = 2.917 ft — exactly 35 in of rise over the 14 ft run. Fractional pitches like 2.5/12 are common on lean-tos precisely because they clear a door or window head with minimum height.

Two shallow-slope cautions before you cut: first, roofing manufacturers set minimum pitches — asphalt shingles generally demand 2/12 or steeper with special underlayment below 4/12, while exposed-fastener metal tolerates gentler slopes per its panel table — so confirm the covering before committing to 2.5/12. Second, long shallow rafters deflect visibly under snow; a 14 ft span is well within rafter territory, but let the span tables and, for anything structural, an engineer make the depth call.

🏚️Shed roof: the full width is the run

Shed roof: the full width is the runPhoto: Unsplash

See What This Roof Looks Like as a Truss

Rafter or truss, the roof plane is the same triangle — so once your rafter numbers are set, it costs nothing to see how the same span behaves as an engineered truss. Compare the truss version of this roof at 4/12 and 8/12 pitch: each link opens the Structural Analysis tab with a Fink roof preloaded at a 24 ft span, showing top-chord forces, heel reactions and web layout for the exact geometry you just calculated.