Estimate rafter length with a horizontal overhang, plus a small birdsmouth plumb and level pair.
| Rafter | – |
| Plumb cut | – |
| Level cut | – |
Use this calculator for a simple gable rafter. Enter the horizontal run to the ridge, the rise, and the horizontal overhang past the wall.
Hypotenuse = √(run² + rise²). Rafter length = hypotenuse + overhang × (hypotenuse ÷ run), which scales the overhang along the slope. Plumb and level marks are a 50 mm seat sketch: level = 50 mm, plumb = 50 × (rise ÷ run). Run, rise, and overhang can be entered in metres (m) or feet (ft). Rafter length follows m or ft; plumb and level follow mm or in.
This does not deduct ridge thickness or a full birdsmouth layout. The 50 mm seat is a sketch, not a code seat depth.
Run: Horizontal run from wall to ridge, in m or ft. Half the building width if the ridge is centred.
Rise: Rise from wall plate to ridge, in m or ft.
Overhang: Horizontal overhang past the wall, in m or ft.
Rafter: Slope length including overhang.
Plumb cut: Plumb-cut mark for a 50 mm seat.
Level cut: Level-cut mark, 50 mm.
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