Rafter Calculator

Common rafter length from span, pitch, overhang and ridge board, with plumb and seat cut angles, stock length and the number of rafters.

A common rafter runs from the ridge to the wall and on to the eaves. Its horizontal run is half the building's span less half the ridge board's thickness, and its length is that run times the slope factor for the pitch, plus the overhang (measured level) times the same factor. For a 24 ft span at 6/12 with a 1½ in ridge and a 12 in overhang, the run is 11.94 ft, the rafter is 13.35 ft to the wall plus 1.12 ft of tail, 14.46 ft in all, so you'd cut it from a 16 ft board. The plumb cut is 26.6° and the seat cut 63.4°, and a 40 ft building at 16 in centres takes 31 rafters a side. Enter your roof and the calculator works it out in feet or metres.

Units
ft
/12
in
in
ft
in
Rafter length
14.46 ft

buy 16 ft lengths

Plumb cut
26.6°

seat cut 63.4°

Ridge height
5.97 ft

above the wall plate line

Rafters
62

31 per side

How it was worked out
  1. Run = half the span less half the ridge board: 12 ft − 0.75 in = 11.94 ft (143.3 in)
  2. Slope factor for 6/12 = 1.118
  3. Rafter to the wall: 11.94 ft × 1.118 = 13.35 ft (160.2 in)
  4. Overhang: 12 in level × 1.118 = 13.42 in
  5. Total rafter length 14.46 ft (173.6 in); the next stock length is 16 ft
  6. Rafters: 40 ft at 16 in spacing = 31 per side, 62 in all

Lengths are along the top edge (the measuring line) of a common rafter on a simple gable roof. Rafter sizes and spacing come from span tables or an engineer, and trusses are often used instead. A common rule is not to cut the birdsmouth seat deeper than about a third of the rafter's depth.

Common use cases

  • Cut rafters for a shed, garage or lean-to
  • Choose rafter stock lengths before buying lumber
  • Find the ridge height for a given pitch
  • Count rafters for a roof

How to use

  1. Choose your units.
  2. Enter the span (outside of the walls), the pitch and the overhang measured level.
  3. Enter the ridge board thickness, the building length and the rafter spacing.
  4. Read the rafter length, stock length, cut angles and the number of rafters.

FAQ

How do I calculate rafter length?

Rafter length = run × slope factor, where the run is half the span less half the ridge board and the slope factor is √(1 + (pitch ÷ 12)²). Add the overhang times the same factor for the tail.

What angles are the rafter cuts?

The plumb cut at the ridge is the roof angle from horizontal: 26.6° for 6/12. The seat cut of the birdsmouth is 90° minus that: 63.4° for 6/12.

What size rafters do I need?

That comes from span tables for your timber grade, spacing, snow and wind loads, or from an engineer. Many roofs now use engineered trusses instead of cut rafters.

Related tools

Results are estimates for planning and buying materials, worked out from the figures you enter and typical values (weights, coverage, sizes) that vary by product and supplier. They are not engineering, building or professional advice. Check your local building code, the product maker's instructions and your supplier's figures before you buy or build, and use a qualified professional for structural, electrical and gas work. Sizes, spans, depths and reinforcement for anything structural must come from span tables, your building code or an engineer.