Rafter Length Calculator
Calculate rafter length from span and pitch, overhang included.
What you'll need to order
- dimensional lumber
- rafter ties
- hurricane straps
- ridge board
Prices vary by region and haul distance. Get quotes from at least two local suppliers.
How this calculator works
rafter = √(run² + rise²) + overhang × pitch multiplier
The rafter is the hypotenuse of a triangle whose base is the run and whose height is the total rise. Overhang runs along the same slope, so it is multiplied by the pitch multiplier rather than added flat.
Worked example
With run (half the span) 12 ft, roof pitch (rise per 12) 6 in, overhang 12 in, this calculator returns 14.53 ft.
Rafter length by pitch
Figures below assume run (half the span) 12 ft × overhang 12 in. Change any measurement in the calculator to get your own numbers.
| Roof pitch (rise per 12) | Rafter length (ft) | Ridge rise above plate (ft) | Plumb cut angle (°) |
|---|---|---|---|
| 3 in | 13.40 | 3.00 | 14.04 |
| 4 in | 13.70 | 4.00 | 18.43 |
| 5 in | 14.08 | 5.00 | 22.62 |
| 6 in | 14.53 | 6.00 | 26.57 |
| 8 in | 15.62 | 8.00 | 33.69 |
| 10 in | 16.92 | 10.00 | 39.81 |
| 12 in | 18.38 | 12.00 | 45.00 |
A 12 ft run with a 12 in overhang. Run is half the building span for a simple gable roof.
Rafter lengths for common spans
| Job | Size | Rafter length |
|---|---|---|
| Shed, 10 ft span | 5 ft × 4 in × 6 in | 5.80 ft |
| Detached garage, 20 ft span | 10 ft × 6 in × 12 in | 12.30 ft |
| Ranch house, 24 ft span | 12 ft × 6 in × 12 in | 14.53 ft |
| Wide house, 32 ft span | 16 ft × 6 in × 16 in | 19.38 ft |
| Steep A-frame, 24 ft span | 12 ft × 12 in × 12 in | 18.38 ft |
| Porch roof, 8 ft span | 4 ft × 3 in × 18 in | 5.67 ft |
Lengths are along the rafter, before the birdsmouth and plumb cuts are made. Order the next standard length up — you cannot add material back once it is cut.
Rafter spacing and what it suits
| Type | Spacing (in on centre) | Notes |
|---|---|---|
| 12 in on centre | 12.00 | Heavy snow loads, or a tile roof |
| 16 in on centre | 16.00 | The residential standard |
| 19.2 in on centre | 19.20 | Five bays per 8 ft sheet |
| 24 in on centre | 24.00 | Common with trusses and thicker sheathing |
Spacing depends on span, species, grade and load. Use the AWC span tables or a truss supplier rather than a rule of thumb — this is the one part of a roof where guessing is genuinely dangerous.
Frequently asked questions
How long should my rafters be?
For a 24 ft span at a 6/12 pitch, each rafter is about 13.4 ft plus overhang. The length is the run multiplied by the pitch multiplier, which is 1.118 at 6/12.
What is a birdsmouth cut?
The notch that seats the rafter on the top plate. Code limits it to a third of the rafter depth, because a deeper notch removes too much of the section carrying the load.
How do I find the ridge board length?
It equals the building length for a simple gable, plus any overhang at the gable ends. The rafter calculation covers the sloped members, not the ridge itself.
Should I use rafters or trusses?
Trusses are cheaper, faster and span further, but they fill the attic with webs. Stick-framed rafters cost more in labour and give usable attic space — which is the whole argument in most renovations.
How do I calculate rafter length?
Square the run, square the total rise, add them and take the square root. A 12 ft run on a 6/12 pitch gives a 13.42 ft rafter before overhang.
Is the run half the span?
Yes, for a symmetrical gable. Measure from the outside of the wall plate to the centre of the ridge. Strictly you should deduct half the ridge board thickness from the run.
What is the plumb cut angle?
The angle where the rafter meets the ridge, equal to the roof angle itself. A 6/12 roof has a 26.6 degree plumb cut. The seat cut at the wall plate is the complement, 63.4 degrees.
How far apart should rafters be?
Sixteen or 24 inches on centre, depending on span, lumber size and snow load. Twenty-four inch spacing is common with 2x8 or larger; 16 inch spacing is required for longer spans or heavy loads.