Rafter Length
Free Rafter Length calculator with clear step-by-step results.
About Rafter Length
The rafter is the hypotenuse of a triangle whose base is half the span and whose height is the rise. The eaves overhang is measured horizontally but adds material along the slope, so it is multiplied by the slope factor rather than added directly.
Formulas
Common rafter length
Length = square root of (run^2 + rise^2), where run = span / 2
Overhang along the slope
Sloped overhang = horizontal overhang x (rafter length / run)
Worked example
With the default values below, Rafter Length returns Rafter length including overhang of 4.677. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Building span (wall to wall)minimum 0.5 m
- 7.2 m
- Rise from wall plate to ridgeminimum 0.05 m
- 2.08 m
- Eaves overhangminimum 0 m
- 0.45 m
- Rafter spacingaccepts 200–1,200 mm
- 600 mm
Results
- Rafter length including overhang
- 4.677 m
- Length from wall plate to ridge
- 4.158 m
- Roof pitch
- 30.02 degrees
- Rafter pairs required
- 13 pairs
Step by step
- 1Values used
- 2Common rafter length
- 3Overhang along the slope
- 4Rafter length including overhang= 4.677 m
- 5Length from wall plate to ridge= 4.158 m
- 6Roof pitch= 30.02 degrees
- 7Rafter pairs required= 13 pairs
Assumptions and limits
- The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
- Figures are rounded for display to 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
- Roof structure design, including member sizing for span, spacing, snow and wind loading, must comply with the applicable building code or be specified by a structural engineer.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
Roof structure design, including member sizing for span, spacing, snow and wind loading, must comply with the applicable building code or be specified by a structural engineer.
Frequently Asked Questions
Is this the length to order?
Order longer. This is the finished length before the plumb cut, birdsmouth and any fascia detail, so add at least 150 mm per rafter for cutting.
What pitch do I actually need?
It depends on the covering: slate and plain tile generally need 30 degrees or more, interlocking concrete tiles work from about 17.5, and membranes can go far lower. Check the manufacturer's minimum.