The roof frame is being priced and the builder needs the exact rafter order length including overhang before the timber quote can be finalised. The structural length is already estimated -- now you need to add overhang and ordering allowances.
Rafter Length Calculator
Roofing
Wall plate to wall plate — full width
Measured horizontally from wall plate — typically 450–600mm
Run = Span ÷ 2 (half span)
Rise = Run × tan(pitch)
Rafter = Run ÷ cos(pitch)
With overhang = Rafter + (overhang ÷ cos(pitch))
Add 100mm minimum at ridge for plumb cut · Add 50mm at seat cut for tolerance.Always verify rafter sizing with a structural engineer for spans over 4m.
Results are estimates for planning purposes. Verify with current standards and a qualified professional.
1 What this calculator does
Calculates the complete rafter order length including the structural length (birdsmouth to ridge plumb cut), eave overhang on the hypotenuse and allowances for cuts. Also calculates ridge height above wall plate and the slope ratio for reference.
2 Formula & professional reasoning
Run = Span / 2 (half the building span)
Pitch in radians = Pitch degrees x Pi / 180
Rafter rise = Run x tan(Pitch)
Structural rafter length = Run / cos(Pitch) (birdsmouth to ridge)
Overhang on hypotenuse = Overhang horizontal / cos(Pitch)
Total rafter length = Structural + Overhang on hypotenuse
Ridge height = Rise (vertical)
The rafter length is the hypotenuse of the right-angled triangle formed by the run (horizontal) and rise (vertical). The eave overhang must also be converted from its horizontal dimension to the hypotenuse dimension -- because the rafter extends on the same slope angle. A 600mm horizontal overhang becomes 600/cos(22°) = 647mm on the rafter. Adding the plumb cut waste (100mm) and seat cut waste (50mm) gives the total order length, which is then rounded up to the next standard length.
3 Worked examples
⚠️ Illustrative example only — not clinical or professional instruction.
Basic
Standard gable roof with 600mm eave
Given: Span: 10.0m | Pitch: 22 degrees | Overhang: 600mm horizontal | Plumb cut allowance: 100mm | Seat cut: 50mm
Working:
Run: 5.0m | Structural rafter: 5.0/cos(22°) = 5.393m | Overhang on hyp: 0.6/cos(22°) = 0.647m | Cut allowance: 0.15m | Total: 5.393+0.647+0.15Answer: Total rafter order length: 6.19m -- order 6.3m lengths
💡 Ridge height: 5.0 x tan(22°) = 2.020m above wall plate. Slope ratio: tan(22°) x 12 = 4.85:12 imperial.
Standard
Steep pitch with large overhang
Given: Span: 8.0m | Pitch: 30 degrees | Overhang: 800mm horizontal
Working:
Run: 4.0m | Structural: 4.0/cos(30°) = 4.619m | Overhang: 0.8/cos(30°) = 0.924m | Cuts: 0.15m | Total: 5.693mAnswer: Total rafter order length: 5.69m -- order 6.0m lengths
💡 Ridge height at 30 degrees: 4.0 x tan(30°) = 2.309m -- a significant roof height. Verify ridge beam and collar tie requirements with engineer.
Advanced
Hip rafter -- longer than common rafter
Given: Building width: 8.0m | Building length: 12.0m | Pitch: 22 degrees
Working:
Common rafter run: 4.0m | Hip rafter diagonal run: sqrt(4.0^2 + 4.0^2) = 5.657m | Hip rafter length: 5.657/cos(22°) = 6.102m | Add overhang and cutsAnswer: Hip rafter structural length: 6.10m vs common rafter 4.31m -- hip is 42% longer
💡 Hip rafters run from the corner of the building to the ridge and are substantially longer than common rafters. Calculate them separately.
4 Sanity check
Eave overhang must be converted to hypotenuse
Overhang on rafter = Horizontal overhang / cos(pitch) | A 600mm horizontal eave becomes 647mm on the rafter at 22 degrees
Always add cut allowances
Plumb cut at ridge: 100mm | Birdsmouth seat cut: 50mm | These are consumed by the cuts and are not part of the span
Hip rafters are longer than common rafters
Hip rafter run = sqrt(half-span^2 + half-length^2) due to the diagonal run
Calculate hip and valley rafters separately.
Standard rafter spacings
Roof tiles: 600mm spacing typical | Metal roofing: 900-1200mm spacing typical
Always check the roofing product specification for maximum rafter spacing.
5 Common errors
| Error | Cause | Consequence | Fix |
|---|---|---|---|
| Adding horizontal overhang to the rafter length instead of the hypotenuse overhang | Not applying the cosine factor to the eave overhang | Rafter too short -- eave does not reach the designed overhang dimension | Always divide the horizontal overhang by cos(pitch) to get the overhang on the rafter slope. Never add horizontal and slope dimensions directly. |
| Not adding cut waste allowances before ordering | Ordering to the calculated length | Rafter exactly the right length but the cuts waste material -- plumb cut at ridge and seat cut at birdsmouth both consume timber | Add 100mm for the ridge plumb cut and 50mm for the birdsmouth seat cut to the calculated total. Round up to the next standard length. |
| Using the same rafter length for hip and common rafters | Calculating only the common rafter length | Hip rafters too short -- incorrect order and delays | Hip rafters have a longer run than common rafters because they run diagonally. Calculate the hip rafter run using Pythagoras (diagonal of the plan rectangle) and then apply the same trigonometry. |
| Not verifying rafter size against span tables for the calculated length | Length calculation done but structural adequacy not checked | Undersized rafter fails under load -- structural deficiency | Use AS 1684 span tables to confirm the rafter cross-section (90x45, 140x45 etc.) is adequate for the calculated rafter length, spacing and roof load. |
6 Reference & regulatory links
7 Professional workflow
Common tools used alongside this one: