Timber & Framing
Rafter Length Calculator
Calculates rafter length from ridge to eaves using span, pitch, and overhang
Updated 26 June 2026 · Live
What this tool does
Calculates rafter length from ridge to eaves using span, pitch, and overhang.
What this tool does not check
- Does not size the rafter section or check deflection.
- Does not calculate wind or snow loads under BS EN 1991.
- Does not check connections at ridge, wall plate, or hip.
Formula Used
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How the rafter length calculator works
Calculates rafter length from ridge to eaves using span, pitch, and overhang. The calculator takes your dimensions and supplier rates, applies a standard Australian trade formula, and returns a quantity with an indicative cost. Every figure is an estimate — site conditions always move the final number.
Typical Australian timber and framing wastage
Timber wastage runs 5–10% on straight cuts, 10–15% for angled or notched framing. Lengths chosen to minimise splicing tend to cost less — a 4.8 m joist often works out cheaper than two 2.4 m pieces. Our defaults reflect common Australian trade allowances, and can be adjusted upwards for non-standard geometry or downwards where experience supports a lower figure.
What this tool does not do
It does not replace a professional quote, factor regional pricing, assess structural adequacy, or confirm National Construction Code (NCC) compliance. Those remain the responsibility of a suitably qualified designer, surveyor, or your building surveyor.
On-site considerations for rafter length
Check grading stamps (C16, C24) on every length — structural timber must be graded to AS/NZS 1748. Moisture content should be below 20% for internal use; above this, shrinkage and movement are significant.
the National Construction Code (NCC) and compliance
Structural framing is subject to the National Construction Code. Span tables in TRADA guidance are indicative only — your building surveyor will want calculations for anything unusual. When in doubt, a pre-application enquiry to the local council can give early clarity, which tends to be less costly than retrospective correction.
Before you order
Buy treated timber (UC3 for exposed, UC4 for ground contact) even for internal joists near wet areas. The premium is small next to the labour cost of a rebuild. Cross-checking the calculator’s output against a supplier quote helps catch differences in pricing assumptions — ask for exact product specifications (grade, finish, batch number) and confirm delivery timescales against your programme.
Adjusting the defaults
Every input in this calculator is editable. Enter your own dimensions, supplier prices, and wastage allowance — the output recalculates instantly. If the defaults feel off for your region or project type, your own numbers always override them.
Using this rafter length calculator alongside other BuildMetricLab tools
This calculator works best as part of a planning workflow. Pair the quantity with our project contingency, labour-hours, and material-cost calculators to build a complete estimate before you pick up the phone to a supplier. All BuildMetricLab tools run entirely in your browser — no sign-up, no data sent anywhere, and every formula is shown on-page so you can audit the maths.
Sources & methodology
Calculates rafter length from ridge to eaves using span, pitch, and overhang. Every result is calculated from the values you enter, and all inputs are editable.
Frequently asked questions
Are rafter length calculator results accurate enough to order materials?
Use them as a starting estimate only. Verifying the final quantity with your supplier or contractor before ordering is good practice — site conditions, wastage and cut-offs all affect the true figure.
What wastage percentage should I use?
The calculator defaults to the typical Australian trade allowance for timber & framing. Increase it for complex cuts, awkward shapes, or first-time DIY. The default wastage allowance reflects common trade practice; values lower than the default may underestimate offcuts.
Does this replace professional advice?
No. This tool is a planning estimator. For works that affect structure, the National Construction Code (NCC), boundary or dividing-fence, gas, electrics, drainage to a sewer, or similar, consult a suitably qualified professional.
Can I change the unit prices?
Yes — every price field is editable. Plug in your supplier's quote to get a total that matches your project.
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