Timber & Framing
Rafter Length Calculator
Calculates rafter length from ridge to eaves using span, pitch, and overhang
Updated 29 April 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 UK trade formula, and returns a quantity with an indicative cost. Every figure is an estimate — site conditions always move the final number.
Typical UK timber and framing wastage
Timber wastage runs 5–10% on straight cuts, 10–15% for angled or notched framing. Order at lengths that minimise splicing — a 4.8 m joist often orders cheaper than two 2.4 m. Our defaults reflect common UK trade allowances — adjust upwards for non-standard geometry, downwards if your experience says so.
What this tool does not do
It does not replace a professional quote, factor regional pricing, assess structural adequacy, or confirm Building Regulations compliance. Those remain the responsibility of a suitably qualified designer, surveyor, or your building control officer.
On-site considerations for rafter length
Check grading stamps (C16, C24) on every length — structural timber must be graded to BS EN 14081. Moisture content should be below 20% for internal use; above this, shrinkage and movement are significant.
Building Regulations and compliance
Structural framing is subject to Approved Document A. Span tables in TRADA guidance are indicative only — your building control officer will want calculations for anything unusual. When in doubt, lodge a pre-application enquiry with your local authority — early clarity is cheaper than a 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. Always cross-check the calculator's output against a supplier quote — 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. Uses UK trade defaults for wastage and density. Prices are illustrative April 2026 values; change them to match your supplier quote.
Frequently asked questions
Are rafter length calculator results accurate enough to order materials?
Use them as a starting estimate only. Always verify the final quantity with your supplier or contractor before ordering — site conditions, wastage and cut-offs all affect the true figure.
What wastage percentage should I use?
The calculator defaults to the typical UK trade allowance for timber & framing. Increase it for complex cuts, awkward shapes, or first-time DIY. Reducing the wastage below the default is not recommended.
Does this replace professional advice?
No. This tool is a planning estimator. For works that affect structure, Building Regulations, Party Wall, 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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