Timber Rafter Span Calculator

Calculator

Enter roof loads and timber properties. Add a proposed span to run a pass/fail check.

Unit system

Metric uses m, mm, and kN/m².

Roof pitch (degrees)

Used to convert horizontal span to slope length.

Proposed horizontal span (optional)

Metric: m. Leave blank to only compute allowable span.

Rafter spacing

Metric: m (e.g., 0.6)

Dead load

Metric: kN/m² (roofing + ceiling + insulation).

Live / snow load

Metric: kN/m² (maintenance or snow).

Timber grade

Choose a grade or enter custom values.

Timber width (b)

Metric: mm

Timber depth (h)

Metric: mm (depth controls stiffness strongly).

Duration factor (Cd)

Typical range: 0.9 to 1.6.

Service factor (Cm)

Use lower values for wet service.

Deflection limit (L/…)

Common limits: L/240 total, L/360 live.

Deflection check load

Controls which line load is used for deflection.

Eave overhang (optional)

Metric: m. Added to cut length only in this tool.

These are illustrative scenarios. Always verify using your local code and project details.

Scenario	Pitch	Spacing	Section	Dead	Live	Grade	Defl.	Expected allowable span (horiz.)
Light roof, moderate pitch	30°	0.60 m	50×200 mm	0.60	0.75	C24	L/240 total	≈ 4.0–4.6 m
Higher snow, tighter spacing	25°	0.40 m	50×200 mm	0.60	1.50	C24	L/360 live	≈ 3.6–4.2 m
Deeper member for stiffness	35°	0.60 m	50×250 mm	0.70	0.75	C24	L/240 total	≈ 5.0–6.0 m
Economy grade check	30°	0.60 m	38×184 mm	0.55	0.75	C16	L/240 total	≈ 3.0–3.6 m
Custom properties (supplier sheet)	20°	0.60 m	63×200 mm	0.60	1.00	Custom	L/240 total	Varies with Fb, Fv, and E

Loads in the table are kN/m² (metric base).

This calculator models each rafter as a simply supported beam with uniform line load along the slope.

Units are handled internally in metric (N, mm, MPa). Imperial entries are converted for calculation.

For final design, include load combinations, notching, connections, bearing, bracing, and local code requirements.