Timber Span Calculator — What Size Joist Do I Need?

This calculator looks up TradeCalculator's own span tables, calculated to Eurocode 5 (BS EN 1995-1-1) with UK National Annex parameters and cross-checked against published industry span tables, to determine the minimum timber section size for your span. It compares your required clear span against the maximum allowable span for each standard section size, grade, spacing, dead-load band and member type. Joist sizes are entered as nominal sawn dimensions (e.g. 47×200mm); the calculation uses the regularised finished section to BS EN 336 (45×195mm), per NHBC Standards 6.4.8.

How it works

For each timber section, the maximum span depends on: timber grade, joist spacing, and load type.

The calculator finds the smallest section where the maximum allowable span exceeds your required span, then recommends one size up as a practical safety margin.

Loading assumptions

• Floor joists: imposed qk = 1.5 kN/m² plus a separate 2.0 kN concentrated-load check; dead-load bands Gk ≤ 0.25 / 0.5 / 1.25 kN/m² excluding joist self-weight, which is added separately. No partition allowance is included: where any partition bears on or crosses the joists, add the BS EN 1991-1-1 allowance or have the floor checked by a structural engineer
• Ceiling joists (no storage): imposed qk = 0.25 kN/m² plus a separate 0.9 kN concentrated-load check; dead-load bands Gk ≤ 0.25 / 0.5 kN/m². No allowance for water tanks, storage or boarded lofts. A ceiling carrying storage or a loft conversion (imposed ≥ 1.5 kN/m²) is sized as a floor instead
• Roof rafters: single-span common rafters, birdsmouthed onto wall plates and tied at ceiling level; valid for pitches of 15–45° and computed at 15°, the most onerous case in the band, so conservative for steeper pitches. Imposed qk = 0.75 kN/m² on plan (snow plus maintenance, covering ground snow load sk up to 0.94 kN/m²) plus a separate 0.9 kN concentrated-load check; dead-load bands Gk ≤ 0.5 / 0.75 / 1.0 kN/m² on the slope. Not valid for raised-tie, collar or room-in-roof designs. Above about 200 m site altitude, or in higher snow zones, confirm the snow load under BS EN 1991-1-3 (UK NA)

All spans assume a minimum 40 mm end bearing; deflection is limited to the stricter of span/333 and 12 mm; values are rounded down to 0.01 m.

Key span values (C16, 400mm centres, floor, Gk ≤ 0.25)

• 47 x 100mm: 1.81m
• 47 x 150mm: 2.77m
• 47 x 200mm: 3.72m
• 47 x 225mm: 4.13m

Standards

Timber grading per BS EN 338. Structural design per Eurocode 5 (BS EN 1995-1-1). Span values are calculated by TradeCalculator to BS EN 1995-1-1 with UK National Annex parameters for the stated load cases, and cross-checked against published industry span tables. Eurocode 5 design is the accepted method for sizing solid-timber members under UK Building Regulations Approved Document A. Snow loading for roofs follows BS EN 1991-1-3 and its UK National Annex. Always confirm sizing with a structural engineer, particularly for unusual loading or non-standard conditions.

Working figures for this are collected in our Timber Joist Sizes & Span Tables reference table.

Working figures for this are collected in our Rafter Span Tables reference table.

Working figures for this are collected in our UK Timber Sizes reference table.