Permissible Clear Spans for Domestic Floor Joists (to BS 5268-7.1)

The allowable clear spans have been calculated in accordance with BS 5268-2:2002 Structural use of timber - Part 2: Code of practice for permissible stress design, materials and workmanship and BS 5268-7.1:1990 Structural use of timber - Section 7.1 Domestic floor joists.

The dead loads shown at the top of span table above do not include the self weight of the joists, however, the joist self weights are included (in addition to the dead loads) in the calculations used to determine permissble clear spans.

Timber to be covered, these span tables do not apply to timber which is fully exposed to the elements.

Notches are not to exceed 0.125 of the depth of the joist and are to be located between 0.07 and 0.25 of the span from the support.

Holes drilled at the neutral axis (centre line) are not to exceed 0.25 of the depth of a joist and are not to be less than three diameters (centre to centre) apart and are to be located between 0.25 and 0.4 of the span from the support.

Strutting or blocking between joists is required as follows;

• Joist spans up to 2.5m - no strutting required
• Joist spans 2.5m to 4.5m - provide strutting at mid-span
• Joist spans over 4.5m - provide 2 rows of strutting at third span positions

Solid timber strutting to be at least 38mm thick and be a minimum of three quarters of the joist depth.

Proprietary strutting is to be used in accordance with manufacturers recommendations.

Wane as allowed in BS 4978:2007+A2:2017 is permitted in all sections included in these span tables.

Joists are to have minimum end bearing of 40mm.

The floor joist span tables are based on the loadings given in the amended version of BS 6399-1 which is an imposed load of either a uniformly distributed load of 1.5 kN/m² or a concentrated load of 1.4 kN. Other span tables are based on the guidance given in BS 5268-7.1 which is a uniformly distributed load of 1.5kN/m² for spans greater than 2400 mm and 3.6 kN load per metre width of floor for spans less than 2400 mm to ensure that very small joist sizes do not result from the calculations for smaller spans. BS 5268-7.1 was published before BS 6399-1 was amended to include the more onerous 1.4 kN imposed concentrated load, so the approach adopted for these span tables uses the recommendations given in BS 6399-1, however, both approaches do result in similar joist sizes being calculated.

These span tables make no additional allowance for partitions or any other loads additional loads. Joists supporting additional loads should be calculated by a competent person.