Permissible Clear Spans for Flat Roof Joists (to BS 5268-7.2)

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 5268-7.2:1989 Structural use of timber - Section 7.2 Joists for flat roofs.

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.

If insulation is placed on top of the flat roof joists and as such the joists are not fastened to the boarding above, the depth to breadth ratio of the joists should not exceed a ratio of 4 to 1.

These span tables apply only to flat roof and roof with a slope of up to 10 degrees.

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.

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 imposed load should be calculated in accordance with BS 6399:Part 3:1988 Code of practice for imposed roof loads, as a rule of thumb for altitudes not exceeding 100m you can use a uniformly distributed load of 0.75 kN/m² and for most other areas exceeding 100m but not exceeding 200m you can use 1 kN/m², for parts of Scotland, Pennines, North East England and heights over 200m you will need to refer to BS 6399 Part 3. Where access to the roof is provided in addition to that necessary for cleaning and repair, the imposed uniformly distributed load should be taken as a minimum of 1.50 kN/m².