Purlins are to have minimum end bearing of 80mm. Wane as allowed in BS 4978:2007+A2:2017 is permitted in all sections included in these span tables. Timber to be covered, these span tables do not apply to timber which is fully exposed to the elements. Holes and notches cannot be drilled or cut into the purlins unless proved adequate by specialist calculations. ![]() The calculations used for these span tables are based on rafters having two equal spans. Since continuous rafters load the purlin more severely, this is the case considered. The rafters may be continuous or may consist of shorter lengths joined at the purlin. ![]() These span tables are based on purlins for a single span, if the purlins are two span continuous or more over intermediate supports you can conservatively consider the longest span between supports. It is also assumed ceiling joists will be used to transmit the horizontal component of thrust occurring at eaves level to complementary rafters. The major axis of the purlin is perpendicular to the rafter slope. These span tables do not apply to trussed rafter roofs. The dead loads shown at the top of span table above do not include the self weight of the purlin, however, the purlin self weight is included (in addition to the dead loads) in the calculations used to determine permissble clear spans. ![]() 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 5268-7.6:1990 Structural use of timber - Section 7.6 Purlins supporting rafters. Joists supporting additional loads should be calculated by a competent person.Dead load per square metre (in kN/m²) supported by purlin These span tables make no additional allowance for partitions or any other loads additional loads. 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. 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. 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. Joists are to have minimum end bearing of 40mm. Proprietary strutting is to be used in accordance with manufacturers recommendations. Solid timber strutting to be at least 38mm thick and be a minimum of three quarters of the joist depth. Joist spans over 4.5m - provide 2 rows of strutting at third span positions.Joist spans 2.5m to 4.5m - provide strutting at mid-span.Joist spans up to 2.5m - no strutting required. ![]() Strutting or blocking between joists is required as follows 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. 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. 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. 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. Dead load per square metre (in kN/m²) supported by joistsĬentre-to-centre spacing of joists (in mm)
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