![]() ![]() 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. Dead load per square metre (in kN/m²) supported by purlin ![]()
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