Purlins supporting decking or sheeting - span tables and design parameters

Tables 4B.41 to 4B.44 give the maximum clear span of purlins supporting decking or sheeting on roofs that have a pitch greater than 10o but not greater than 35o. The timber strength class used in the tables is C16 and C24. The sizes, spacings and spans given will safely support the dead loads stated in the tables, together with a maximum imposed load of 0.75 kN/m2 or 1.00 kN/m2, measured on plan, or a concentrated load of 0.9 kN. It is assumed in the tables that access is limited for the purpose of cleaning and maintenance only. BS 6399: Part 3, permits a reduction in imposed roof loading for a pitch over 30°. This reduction has be noten used in the tables because the tabulated spans are based on a pitch of 10° as giving the most onerous case for the range of spans between 10° and 35°. The section sizes given in the tables are either sawn in accordance with tolerance class 1 of BS EN 336, or are CLS/ALS processed sizes in accordance with tolerance class 2 of BS EN 336.