Rafters and purlins supporting rafters - span and load tables

Tables 4B.11 to 4B.34 give the maximum clear span of common or jack rafters and the purlins that provide support to the common or jack rafters using timber strength class 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 or a concentrated load of 0.9 kN. The tables are presented in sets to cover a range of roof pitch from 15° to 45° for each load combination and strength class of timber. In calculating the rafter sizes for these tables no account has been taken of permitted reduction to the imposed loads for a pitch greater than 30°, as recommended in BS 6399: Part 3. This is because the members have been sized assuming a pitch of 30° as being the most onerous for the pitch range 30° to 45°.