Flat Roof Timber Joist Sizes

The construction of the flat roof is similar to that of the timber upper floor.

Flat roof timber joist sizes.

They also tell you what the maximum spacing should be between each section or timber member size of joists mm clear span c16 joist m clear span c24 joist m 47 x 95 47 x 120 47 x 145 47 x 170 47 x 195 47 x 220 1 77 2 40 2 89 3 38 3 87 4 36 2 05 2 67 3 22 3 77 4 31 4 85 assumptions at 400mm centers. Dead load per square metre in kn m supported by joists. Not more than 0 5 more than 0 5 but not more than 0 75 more than 0 75 but not more than 1. Joists are placed on edge spaced at 400 600mm centres supported by external and internal load bearing walls.

In the majority of cases the covering will be plywood or osb timber which is more commonly called stirling board. For pitched roofs flat roofs and ceiling joists. Timber and concrete upper floors. Usually 400 450 or 600mm but any value can be used up to 610mm max usually 400 450 or 600mm but any value can be used up to 610mm max 1 2 timber strength class and joist size.

Trada and approved document part a give guidance on sizes of joists for flat roofs relating to spans and loading. Where the tables do not apply or where there are concentrated loads floor joists should be designed by an engineer in accordance with technical requirement r5. Surveyors and structural engineers utilise data from tables below to help calculate the correct size strength and centres of roof timbers for the required spans and loadings. These joists are spaced at a specified distance apart usually 400mm depending on the width or span they are covering and the surface that is to be applied.

Centre to centre spacing of joists in mm joist size width depth mm 400 450 600 400 450 600 400 450 600. Permissible clear span metres. Surveyors span tables for designing roof rafters. Should refer to trada document span tables for solid timber members in floors ceilings and roofs excluding trussed rafter roofs for dwellings or ask your lbc surveyor for advice.