Flat Roof Timber Joist Sizes

Permissible clear span metres.

Flat roof timber joist sizes.

Centre to centre spacing of joists in mm joist size width depth mm 400 450 600 400 450 600 400 450 600. Not more than 0 5 more than 0 5 but not more than 0 75 more than 0 75 but not more than 1. Flat roof span tables allow for a maximum roof pitch of 5º. Posts these span tables are designed for a minimum post size of 50 x 50 x 2 5mm up to a maximum height of 3 0m.

For pitched roofs flat roofs and ceiling joists. Dead load per square metre in kn m supported by joists. Timber and concrete upper floors. Timber floor joists shall be adequate for the spans and loads and be correctly installed.

This is the distance between joist centres. Surveyors span tables for designing roof rafters. Solid timber joist sizes are provided in the bs 8103 3 span tables. Joists are placed on edge spaced at 400 600mm centres supported by external and internal load bearing walls.

The construction of the flat roof is similar to that of the timber upper floor. In the majority of cases the covering will be plywood or osb timber which is more commonly called stirling board. The sizing and spacing will depend upon the loads imposed on them and the required span. 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.

Common joists sizes are 200 x 50mm 175 x 50mm and 150 x 50mm. 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. 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. 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.

Use recommended minimum roof pitches as specified by roof sheet supplier. The joists will normally be placed at 400mm centres but no more than 600mm centres. 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. Trada and approved document part a give guidance on sizes of joists for flat roofs relating to spans and loading.