Flat Roof Timber Joist Sizes

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.

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. Timber floor joists shall be adequate for the spans and loads and be correctly installed. Joists are placed on edge spaced at 400 600mm centres supported by external and internal load bearing walls. Surveyors span tables for designing roof rafters.

Permissible clear span metres. 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. 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. 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. The construction of the flat roof is similar to that of the timber upper floor. Not more than 0 5 more than 0 5 but not more than 0 75 more than 0 75 but not more than 1. 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. Wherever possible joists should span the shortest distance. 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.

Flat roof span tables allow for a maximum roof pitch of 5º. The joists will normally be placed at 400mm centres but no more than 600mm centres. Use recommended minimum roof pitches as specified by roof sheet supplier. 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.

Dead load per square metre in kn m supported by joists. Common joists sizes are 200 x 50mm 175 x 50mm and 150 x 50mm. Solid timber joist sizes are provided in the bs 8103 3 span tables. The sizing and spacing will depend upon the loads imposed on them and the required span.