Flat Roof Timber Joists

The basic construction of a flat roof starts with roofing timber that is laid across two upright items usually two walls.

Flat roof timber joists.

Trada and approved document part a give guidance on sizes of joists for flat roofs relating to spans and loading. The rafters in a flat roof serve as ceiling joists for the space below. If insulation is placed on top of the flat roof joists and as such the joists are not fastened to the boarding above the depth to width ratio should not be greater than 4. Notches are not to exceed 0 125 of the depth of the joist and are to be located between 0 07 and 0 25 of the span from the support.

These span tables apply only to flat roof and roof with a slope of up to 10 degrees. It s often chosen as the standard timber for roof extensions or joist refurbishment as it s the most cost effective internal structural timber. A cavity closer should be provided to the top of the walls to prevent fire spread and to stop damp air entering the roof space. For additional stability you can also use steel joists that are built into one or both of the walls.

Joists are placed on edge spaced at 400 600mm centres supported by external and internal load bearing walls. This will give additional stability to the roof structure. 1 2 timber strength class and joist size. To meet the requirements of the c16 timber grade timber is machine graded or visually graded.

The construction of the flat roof is similar to that of the timber upper floor. Timber to be covered these span tables do not apply to timber which is fully exposed to the elements. On a basic level flat roof construction comes in the form of roofing timbers or joists as they are more commonly known or steels that are laid across two uprights walls in most cases. The steels or rsj s rolled steel joists are often built into one or both walls to give added stability to the roof structure.

The size of rafters for a flat roof depends on the horizontal span of the building the type and grade of timber used and the load on the roof. They are nailed securely to exterior wall plates and to each other. 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.