Flat Roof Timber Joists

Where a new flat roof abuts an existing external wall the joists can be hung on metal joist hangers which are fixed to a horizontal timber plate wall bolted to the wall.

Flat roof timber joists.

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. For additional stability you can also use steel joists that are built into one or both of the walls. The construction of the flat roof is similar to that of the timber upper floor. This will give additional stability to the roof structure.

C16 timber is commonly used in floor and roof joists for internal building work. It s often chosen as the standard timber for roof extensions or joist refurbishment as it s the most cost effective internal structural timber. 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 rafters in a flat roof serve as ceiling joists for the space below. Trada and approved document part a give guidance on sizes of joists for flat roofs relating to spans and loading. These span tables apply only to flat roof and roof with a slope of up to 10 degrees. 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.

Depth to width ratio should not be greater than 6. To meet the requirements of the c16 timber grade timber is machine graded or visually graded. 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. 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.

1 2 timber strength class and joist size. The basic construction of a flat roof starts with roofing timber that is laid across two upright items usually two walls. 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.