Flat Roof Timber Joists

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

Flat roof timber joists.

For additional stability you can also use steel joists that are built into one or both of the walls. It s often chosen as the standard timber for roof extensions or joist refurbishment as it s the most cost effective internal structural timber. The rafters in a flat roof serve as ceiling joists for the space below. They are nailed securely to exterior wall plates and to each other.

Trada and approved document part a give guidance on sizes of joists for flat roofs relating to spans and loading. The steels or rsj s rolled steel joists are often built into one or both walls to give added stability to the roof structure. To meet the requirements of the c16 timber grade timber is machine graded or visually graded. 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.

These span tables apply only to flat roof and roof with a slope of up to 10 degrees. Depth to width ratio should not be greater than 6. Joists are placed on edge spaced at 400 600mm centres supported by external and internal load bearing 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.

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

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. This will give additional 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.