Flat Roof Joist Construction Details

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

Flat roof joist construction details.

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. Trada and approved document part a give guidance on sizes of joists for flat roofs relating to spans and loading. 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 construction of the flat roof is similar to that of the timber upper floor.

Getting the balance right here is crucial as too much weight will inevitably lead to an unstable roofing structure. 1 2 timber strength class and joist size. In addition to floor and roof framing details this guide includes recommendations for cantilevers and placement of web holes. Depth to width ratio should not be greater than 6.

Maximum clear span of flat roof joists in metres for sc3 timber spacing between joists size. I joists in floor and roof applications. I joist construction details. Flat roofing under construction plywood decking is then used to cover the joists with the width in between also determined by the weight being placed upon them.

The steels or rsj s rolled steel joists are often built into one or both walls to give added stability to the roof structure. A waterproof exterior layer is essential and there are a number of options including bitumen based felt or asphalt epdm rubber and pvc based systems as well as cold applied liquid systems bitumen coverings. Joists are placed on edge spaced at 400 600mm centres supported by external and internal load bearing walls. For additional stability you can also use steel joists that are built into one or both of the walls.

Ceilings and roofs excluding trussed rafter roofs for dwellings or ask your lbc surveyor for advice. This will give additional stability to the roof structure. The most important thing to know about a flat roof says tom is that it s not flat to prevent water from pooling and eventually invading the home flat roofs are always built on a slight incline at least 1 8 inch per foot.