Flat Roof Joist Detail

Trada and approved document part a give guidance on sizes of joists for flat roofs relating to spans and loading.

Flat roof joist detail. The construction of the flat roof is similar to that of the timber upper floor. Detail for attachment to metal panel siding or fascia. Usually 400 450 or 600mm but any value can be used up to 610mm max. This is the distance between joist centres.

Nail to backer block and joist with 2 rows of 10d nails 6 o c. X 5 8 bolt brass alum. Figure 3 i joist cantilever detail for balconies 1 1 2 x l4 minimum 4 maximum where l is length l of cantilever 2 x 8 min. 1 enter joist details 1 1 span and joist spacing.

In the warm flat roof construction there is insulation packed into every possible space where air can flow. 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. There is no need for bearing posts under the ridge board which is nonstructural. The full version allows any size.

This demo version is fixed at 1m clear span. Continuous ceiling joists or collar ties span from rafter to rafter. In a typical roof framed with dimension lumber the rafters rest on the exterior wall top plate at the lower end and bear against a ridge board at the top. 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.

I joists are comprised of top and bottom flanges which resist bending united with webs which provide outstanding shear resistance. Cantilever nails may be used to attach backer block if length of nail is sufficient to allow clinching. The major difference between the warm flat roof and the cold flat roof has to do with the air flow between the roofing timbers known as joists that help to hold up the roof. The flange material is typically laminated veneer lumber lvl or solid sawn lumber and the web is made with plywood or osb.

Joists are placed on edge spaced at 400 600mm centres supported by external and internal load bearing walls. This detail for cmu installation only. Continuous insulation on top of sheathing exterior ci with fiber fill in joist cavity this is how to detail an unvented roof assembly for flat and low slope roofs. 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.

Flat roof joist calculator.