Flat Roof Joist Layout

Let s consider a basic gable wood roof framing design.

Flat roof joist layout.

Trada and approved document part a give guidance on sizes of joists for flat roofs relating to spans and loading. Discover the pros and cons to multiple roof designs. Joists are placed on edge spaced at 400 600mm centres supported by external and internal load bearing walls. The construction of the flat roof is similar to that of the timber upper floor.

Floor beam span tables of residential structural design we will now look at roof rafter and beam design. The steels or rsj s rolled steel joists are often built into one or both walls to give added stability to the roof structure. Frame the roof one important and perhaps not so obvious fact is that a flat roof is not actually flat. The 8 step process for diy installation step 1.

Size of roof joist 400mm 450mm 600mm 47 x 97 1 84 1 81 1 74 47 x 120 2 43 2 39 2 27 47 x 145 3 08 3 02 2 87 47 x 170 3 75 3 67 3 42 47 x 195 4 41 4 31 3 92 47 x 220 5 04. 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. Here we have a wood framed gable roof. 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.

Add the plywood once you ve added the framing it s time to add a layer of 5 8 inch plywood sheathing. 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. There are two types of flat roof system and they differ according to whether the insulation is above or below the structural roof deck. The image below shows a cross section of our simple 12 x 13 house from the joist span tables section of this tutorial.

In the past most were constructed by packing the insulation directly above the internal ceiling finish in between the joists with a ventilation gap above. Flat roof with no permanent access and altitude not exceeding 100m 0 75 kn m udl 0 9 kn point load most areas exceeding 100m but not exceeding 200m excluding parts of scotland pennines north east england refer to bs 6399 3 1 kn m udl 0 9 kn point load.