Flat Roof Joist Layout

Learn the 1 rafter design that will guarantee your roof that is safe and strong for years to come.

Flat roof joist layout.

The 8 step process for diy installation step 1. 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. Joists are placed on edge spaced at 400 600mm centres supported by external and internal load bearing walls. Here we have a wood framed gable roof.

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. 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. Frame the roof one important and perhaps not so obvious fact is that a flat roof is not actually flat. The construction of the flat roof is similar to that of the timber upper floor.

There are two types of flat roof system and they differ according to whether the insulation is above or below the structural roof deck. Continuing on from part 3. The steels or rsj s rolled steel joists are often built into one or both walls to give added stability to the roof structure. 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. Let s consider a basic gable wood roof framing design. 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. The image below shows a cross section of our simple 12 x 13 house from the joist span tables section of this tutorial.

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. This is our ultimate roof rafter guide for people designing and building an off grid cabin or tiny home.