Flat Roof Joist Layout

There are two types of flat roof system and they differ according to whether the insulation is above or below the structural roof deck.

Flat roof joist layout.

Add the plywood once you ve added the framing it s time to add a layer of 5 8 inch plywood sheathing. 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. Floor beam span tables of residential structural design we will now look at roof rafter and beam 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.

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. 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. Let s consider a basic gable wood roof framing design. Frame the roof one important and perhaps not so obvious fact is that a flat roof is not actually flat.

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. The steels or rsj s rolled steel joists are often built into one or both walls to give added stability to the roof structure. The construction of the flat roof is similar to that of the timber upper floor.

Discover the pros and cons to multiple roof designs. This is our ultimate roof rafter guide for people designing and building an off grid cabin or tiny home. Trada and approved document part a give guidance on sizes of joists for flat roofs relating to spans and loading. Continuing on from part 3.

The image below shows a cross section of our simple 12 x 13 house from the joist span tables section of this tutorial. Learn the 1 rafter design that will guarantee your roof that is safe and strong for years to come.