Flat Roof Joist Design

Continuing on from part 3.

Flat roof joist design. Continue reading below our video of the day. A flat roof truss is also known simply as a flat truss and is not completely flat but rather slightly sloped in order to allow water to run off of the roof. Here we have a wood framed gable roof. There are two types of flat roof system and they differ according to whether the insulation is above or below the structural roof deck.

Floor beam span tables of residential structural design we will now look at roof rafter and beam design. Hot tar and gravel also known as the built up roof bur this type of flat roof features three or more layers of waterproof material mixed in an alternating fashion with hot tar and a layer of smooth stone like gravel. Boarding to be fastened on top of flat roof joists with a depth to width ratio greater than 4 the allowable clear spans have been calculated in accordance with bs 5268 2 2002 structural use of timber part 2. The image below shows a cross section of our simple 12 x 13 house from the joist span tables section of this tutorial.

They re simple in form and function and are a good example of the modernist principle of clean lines. 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 steels or rsj s rolled steel joists are often built into one or both walls to give added stability to the roof structure. Let s consider a basic gable wood roof framing design.

Flat roofs can cantilever into daring overhangs for shade protection and solar gain control. In order to make this correctly you will have to work out exactly what you need to do with your truss in order to create a flat roof. Spantec has prepared these tables for use by people skilled in the design and specification of the. 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.

Code of practice for permissible stress design materials and workmanship and bs 5268 7 2 1989 structural use of timber section 7 2. Flat roof span tables allow for a maximum roof pitch of 5º. There are several methods and materials for the design of the exterior surface of a warm or cold flat roof. Trada and approved document part a give guidance on sizes of joists for flat roofs relating to spans and loading.

The construction of the flat roof is similar to that of the timber upper floor. Joists are placed on edge spaced at 400 600mm centres supported by external and internal load bearing walls. Flat roofs create long horizontal planes reminiscent of the broad horizon line seen often in nature.