Flat Roof Joist Design

Flat roofs create long horizontal planes reminiscent of the broad horizon line seen often in nature.

Flat roof joist design. 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. Code of practice for permissible stress design materials and workmanship and bs 5268 7 2 1989 structural use of timber section 7 2. Continuing on from part 3. Joists are placed on edge spaced at 400 600mm centres supported by external and internal load bearing walls.

Floor beam span tables of residential structural design we will now look at roof rafter and beam design. Trada and approved document part a give guidance on sizes of joists for flat roofs relating to spans and loading. The steels or rsj s rolled steel joists are often built into one or both walls to give added stability to the roof structure. Use recommended minimum roof pitches as specified by roof sheet supplier.

Here we have a wood framed gable roof. 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. They re simple in form and function and are a good example of the modernist principle of clean lines. This includes structural joists to hold up a roof deck usually of timber boarding a suitable membrane to reduce the risk of condensation and a layer of insulation to keep the warmth in.

The image below shows a cross section of our simple 12 x 13 house from the joist span tables section of this tutorial. Flat roofs can cantilever into daring overhangs for shade protection and solar gain control. 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. Let s consider a basic gable wood roof framing design.

Spantec has prepared these tables for use by people skilled in the design and specification of the. The construction of the flat roof is similar to that of the timber upper floor. 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. Posts these span tables are designed for a minimum post size of 50 x 50 x 2 5mm up to a maximum height of 3 0m.

There are several methods and materials for the design of the exterior surface of a warm or cold flat roof. Continue reading below our video of the day. 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.