Flat Roof Joist Design

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 design.

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. Flat roofs create long horizontal planes reminiscent of the broad horizon line seen often in nature. Flat roofs can cantilever into daring overhangs for shade protection and solar gain control. 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.

The construction of the flat roof is similar to that of the timber upper floor. Trada and approved document part a give guidance on sizes of joists for flat roofs relating to spans and loading. 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. 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.

The steels or rsj s rolled steel joists are often built into one or both walls to give added stability to the roof structure. Joists are placed on edge spaced at 400 600mm centres supported by external and internal load bearing walls. 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.