Flat Roof Construction – Part 4

This is Part 4 of our Guide To Roof Construction. It covers flat roof materials and flat roof construction of warm and cold roofs.

Also See:
Part 1 – A Guide To Roof Construction: covers an overview of flat and pitched roofs.
Part 2 – Pitched Roof Construction: covers pitched roof materials, basic pitched roof designs, cold and warm pitched roofs and pitched roof construction.
Part 3 – Pitched Roof Detail: covers pitched roof detail: roof ridges and ridge tiles, roof hips and hip tiles, roof valleys, roof verges and lead flashings.

A flat roof is generally defined as having a pitch not greater than 15° to the horizontal. A truly flat roof would not allow rainwater to drain away so most flat roofs have a fall on them so that the rainwater naturally flows to collection points.

Whilst you can have tiled roofs of  10-15° these are really flat roof construction with a cosmetic covering of tiles.
Flat Roof Materials: Waterproofing Layer

There are different types of roofing material used in the waterproofing layer of a flat roof construction:

• EPDM is a type of rubber which comes in continuous sheets. (EPDM stands for ethylene propylene diene monomer if you are desperate to know)
• Roofing felt is a bitumen coated sheet with a mineral coated surface to add colour and provide the bitumen with protection from the sun
• Fibre glass is a rigid material 

Flat Roof Construction

Above is a typical construction of a warm flat roof. It consists of six basic elements

1. A Waterproof layer which resists water.
2. Thermal insulation which retains heat within the building rather than allowing it to escape through the roof.
3. A vapour barrier to prevent moisture escaping
4. A timber deck, often of plywood sheeting, which allows the roof to support a load such as someone walking on it
5. Wooden roof joists which are supported by the walls
6. Plasterboard ceiling

A cold roof consists of the same elements mentioned above but in a different order.