How flat roof is constructed

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.

The joists are then either laid on top of the steels or cut into them. These joists are spaced at a specified distance apart, usually 400mm depending on the width or span they are covering and the surface that is to be applied.

In the majority of cases the covering will be plywood or OSB timber which is more commonly called Stirling board.

When timbers are used in flat roofing they obviously need to be waterproofed so boards are normally laid across the joists and then a felt or rubber is used to cover the boards to make them waterproof. Where two boards meet and the joint is not on a joist, tongue and grooved decking boards make the joint strong enough to support the flat roof load.

Never confuse the word flat, with the word level. Flat roofing should always be flat, but never level…. not in just one direction anyway.

Water must be allowed to flow off flat roofs as puddles of water are very heavy and they soon force a dip into the timber roof decking which then becomes bigger allowing more water to puddle until the roof can bear the weight no longer and leaks occur.

When is a Flat Roof Needed?
Sometimes town planners will not allow the height required for a pitched roof. This is because it may obscure a neighbours view or light. Sometimes a pitched roof is not appropriate for an extension because it would mean obscuring light to the home owners own windows. Usually however, a flat roof is constructed when an extension butts up to an existing building.

To use a pitched roof in these circumstances would mean adding roof valleys or box gutters and make draining the roof difficult and expensive. Using a lean-to flat roof allows water to run easily away from the building into new gutters which can be connected to existing drainage systems or even new soakaways.

Occasionally a new flat roof will be required to replace an old roof, for example asbestos or corrugated tin roofs which are either not allowed under building regulations or are in a complete state of disrepair.