Firrings

Timber firrings are tapered lengths of timber used to create a fall on a flat roof. Although people call them “flat roofs”, the roof still needs a slight slope so rainwater can drain towards a gutter or outlet. Firrings are fixed on top of the roof joists, then the roof deck is fitted over them to create the correct angle before the waterproof covering is installed.

You need firrings because a completely flat roof can hold standing water, which may shorten the life of the roof covering and increase the risk of leaks. Firrings help direct rainwater away from the roof surface. They are a simple way to build a consistent slope into timber flat roof structures without having to cut or alter the main roof joists.

Many flat roof projects are designed with a fall of around 1:40 so the finished roof still achieves a suitable drainage fall after construction tolerances, roof decking and slight deflection are taken into account. The exact fall can depend on the roof covering, roof size, outlet position and project specification, so always check your plans, architect’s drawings or roofing manufacturer’s guidance before ordering.

The size of firring you need depends on the depth of fall required and the length of the roof run. As a simple example, a longer roof usually needs a deeper firring at the high end to achieve the same fall. You will need to know the direction of the fall, the distance from the high point to the drainage point, and the required finished roof level. For structural or building control work, check the specification before ordering.  

Timber firrings are fitted with the thicker end at the high side of the roof and the thinner end pointing towards the gutter or drainage outlet. This creates the slope needed for water to run off. Before fixing them, it is worth laying them out in position and checking the fall direction carefully, especially on roofs with multiple outlets, abutments, rooflights or parapet walls.

Treated timber firrings are usually the better choice for roof construction because they offer improved resistance to moisture, decay and insect attack. Even though the firrings are normally covered by roof decking and a waterproof system, they are still part of an external roof structure. Treated firrings provide extra protection, especially during installation or if the roof is exposed before the final covering is fitted.  

Yes, timber firrings can be used under common flat roof systems such as EPDM rubber, felt, GRP fibreglass and liquid-applied coverings. The firrings create the fall in the timber structure, while the roof deck and waterproofing system sit above. The important part is making sure the fall, deck type and roof covering are all suitable for the project and installed to the manufacturer’s guidance.