Timber firrings are an essential element of flat roof design. By definition you would assume flat roof is exactly that, a flat and level roof. But in reality flat roofs are rarely level, they require a fall or an incline to be built into them to manage and control where rain water goes.
This fall also protects the roofing material by ensuring water does not pool or remain on the roof where it can cause water ingress problems over time leading to damage of the buildings structure.
What Is a Timber Firring Strip?
A timber firring is a long accurately cut timber wedge. Viewed from the side it’s a right angled triangle. Its 90 degree sides form the base and high side of the triangle and its sloping upper side forms the fall of the roof. Usually cut from standard sized timber lengths with their width matching the supporting roof joists, often nominal 2” or 47mm in width.
What Are Flat Roof Firrings Used For?
The primary job of flat roof firrings is water management. Despite their name, "flat roofs", they are actually low-pitch roofs. If a roof was built perfectly flat and level, natural timber sagging and surface tension would cause standing water or pooling on the roofs surface. Timber firrings set out and support the sloping roof surface.
They simplify the awkward task of creating a uniform slope with a perfectly flat upper surface while leaving the underside of the joists true and level to receive the ceiling. When fixed to the joists they form a support for the roof’s surface that remains stable over the buildings life.
By utilising roof firrings fixed onto the top of flat roof joists, you achieve three critical requirements for a successful long lasting roof:
- Efficient Rainwater Drainage: The slope they create directs rain water smoothly off the roof deck and into gutters or outlets.
- Extended Roof Life: Eliminating standing water prevents premature degradation of roofing membranes like EPDM, felt or fibreglass.
- Level Internal Finishes: Because the angle is created entirely above the joists, your interior plasterboard ceiling remains flat and level.
Roof Design: Warm Roof vs. Cold Roof Configurations
Most modern build homes are designed to have a warm roof system (the insulation sits above the main structural roof deck). In these cases your firrings can be fixed to the joists, which is a popular choice. However in some modern warm roofs, flat timber joists are used without firrings and the fall is instead created using pre-cut tapered insulation boards.
If you have a traditional home or an extension then it is likely you have a cold roof system (the insulation sits between or below the joists). In these cases firrings are fixed directly to the top of the joists to create the fall, but they also serve a second critical purpose: maintaining a continuous 50mm cross-ventilation gap (UK Building Regulations requirement (BS 5250)) above the insulation. The VCL must be installed on the warm side of the insulation (directly above the ceiling plasterboard) to prevent moisture-laden air from rising into the ventilated cavity created by the firrings.
Sizing Timber Firring Strips
Timber firring strips come in a variety of lengths, drops, and profile sizes to match different roof spans:
- Single Taper Firrings: A timber piece that tapers from a set thickness (such as 75mm) down to 0mm or a thin edge at the end.
- Firring Length: Firrings are commonly supplied in lengths of 2.4 metres, 3 metres or 3.6 metres.
Standard widths usually match common joist thicknesses, typically 47mm (2 inches), ensuring a flush fit over standard structural timber. While listed as tapering to "0mm", physical off-the-shelf firrings rarely finish at a razor-sharp point because extreme feather-edges split easily during transport and fixing.
Calculating the Right Fall for Your Roof
To ensure adequate drainage, building regulations in the UK generally recommend a minimum finished fall of 1:80. However, to achieve this after natural timber deflection and building tolerances, most design engineers plan for a 1:40 fall.
To calculate the size of the firring strips you need:
- Measure the length of the run (the distance the water needs to travel).
- Divide the total span length by the target fall ratio (e.g., a 4000mm span divided by 40 requires a 100mm drop from high end to low end).
- Select a firring strip that tapers 100mm.
As specialists in timber firrings, we’re on hand to answer any questions you have ensure you get things right. With various calculations and building regulations it can feel complicated to feel confident you have the right things ordered. As timber specialists we understand the importance of timber firrings being installed correctly so will always do what we can to assist.
Timber Firrings - FAQs
How are timber firring strips installed on roof joists?
Timber firring strips are laid flat along the top edge of each structural joist and secured using wood screws or nails spaced every 300mm to 400mm. The decking boards (usually OSB3) are then screwed down through the firring and directly into the joist below using separate screws.
These decking screws must penetrate into the main joist by at least 45mm. However, as the firring height changes along the fall, screw lengths must vary across the slope—you will need longer screws at the thick 100mm end (e.g., 160mm screws) and significantly shorter screws at the thin end (e.g., 65mm screws) to achieve the full 45mm penetration.
Which way up should roof firrings be fixed?
The thick edge of the firring strips go at the highest point of your roof slope (often against a house wall or the roof ridge), and the thin end points toward the lowest edge of the roof where gutters or drainage outlets are located.