Quick answer: timber firrings are tapered strips of timber used to create the fall on a flat roof. They are normally fitted on top of level roof joists, with the roof deck fixed over them, so rainwater runs towards the gutter or outlet.
What do firring strips do?
Firrings create a slope without having to install the main roof joists on a slope. The thick end sits at the high side of the roof and the taper runs down towards the drainage point.
Current BS 6229:2025 guidance requires a minimum completed fall of 1:80 for most flat roofs, with the design fall allowing for construction tolerances and deflection. Bauder explains the current BS 6229:2025 fall guidance here.
What does 3in to 0in or 4in to 0in mean?
It describes the taper. A 3in-to-0in firring starts at roughly 3in deep and reduces towards zero along its length. A 4in-to-0in firring starts deeper, so over the same length it creates a steeper fall.
The actual fall depends on both the taper and the length of the firring.
Where are firrings fitted?
They are normally fixed on top of the roof joists, running from the high side of the roof towards the gutter or outlet. The OSB, plywood or other roof deck is then fixed over the firrings.
Firring strip vs tilt fillet
They do different jobs. Firring strips create the main fall across the roof. A tilt or angle fillet is used around perimeter edges and abutments.
Which firring strip do I need?
Choose the taper from the required roof fall, the length of the roof run and the position of the drainage point. The waterproofing system and roof design also need to be taken into account.
For the calculation itself, use our flat roof fall and firring size guide. It shows how to convert 1:80, 1:60 and 1:40 falls into millimetres across your roof run before choosing the firring.
We stock 3in-to-0in timber firrings and 4in-to-0in timber firrings. For the rest of the roof build-up, see the Build a Flat Roof collection.
