A flat roof that leaves water standing after every shower is not doing its job. Firring strips for flat roofs create the planned slope that moves rainwater towards an outlet, gutter or drainage channel, while allowing the structure below to remain level. Get the fall, timber section and roof build-up right at the start and the deck, membrane and drainage details all become easier to manage.
What firring strips do on a flat roof
A firring strip, sometimes called a firring piece, is a length of timber cut to a gradual taper. It is fixed over level joists or structural members to form a fall across the roof deck. The thick end raises the deck, while the thin end sets the low point at the drainage outlet.
They are commonly used on traditional timber flat roofs where the joists need to remain level for ceiling finishes, insulation details or the supporting structure. Rather than building the slope into the joists, the roofer or carpenter creates it above them with tapered timber.
Do not confuse firring strips with lightweight furring battens used to level walls or create a service void. Roof firrings must be specified for the load and environment involved. They carry the roof deck and contribute to the overall build-up, so poorly sized, twisted or inadequately fixed timber can cause an uneven deck and ponding water.
Choose the right fall before ordering timber
The first decision is the finished roof fall. For many flat roof systems, a design fall of 1:40 is used to allow for construction tolerances, timber movement and deflection, with a minimum finished fall of 1:80 often expected at the waterproofing layer. The exact requirement depends on the membrane manufacturer, roof design and drainage arrangement, so check the system specification before cutting timber.
A 1:40 fall means the roof drops 1mm for every 40mm of horizontal run. Over a 4m run, that is a 100mm difference in height. At 1:80 over the same distance, the difference is 50mm. Those numbers may look straightforward, but they affect parapet heights, door thresholds, insulation thickness, drainage positions and the upstand available for the membrane.
Work from the drainage point back
Set the low point first. This is normally the outlet, gutter edge or internal valley, not simply the lowest convenient side of the building. Then calculate the rise back to the highest point using the design fall and the true horizontal distance.
For example, if a roof is 3.6m from its high point to an outlet and requires a 1:40 design fall, the total rise is 90mm. If the firring is to finish at 20mm at the outlet end, it needs to be 110mm at the high end, before allowing for any changes in deck or insulation build-up.
A roof with more than one outlet needs more thought. Firrings may need to run in different directions to form a central valley, a hipped fall or separate drainage zones. This is where a simple side-to-side taper can become the wrong solution. Mark falls on a roof plan before material is ordered, particularly where there are rooflights, abutments or parapet walls.
Timber grade, treatment and section size
Use straight, dry and consistent timber. Treated carcassing timber is a practical choice where the roof detail calls for it, especially when there is potential for moisture during construction or in a ventilated roof zone. However, treatment does not correct a poor roof design or make unsuitable timber structurally capable.
C16 and C24 are structural strength classes for graded softwood. If the tapered timber is only creating the fall over joists that have already been designed to carry the roof load, the firring pieces are often specified as suitable carcassing rather than designed as primary structural members. If the firrings are spanning unsupported distances, carrying concentrated loads or replacing part of the structural joist design, speak to the project engineer or timber designer first.
The starting section must be deep enough to produce the required taper without leaving an impractically thin feather edge. A very thin end is prone to splitting during fixing and can be difficult to keep level. In some details, the low end is kept at a sensible minimum thickness and the higher end is increased accordingly.
Timber moisture content also matters. Fresh, wet timber can shrink, cup or twist as it dries, telegraphing movement through the deck and roof covering. Store firrings flat, off the ground and protected from prolonged rain before installation. Reject lengths with pronounced bow, twist, large splits or damage around the fixing line.
Installing firring strips for flat roofs
Firring strips should run in the direction of the intended fall, normally from the high point down towards the outlet. Fix each piece securely to the joist below, keeping the taper consistent from one joist to the next. A string line, laser level or straightedge is worth using before the deck goes down. Small errors repeated across a roof can create a low spot exactly where water needs to keep moving.
Use fixings long enough to achieve sound penetration into the supporting joist after allowing for the firring thickness. Fixing type and spacing should suit the timber, the roof design and exposure during construction. Pre-drilling near thin ends can reduce splitting where required.
Check the deck, not just the timber
Once the firrings are installed, check the actual plane formed by the roof deck. The deck is the surface that supports the waterproofing system, so it is the finished slope that matters. Structural plywood, OSB3 or another specified roof deck material must be fitted to the correct thickness, supported at joints and fixed in line with the board and membrane requirements.
Avoid assuming that a nominal 1:40 timber fall guarantees a 1:40 finished fall. Joist deflection, board joints, insulation compression and workmanship can all reduce it. Check the deck for dips before the roof covering is applied, when corrections are still manageable.
The waterproofing system should be installed only once the deck is dry, clean and properly prepared. Felt, EPDM, GRP, liquid systems and single-ply membranes all have their own substrate, primer, edge and upstand requirements. The fall formed by firrings is only one part of a watertight roof.
Firrings or tapered insulation?
Firring strips are not always the best way to create a fall. On a warm roof, tapered insulation can form the drainage slope above the structural deck. This can simplify the timber structure and maintain insulation continuity, but it may cost more and needs a carefully designed layout to avoid cold spots and awkward perimeter details.
Timber firrings can be the more economical choice for straightforward extensions, garages and outbuildings where the roof structure is accessible and the required fall is simple. They also suit projects where the roof deck needs to be laid to a defined slope before insulation or covering work begins.
The trade-off is build-up height. A steep or long fall can create a substantial rise at one end of the roof. That may affect flashing heights, fascia depth, neighbouring windows or planning constraints. In these cases, tapered insulation, sloping joists or a revised drainage layout may give a cleaner result.
Information to have ready when buying
A timber merchant can machine or supply material more accurately when the roof information is clear. Have the roof length and width, joist centres, direction of fall, drainage location, desired gradient, minimum and maximum firring depths, and required timber lengths to hand. Also confirm whether the roof is a warm roof, cold roof or another specialist build-up.
For multi-fall roofs, provide a sketch with dimensions rather than relying on a verbal description. It prevents wasted timber and helps ensure each taper is cut for its correct position. If standard treated carcassing is being cut on site, allow enough length and section size for clean, accurate cuts and a sensible minimum thickness at the low end.
Wel Timber & Building Supplies can help trade customers source the carcassing, structural sheet material and roof components needed to keep the job moving, with local delivery across Hertfordshire and nearby London areas.
Before the roof deck is fixed, put a level across the firrings, confirm the outlet is genuinely the lowest point and check every upstand still has enough height for the chosen membrane. That ten-minute check is cheaper than returning to a roof after the first heavy rain.
