Roof Furring Strip Calculator

How the roof furring strip calculator works

Furring strips (called battens or counter-battens in much of the world) are the thin lengths of timber fixed across rafters, studs or sheathing so that metal roofing, tiles, board-and-batten siding or rainscreen cladding has something straight to fasten to. This calculator counts how many you need. It turns the surface area into a total length of strip, adds your wastage allowance, divides by the length of one strip and rounds up.

The division by spacing is the step that trips people up when they check the maths by hand. Strips at 16 in centres each look after 16 in (1.333 ft) of surface, so 215 ft² needs 215 × 12 ÷ 16 = 161.25 ft of strip. Add the default 10% and that becomes 177.38 ft. Divided by 8 ft strips it is 22.17, which rounds up to 23 strips, or $92 at $4 each. The metric side runs the same logic in metres: 20 m² at 400 mm centres is 20 ÷ 0.4 = 50 m, then 55 m with 10% added, and 55 ÷ 2.4 = 22.9, so 23 strips at £3 comes to £69. That is why the formula on this page converts the spacing to feet (÷ 12) or metres (÷ 1,000) before anything else happens.

One simplification matters on site. Area ÷ spacing counts the gaps between rows, not the rows, so every surface comes out one row short. Take a roof slope 12 ft from eaves to ridge and 20 ft long, battened at 24 in centres. The area method gives 240 ÷ 2 = 120 ft of strip. Count the rows instead and you get six spaces, seven rows and 140 ft. The missing 20 ft is the top row. The fix is to add one row of area before you type it in: 20 ft × 2 ft = 40 ft², so enter 280 ft² and the run comes out at 140 ft. In metric, a 3 m × 6 m slope at 600 mm gives 30 m by area and 36 m by rows; entering 18 + (6 × 0.6) = 21.6 m² closes the gap.

Wastage for furring strips

The default is 10%. Most furring waste comes from end cuts. Strips have to be joined over a rafter or stud, so a row that doesn't divide neatly into strip lengths leaves an offcut, and hips, valleys, windows and doors add angled cuts on top. Plain walls and simple gable roofs can run at 5%. Hip roofs, dormers, lots of openings, or a delivery of bowed timber that has to be culled should push you towards 15% or more. The wastage figure isn't the place to cover the missing top row described above; correct the area for that and keep wastage for genuine offcuts.

Strip length changes the waste as well. Choose a length that is a whole multiple of the framing centres and every joint lands on a support: 8 ft spans exactly 6 bays at 16 in or 4 bays at 24 in, and 2.4 m does the same at 400 mm and 600 mm. A 10 ft strip on 16 in framing is 7.5 bays long, so each one gets trimmed to 9 ft 4 in and the rest is scrap.

What this tool does not do

It counts timber. It doesn't size the strips, choose fixings, check that the roof or wall can carry the cladding, or tell you what spacing your cladding needs. Spacing for metal roofing comes from the panel manufacturer's installation guide, and it usually tightens as panels get thinner and as design snow and wind loads rise. Tile battens follow the gauge in the tile manufacturer's literature. Siding and rainscreen systems publish their own maximum support centres. Enter the figure from those documents, not a rule of thumb.

It also leaves out fixings, counter-battens (a second layer running the other way, common under tiles and behind vented cladding), membranes, flashings and trims. Those are separate take-offs. Where the strips carry snow, wind uplift or heavy tiles, have the spacing and fixing confirmed by the cladding manufacturer or a structural engineer.

On-site considerations for roof furring strips

Battening a roof is work at height. In the United States, OSHA's 29 CFR 1926.501 requires fall protection for construction work 6 ft (1.8 m) or more above a lower level. In Great Britain the Work at Height Regulations 2005 apply wherever a fall could cause injury, and the HSE's work at height guidance sets out how to plan for it. Other countries have their own rules, so check the local ones before anyone goes up.

Snap a chalk line for every row rather than measuring off the row below; small errors add up across a slope, and metal panels and tiles show a wandering batten badly. Fix into the rafter or stud, not only the sheathing, unless the cladding system says otherwise, and stagger the joints so they don't all fall on one rafter. Behind wall cladding, furring also creates a drainage and ventilation gap. Building Science Corporation's BSD-013 on rain control explains the reasoning: a gap of around 6 mm (1/4 in) acts as a capillary break, and a continuous air space lets water drain away and the back of the cladding dry. Vertical strips drain freely. Horizontal ones behind cladding need breaks or counter-battens so water doesn't sit on their top edge.

Codes and compliance

Building codes rarely regulate furring on its own. They regulate what it holds: the roof covering, the wall cladding, the weather barrier behind them and the fixings that resist wind uplift. In the US that means the IRC or IBC as your jurisdiction has adopted it; in the UK, the Building Regulations alongside BS 5534 for slating and tiling; elsewhere, the national code. Manufacturers' installation instructions often form part of compliance, so a spacing wider than they allow can void the warranty and fail an inspection. If the job needs approval, ask the local building department or building control body before you order.

Before you order

Check the real stock sizes at your supplier. Nominal 1×3 furring in the US measures about 3/4 in × 2-1/2 in, and 1×4 about 3/4 in × 3-1/2 in. UK roofing battens are sold in sawn sizes such as 25 × 38 mm and 25 × 50 mm. Lengths vary by market and by yard, and if your supplier only stocks 3.6 m or 4.8 m lengths the count changes, so re-run the calculator with that length. Sight along a few strips before accepting a delivery. Bowed or twisted furring is hard to use on a roof, and every cull comes out of your wastage allowance. Use treated timber for roof battens and for furring behind cladding, since both can get wet.

Adjusting the defaults

The US side opens at 215 ft², 16 in centres, 8 ft strips, 10% wastage and $4 a strip. The metric side opens at 20 m², 400 mm, 2.4 m, 10% and a price of 3 a strip, shown as £3 in the UK. Treat them all as placeholders. Measure the surface you are actually battening (for a roof, the sloping area, not the plan footprint), take the spacing from your cladding instructions, the length your supplier stocks and the price from a current quote. On the metric side the currency symbol follows your browser's region (pounds, euros, Australian or Canadian dollars), and it falls back to pounds elsewhere. If the symbol isn't yours, enter your local price per strip anyway: the count and the total still come out right, and only the symbol is off.

Using this roof furring strip calculator alongside other BuildMetricLab tools

A furring count is one line of a roofing or cladding estimate. The roof pitch calculator gives the rafter length (its hypotenuse) from rise and run, and that length multiplied by the length of the roof is the sloping area to enter here. The underlayment calculator covers the membrane under the strips, and the shingle calculator handles a shingle roof. Everything runs in your browser. Nothing you type is sent anywhere, and the formula sits on the page so you can check the arithmetic yourself, exactly as the worked examples above do.