Falls and firrings
Firring sizes, stock timber and number of firrings
- where
- firring depth at the deep end (mm)
- depth at the shallow end (mm)
- length of each firring (m)
- design fall ratio, 1 in G
- stock depth to rip two firrings from (mm)
- number of firrings
- roof width across the joists (m)
- joist centres (mm)
- total firring length (m)
Explain this equation
Sizes the tapered firrings that create the fall on a flat roof, and counts how many you need. Use it before ordering timber for a new or re-decked flat roof.
Each firring starts at d_0 deep and gains the full fall over its length L, so the deep end is d_0 plus 1000 L / G. Two firrings can be ripped diagonally from one piece, top-to-tail, so the stock needs to be d_0 + d_1 deep. One firring sits on each joist: the width divided by the centres, rounded up, plus one for the end joist.
The fall part is the same proportion as any flat-roof fall; the stock depth follows because a diagonal cut leaves the thin end of one firring against the thick end of the other.
Sense check: 4 m at 1 in 40 from 10 mm gives a 110 mm deep end, so 120 mm stock. A 3 m roof at 400 centres needs 9 firrings, 36 m in all.
Watch out: allow for the saw kerf when ripping, and check firrings run in the direction of fall, which may be across the joists rather than along them.
Sizes tapered firring pieces to give the design fall, and works out how many you need along the roof.
Enter your values to see the result.
Good to know
A firring cut diagonally from a rectangular length gives two identical tapers, so the stock depth is the shallow plus the deep end depth. Design at 1:40 (BS 6229) so the finished roof stays at 1:80 or better. Firrings normally run along each joist, so the count is one per joist space plus one.
Common mistake
Cutting firrings for a 1:80 design fall. After deflection the finished fall will be flatter than 1:80 and the roof will pond.
Limits
Firrings along the joists only. Firrings across joists, counter-battens and tapered insulation schemes need their own layout. Saw kerf is ignored.
Sources
Results are calculations from the published methods and figures shown, and depend on the data you enter. Follow the roof covering manufacturer’s fixing specification, BS 5534, BS 8000-6, BS 6229, the Lead Sheet Training Academy guidance and Building Regulations. Wind uplift and structural design need a competent designer.