Sawn Timber Firrings creates a fall on a flat roof system. Typically these would be nailed onto a ceiling or roof joist to create a sloped roof. Despite its name, a flat roof (at least according to building regulations) should have a slight slope to reduce "ponding" or pooling of water on the roof. Building regulations encourage a minimum fall of 1:40. The minimum finished fall required according to BS6229 is 1:80. Each roofing material has a minimum recommendation, which should be specified by either the architect or the roofing product manufacturer.

A quick way to measure the correct size firring is to use the following formula:

• Length of run divided by fall.
• For a 3600mm roof, 1:40 fall (Building Regs) - Take 3600mm divided by 40 = 90mm Fall Required
• For a 3600mm roof, 1:80 Fall (BS6229) - Take 3600mm divided by 80 = 45mm Fall Required

90mm fall over 3600mm would be in line with building regulations 1:40 fall. Note: not all firrings need to fall to 0mm. A 100mm to 10mm timber firring would be suitable for use as a 90mm drop. Some roofers may specify a thicker end firring (i.e 100mm to 10mm) rather than a "to 0mm" firring. 

Where a reference to building regulations or to BS6229 is made, this is based on reference material freely available and may not be the most up-to-date revision. For the latest regulations and guidelines, you should always check with the relevant government department or professional body.