Minimum Fall For Flat Roof

Cold roofs and warm roofs.

Minimum fall for flat roof.

In general the minimum slope for a flat roof is 1 0 but as stated earlier this is only possible in theory. The minimum pitch for a roof is 1 4 12 which translates to 1 4 inch rise to 12 inches of run. There are two types of flat roof. This is the basis for calculating the angle which is approximately a 1 19 degree slope.

This applies to general roof areas along with any internal gutters. Since they vary in quality as well as method of installation the choice preferred for a flat roof will ultimately have an effect on the final slope of the flat roof. The short simple answer is 2 12 has traditionally been considered the absolute minimum acceptable roof slope suitable for asphalt shingles. My flat roof is 1 7 degrees.

As the single ply roofing association spra points out this type of flat roofing is strong durable completely sealed and usually lasts at least 25 30 years. It takes the manufacturer s recommendations the unique conditions of the roof as well as the skills and the expertise of the installer to arrive at the right slope. According to the international building code the typical built up flat roof that uses tar or asphalt goes by the guideline that for every foot 12 inches of a flat roof a minimum of inch must step up or down. I calculate that to be 0 7 degrees.

The boards should be laid onto a vapour control layer and be covered with the waterproofing system. The recommended minimum fall for a flat roof is 1 80 0 72. However you can only use this pitch with built up roofing or specialized synthetic roofing. As opposed to old built up felt roofing with joins new single ply waterproof membranes are now most commonly used.

This applies whether the waterproofing is a polymeric single ply membrane a reinforced bitumen membrane or another type listed in bs 6229 2018 the code of practice for flat roofs with continuously supported flexible waterproof coverings. According to bs 6229 bs 8217 flat roofs should be designed with minimum falls of 1 40 to ensure a finished fall of 1 80 can be achieved allowing for any inaccuracies in the construction. Good practice guidance recommends that flat roofs defined as 10º should not be completely flat and should be designed with adequate finished falls to ensure the completed roof drains freely. I read that an edpm rubber roof requires a minimum fall of 1 in 80 in order to be able to shed water.