Minimum Fall For Flat Roof

Cold roofs and warm roofs.

Minimum fall for flat roof.

The minimum pitch for a roof is 1 4 12 which translates to 1 4 inch rise to 12 inches of run. I calculate that to be 0 7 degrees. Proprietary preformed tapered insulation boards can provide drainage falls to a warm decked roof manufacturers recommend falls not less than 1 60. 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.

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. I read that an edpm rubber roof requires a minimum fall of 1 in 80 in order to be able to shed water. Asphalt shingles continue to be the most widely installed roof covering option in north america. As opposed to old built up felt roofing with joins new single ply waterproof membranes are now most commonly used.

The boards should be laid onto a vapour control layer and be covered with the waterproofing system. I was thinking to also install a sheet of polycarbonate on the roof or even better perspex glass like window onto the sky. 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. In general the minimum slope for a flat roof is 1 0 but as stated earlier this is only possible in theory.

My flat roof is 1 7 degrees. 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. 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. There are two types of flat roof.

The short simple answer is 2 12 has traditionally been considered the absolute minimum acceptable roof slope suitable for asphalt shingles. 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. This applies to general roof areas along with any internal gutters. However you can only use this pitch with built up roofing or specialized synthetic roofing.