Falls and drainage

It is a requirement of Building Regulation Part H that adequate provision is made for rainwater to be carried from the roof of the building.

It is generally considered good practice for flat roofs to be designed to clear surface water as rapidly as possible. According to BS6229 & BS8217, 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.

Water ponding on membrane roofs should be avoided because:

  • It encourages the deposition of dirt and leaves which can be unsightly, may obstruct outlets and /or become a slip hazard.
  • In the event of damage, the interior may suffer greater water ingress.
  • It increases the dead load and may cause progressive deflection of the deck.
  • Ice may be a slip or wind hazard during the winter months.
  • Roofs with extensive ponding require increased maintenance input.

Falls in the structure can be achieved by adjusting the height of the supporting beams or purlins, by using tapered supports, or by the addition of firring pieces before the deck is laid. In the case of a cast in-situ concrete slab, falls are normally provided by use of a screed.

Tapered insulation

Tapered insulation systems are a lightweight, convenient and cost effective alternative method of providing falls to the roof and can be used with our reinforced bitumen membrane (RBM) and synthetic single ply roof systems.


Drainage needs to be provided via internal rainwater outlets and downpipes or via external guttering systems or hoppers. Even if a roof is very small, it is recommended there are at least two drainage points in case one becomes blocked. Internal gutters linking internal outlet positions should be at least 500mm wide.

BS EN 12056-3 and the Building Regulations Approved Document Part H contain relevant design information to enable rainfall calculations to be undertaken and give design principles for gutters and downpipes. Alternatively, most drainage component manufacturers will make recommendations regarding the type, size and location of their products for any given roof project.


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