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. 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. This applies to general roof areas along with any internal gutters.

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Some Bauder waterproofing systems are BBA approved for use with zero falls but back falls are not acceptable and should be corrected. In order to ensure a finished surface with zero fall, a design fall of 1:80 should be used and a detailed structural analysis should account for construction tolerances, settlement and for deflection under load. Where areas are found by a site level survey to have negative falls, i.e. will hold water, remedial action should be taken, e.g. localised screed or additional rainwater outlet.

To prevent ponding caused by lap build-ups around rainwater outlets, these should be recessed or fitted in sumps, where practical. Particular attention should be paid to the controlled disposal of rainwater from green roofs, designed to support planting, and blue roofs, which are designed to attenuate the drainage of stormwater from the roof.

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.