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It is good practice to design a flat roof to clear surface water as fast as possible. In fact, today it is extremely unlikely to see a flat roof installed which has not been designed with falls.
The ponding of rainwater is often seen on old flat roofs (this is one of the common GRP roofing problems). There are several problems of ponding occurring on a flat roof. First, if there is a waterproofing failure this excess water will feed through into the building. Second, it increased the dead load of the roof. And finally, it is not aesthetically pleasing.
Falls can be formed through the structure of the roof or can be formed within the specification itself above the deck. To form falls in the structure you can add firing pieces prior to laying the deck, alternatively you can use tapered supports or adjust the height of the supporting beams and purlins.
The use of firring pieces is most often used with decks such as precast concrete, metal decking, woodwool and timber. If there is in-situ cast concrete slab, falls are normally created with a screed.
Preformed tapered insulation boards can also be used in some circumstance to provide falls on a level roof deck. However, if complex falls is required preformed tapered insulation boards are probably not suitable.
The minimum fall on a flat roof should be 1:80. For this to be achieved the roof must be designed to allow for any construction inaccuracies. For example, a deflection can produce ponding on the flat roof.
To counter this a lot of designed will choose to plan for a fall of 1:40 or 1:60 in the assumption that this will lead to a fall of at least 1:80 once the flat roof is installed.
However, in some circumstances such as roofs which are screeded this causes other issues. One is the cost of the roof goes up as does the thickness of the roofing system. Here the designed should use a different method to ensure the flat roof has a fall of at least 1:80. One way to do this would be to take 1 in 80 as the finished fall and allow an adjustment for any construction inaccuracies encountered. Here you would give approximately 15mm for metal decks and 25mm for concrete roofs.
Once this has been done it must be considered how any deck deflection will impact the drainage flow of the roof. This could make drainage better or worse depending on where the deflection is in relation to the rainwater outlets.