Where required, falls of not less than 1 in 80 must be provided for water to clear the roof. To allow for normal construction tolerances and achieve a finished fall of 1:80, a design fall of at least 1:50 (2.0%) is required, if freedom from ponding on the finished roof is to be achieved.
Falls may be formed in the structure or can be created within the specification above the deck. Falls in the structure can be achieved by adjusting the height of supporting beams or purlins, by using tapered supports or by the addition of firring pieces before the deck is laid. The latter method is normally used with decks such as woodwool, timber, precast concrete and metal decking. In the case of an in-situ cast concrete slab, falls are normally provided by the use of a screed.
Preformed tapered insulation boards can also be utilized to form drainage falls. A tapered insulation scheme can provide a practical alternative to screeding, particularly on roof refurbishment projects.