Flat roofs can be a continuing problem with damp penetration and short lives. Illustrated here are three examples; one of a flat roof with no laid incline, one with ponded rainwater due to inadequate incline and one with a slight pitch to the roof. The roof with chippings has moss growth and poor run off for water. In contrast the roof with some incline allows the water to run off .
Ponding of rainwater on a poorly laid roof can lead to short term erosion of the felt and damp problems.
Its best to get Chiltern Associates to perform a building survey and assess all of the flat roofs on your proposed purchase so you know for the future.