In the 1960s, a large number of home extensions had flat roofs. This type of roof generally comprised a timber or chipboard ‘deck’ overlain with three layers of felt bonded together with bitumen. This type of roof is nowadays known as the cold deck flat roof and is
highly prone to internal condensation problems. Water vapour condenses inside the flat roof void staining the plasterboard ceiling and dampening the flat roof joists and making them prone to rotting. It is worth noting that cold deck flat roofs are no longer recommended
by NHBC Standards. The reasons given are the difficulty in providing
(1) effective vapour control layer at ceiling level (2) required level of ventilation (3) unobructed 50mm ventilation over insulation (4) ventilation at both ends of each joist void.