In todays modern buildings, humidity can penetrate into the roof space, posing a threat to the roof structure from moist air forming condensation under the waterproofing layer, leading to the development of mould, rot or blisters. Moisture entering the roof space via defective waterproofing can also create these same problems. Cold roofs need to be ventilated so as to avoid the threat of this kind of damage. On unventilated warm roofs, with bituminous waterproofing membranes, moist air entering the roof space can lead to blisters forming on the waterproofing membrane. Eventually, these may crack or split, leading to damage of the deck and sub-structure.