Cold roofs

Until perhaps the 1970s, this was a common approach to the design of flat roofs. The diagram below shows the insulation placed immediately above the ceiling and between the joists. A vapour control layer (below the insulation) and adequate ventilation of the void (above the insulation) is necessary in order to prevent moisture vapour condensing on the colder timbers.

In practice, achieving adequate (cross) ventilation is difficult with many flat roofs and so this form of roof is rarely used nowadays.

Note: firring pieces are timbers cut at an angle to produce the fall. Reflective chippings are used in order to reflect ultraviolet light from the sun because this would otherwise cause degradation of the waterproof covering.