Describes an investigation, both theoretical and experimental, into the use of roof vents to exhaust the hot gases produced by a fire in a large single-storey building. Shows how the rate of discharge of hot gases and of heat through a vent, the temperature near the ceiling and the depth of the layer of hot gases may be calculated, when the area of the fire is small and its heat output is known. The research programme was carried out jointly with Colt Ventilation and Heating Ltd.