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A method of smoke control has been advocated in which smoky gases generated by a fire are extracted at ceiling level from the layer they form there because they are buoyant. However too high an extraction rate at a given point will draw up air from underneath the layer into the extraction duct and this will markedly reduce the actual amount of smoky gases removed. This note reports experiments showing that the maximum extraction rate before air is drawn up decends mainly on the layer depth and temperature and is not sensitive to the area or shape of the extraction opening over the range of areas of major practical importance. An expression, derived from large and small-scale experiments, is given for this maximum extraction rate. In practice, to achieve a rate of removal of smoke equal to the rate at which a fire is producing it, extraction at a number of well-separated points may be necessary. A very simple expression has been derived from this work for the maximum size for a vent in the form of a simple opening in a flat roof, if entrainment and hence inefficient extraction are to be avoided.