In the initial stages of a fire heat, smoke and hot gases rise to the ceiling or roof space, then spread and form a layer which accumulates downwards. Early automatic opening of roof ventilators is encouraged to continuously remove the heat and smoke rising from the fire in an attempt to achieve a point of equilibrium at which point the production rate of hot gases is equal to the rate of discharge through the ventilators. The objective being to provide a clear layer of good visibility near floor levels permitting safe and orderly evacuation of people and to enable fire fighters to see and tackle the blaze in addition to reducing temperatures, prevent from flash-over and the risk of building collapse. The smoke and heat ventilation system is usually activated by a smoke and heat detection system.