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October 2004: Smoke Vents

Roof vents were originally used as an aid to fire fighting in unsprinklered buildings. When a fire occurs, the smoke and hot gases rise until they reach the ceiling. They then spread outward and down towards the floor level, making manual fire fighting difficult. Roof vents, helped by draft curtains, can sometimes relieve smoke accumulation in unsprinklered building by containing the spread along the ceiling and by permitting the smoke to escape. The smoke rises through the vent because it is much hotter than the outside air. If this difference in temperature decreases the rate of smoke removal will also decreases.

Vents in sprinklered buildings can have a neutral or negative effect by increasing burning intensity and spread. Passage of hot air and smoke through the vents causes fresh air to enter the building through other available openings, resulting in greater fuel consumption and an increased water demand. Sprinkler water also absorbs much of the heat from the gases, cooling them and thereby decreasing the rate of smoke through the vents.

Since vents are seldom beneficial in achieving fire control, their use in sprinklered buildings is best limited to situations in which their benefits outweigh their detrimental side effects. Such situations include concentrations of flammable liquids where fire partitions are impractical.

Venting, if required by the authority having jurisdiction, should be arranged so that the operating device, normally a fusible link, should be rated to operate at one temperature rating higher than the temperature rating for the sprinklers in the area. This will allow the sprinklers to gain control of the fire. The provision for manual operation of the vents should be in place to allow the fire department to operate them once the combination of sprinklers and manual fire fighting has gained control of the fire.

If you would like further information regarding the use of roof vents in sprinklered buildings, please contact Risk Logic Inc.