Ventilation is the systematic removal of smoke and heated fire gases from a burning structure. As the first article discussed, ventilation is performed in four basic situations. These situations include fire control, fire attack, rescue and overhaul. Firefighters have a choice of basic ventilation types to accomplish the objectives of these situations. The basic types are vertical, horizontal, and forced ventilation. The focus of this article will be vertical ventilation and the advantages, disadvantages, resources necessary, and the most effective application will be examined.
Vertical ventilation is the process of allowing heat and smoke to travel upwards and out of a structure. This is performed by cutting vents or making use of existing openings in the roof of the structure. To accomplish vertical ventilation effectively requires a large number of resources. At a minimum, these resources include two well-trained and fully protected firefighters, cutting equipment, ladders, and a charged line. Once these resources have been assembled and moved to the roof, vertical ventilation normally involves first opening existing openings and then making roof cuts as close as safely possible to the seat of the fire. Firefighters making roof cuts should remember that one large vent is more effective than several small vents and once vertical ventilation is completed, return immediately to the safety of the ground.
The advantage to vertical ventilation is found in basic fire behavior. Due to convection, the first choice of heat is to travel straight up. Vertical ventilation allows this natural movement to occur and is, consequently, the most effective form of ventilation. When vertical ventilation is properly performed, it can greatly reduce the mushrooming of gases and improve the conditions inside the structure. Unfortunately, there are a number of disadvantages to vertical ventilation. First, firefighters are subjected to all the dangers of being on the roof of a structure. These include structural collapse, disorientation, and falls. Second, vertical ventilation takes time to perform and is often impractical. Many roofs are extremely difficult to breach and fires not on the top floor may see little benefit to the operation. Third, as described above, vertical ventilation demands a large number of resources. Most departments do not initially have the firefighters necessary to perform vertical ventilation in the early stages of an incident. Even though these disadvantages exist, there are times when vertical ventilation is preferred.
When departments have adequate personnel, vertical ventilation and other operations can be performed simultaneously. Departments that do not have these resources must still recognize when vertical ventilation can assist the operation. Of the four situations discussed, fire control is most effectively accomplished with vertical ventilation. Fire attack, rescue, and overhaul have other ventilation options that departments with limited personnel can perform. The objective of fire control is to stop the horizontal spread of the fire and vertical ventilation makes this possible. This is especially true with attic fires and fires in balloon construction. Although it will often be too dangerous to vent an attic fire, fires in balloon construction can benefit from early vertical ventilation. Fires that have penetrated into the walls of such structures will quickly travel to the attic. From there, the fire will rapidly spread throughout the structure. If vertical vents have been placed early, before the fire has weakened the roof, the effects of fire spread will be reduced.
Original article found at: http://www.hcdps.com/fire101/9_01.htm