Fire Extension

Typically, the preferred life-safety tactic is to extinguish the fire rapidly, protecting the occupants and preventing or limiting fire extension to other areas of the involved building or to external structures. The likelihood of a fire spreading to exposures greatly increases when the fire has reached an advanced stage by the time firefighters arrive and when the fire is in a large-area compartment where it cannot be immediately extinguished. Even when the fire is quickly brought under control, however, it will often have extended into concealed spaces and other areas of the building.

We define two types of exposures: internal and external. Internal exposures are areas in the structure where the fire began or in buildings directly connected to the building of origin that were not involved in the initial fire ignition. External exposures are buildings, vehicles, or other properties outside the building of origin.

The most effective way to protect internal exposures is to advance hose lines into areas where the fire is most likely to spread while making certain all doors to adjoining rooms and exits are closed. Most often, fires spread upward. For example, basement fires can extend through wall spaces into the attic. For this reason, a crew should always check the attic with a hose line on hand. In multistory buildings, placing a hose line on the floor immediately above the fire floor is a good practice.

Sometimes, fire will travel along unpredictable pathways, and ventilation crews must be aware of this. Improper ventilation can cause a fire to spread into internal or external exposures, while proper ventilation will pull the fire, heat, and toxic gases away from uninvolved areas.

Thermal imaging cameras can be invaluable in checking concealed spaces in the building of origin and attached structures. If there is any doubt about fire entering a concealed space, however, that space must be opened and inspected. For offensive operations, the water supply must be able to supply the necessary rate of flow plus backup lines, as well as other hose streams used to protect exposures.

If the incident commander's (IC's) size-up indicates that a defensive operation is appropriate, operations are begun from the exterior. When exterior exposures are threatened or involved in fire, master streams are usually the streams of choice. In most cases, options for protecting internal exposures during a defensive attack are limited, so the primary strategic objective is to protect external exposures while extinguishing the main body of fire.

The most effective way to protect exposures is to extinguish the main body of fire. Because rapid extinguishment is not always possible, however, several other variables must be considered. For instance, the distance between exposed structures and the volume and location of the fire have much to do with setting the priorities for exposure protection. In addition, radiant heat increases as the size of the flame front increases. Therefore, knocking down the main body of fire can reduce the flame front, thus reducing radiant heat energy.

Setting priorities for exposures involves many factors, including fire hazard to occupants in the exposure; proximity to the fire building; wind direction; the height of the exposure compared to the level of fire involvement in the fire building; and the hazard presented by the exposed occupancy, such as explosives or chemical storage.

The IC must determine the best method of protecting potential exposures. Available staffing, water supply, and apparatus availability will greatly affect the IC's decision.

Radiant heat will travel through transparent materials such as water, so directing a water stream between buildings is less effective than applying water directly to the exposed building. Class A foam, especially the compressed-air type, tends to cling to exposures and do a better job of pre-wetting surfaces. When evaluating exposures, the IC must also consider the location of fire apparatus and equipment parked near the fire building.

An offensive attack is preferred whenever it can be conducted safely and effectively. However, defensive attacks are sometimes the only safe way to contain a large, uncontrolled fire or a fire in a building whose structural integrity has been seriously compromised. The key to making the right decision is a good risk-versus-benefit analysis.