Features Structural Training
Trainer’s Corner: Containing the fire spread

Most fires start in the contents of a structure. That contents fire, if not quickly extinguished, spreads further into the building, through the walls, common roof and or attic spaces, and even from one structure to an adjacent building.

Fire spreads by four principal means: convection; conduction; radiation; and direct flame impingement, or radiation.

In a recent series of full-scale laboratory experiments at the National Institute of Standards and Technology (NIST), it took fewer than five minutes for flames from a simulated house with combustible exterior walls to ignite a similar “house” two metres away. In fewer than five minutes, flames shattered the window of the home with the original fire, spread across the gap and ignited the exterior of the second structure. 

In a structure fire the concealed flames must be located and cut off, in addition to extinguishing the original contents fire. To do this effectively, incident commanders must know the different ways fire can spread in a structure.

If the fire grows, it spreads by four principal means: convection; conduction; radiation; and direct flame impingement (sometimes actually considered a form of radiation). Just as water and electricity seek the route of least resistance, so does fire. 

■ Fire spread
Fire and smoke spread by convection is the most dangerous and causes a large proportion of injuries and deaths. When fire starts in an enclosed space such as a building, the smoke and heat rising from the fire becomes trapped by the ceiling and then spreads in all directions to an ever-deepening layer in the entire space. This ever-increasing superheated thermal layer will eventually cause everything in that compartment to burst into flame (flashover). Do not be fooled into thinking this is a long, drawn-out process. Flashover can happen within three minutes.  

Some materials, such as metal, can readily absorb heat and transmit it to other rooms by conduction, where it can set fire to combustible items that are in contact with the heated material. Radiation transfers heat in the air in the same way that an electric base board heater heats a room. Material close to a fire will absorb the heat, and, if it’s not extinguished, will begin to smolder and then burn.

If incident commanders know the basic construction types in their communities, they have a better chance of determining how a fire may spread in those particular structures.

When the hydroelectric station in Kemano (a company town built by Alcan in northwestern B.C.) was automated, the town became obsolete. It was donated to B.C.’s fire services for fire research and training. The 40 abandoned houses filled with donated furniture and props made an ideal, full-scale experimental fire research site. It was in one of those houses, a two-story Type V building, that I discovered the “white room”. The entire second floor (three bedrooms and a bathroom) had heavy fire damage – it was fire charred and blackened. The door of the third bedroom had extensive fire damage but was closed. When I opened the door I was shocked to find an almost pristine baby’s room. The white walls and white baby crib were untouched by the fire. There wasn’t even any smoke damage. A closed door restricted the fire spread enough that it could very well have saved a life.

Some construction types burn much more readily than others. Size-up must not only consider the construction type, but also the building’s contents; both add fuel to a fire. Far too often when there is a line-of-duty-death we hear reports that the firefighters didn’t know where the fire was or that the fire spread more quickly than expected. (See sidebar.) Each construction type has its own fire-spread problems.

■ Type 1 construction
Type I fire-resistive construction was originally designed to contain fire inside the building to one floor. However, there are two avenues by which fire and smoke can spread throughout a modern fire-resistive building; the central air conditioning ducts and auto-exposure, (flames extending vertically from window to window).

Air ducts of a central air conditioning system penetrate every fire barrier in the modern Type I building.

One of the IC’s first actions at a fire inside a fire-resistive building should be to order the air system shut down.

Auto-exposure is when flames erupt out of a heat-shattered window and melt and break the glass window directly above. Once the window above is broken and falls away, flames can enter and ignite ceiling tile, curtains or furnishings. Even if the windows do not melt or break from heat, a small-concealed space between the exterior wall and the end of the floor slab can allow vertical spread of fire and smoke from floor to floor above and near a window. To combat fire spread by auto-exposure, the
IC should order an aerial master stream into operation.

A water stream directed against the exterior wall between the top of one window opening and the bottom of the window above can slow down fire spread.

■ Type II construction
When a fire occurs inside a Type II building, flames rising to the underside of the steel roof deck may conduct heat through the metal and ignite the roof covering above. After a fire has been extinguished inside a Type II building, the IC should go to the roof and examine the roof covering directly above for extension.

■ Type III construction
The major recurring fire spread problem of Type III construction is concealed spaces and poke-through holes. Concealed spaces are created by wood studs, floor joists and suspended ceilings. Poke-through holes are created by small openings for utilities. These small openings around pipes and wires allow fire to spread into concealed spaces. Flames can spread vertically several storeys or horizontally to adjoining spaces. When firefighters search for hidden fire in this type of building they should try to cut off the rising fire. For example, if they discover fire in a floor, open up the wall above it. If they discover fire in a wall, open up the ceiling. If they discover fire in a ceiling, open up the baseboards on the floor above. By doing this, convection currents of flame and heat can be cut off and revealed so they can be extinguished.

■ Type IV construction
One difference between a heavy-timber building and ordinary construction is that a heavy-timber building does not have plaster walls and ceilings covering the interior wood framework. The exposed wood timber girders, columns, floor beams and decks, if ignited in a fire, create large radiated heat waves after the windows break during a blaze. Flames coming out of the windows may spread fire to adjoining buildings by radiated heat. As the fire grows, apparatus will have to be repositioned away from the radiated heat waves. Large water supply sources must be located and master streams set up to protect nearby buildings.

Although heavy-timber construction is often called slow burning, a better term is long burning. These buildings may have self releasing floors lightly tied to columns so the floors will fall without pulling down adjacent floor sections. This is good for the building, but bad for firefighters. A collapse danger zone must be designated to protect against a building collapse. Expect the floors to collapse first and then the walls to push outward falling into the street. Note: Water curtains are of little use in stopping radiant heat. In order to protect an exposure the water must be applied to the exposure to reduce the temperature of the exposure itself.

■ Type V construction
Wood-frame (Type V) construction is the most combustible of the five building types. When sizing up a fire in a wood-frame building, the outside walls must be considered for the fire spread. Flames can spread out a window and then along the outside wood walls (in addition to the interior fire spread). To combat fire this type of fire the IC must position a hose line or master stream outside the structure in addition to the attack line inside the structure. Outside streams must stand by or extinguish exterior siding fire from spreading to adjoining structures.

In our next issue we will look at advancing the attack line.

Until then stay safe and remember to train like their lives depend on it, because they do.

(Special thanks to retired Deupty Chief Vincent Dun of the FDNY, who has so graciously given me access to his training materials.)

Ed Brouwer is the chief instructor for Canwest Fire in Osoyoos, B.C., and the training officer for West Boundary Highway Rescue. The 19-year veteran of the fire service is also a fire warden with the B.C. Ministry of Forests, a wildland interface fire suppression instructor/evaluator and a fire-service chaplain. Contact Ed at ed@thefire.ca