Construction Concerns: Supervisors and Concealed Spaces

Article and photos by Gregory Havel

Emergency services supervisors are assumed to hold these positions because they are better trained and better educated than the majority of their organization members. We can hope that they have the fortitude to assume the role of “designated grown-up” when things become chaotic at the station or at the scene of an emergency. These statements should be true at all levels, from team leader or company officer through chief officers, including the chief of the department and incident commander. 

In firefighting, the great majority of our fires involve a single room and contents in residential occupancies. The operating procedures we use at these room-and-contents fires can become so habitual that we use these same means and methods when responding to residential fires that have extended beyond the room of origin and at fires in commercial and industrial occupancies. In both of these cases, these methods are likely to be inadequate and inappropriate.

 

The key to fires that extend beyond the room of fire origin is the “concealed space.” These are often overlooked when the fire has extended into the hallway and up the stairway, simply because they are concealed. We don’t see them, so we don’t think about them enough when we are excited by the firefight.

 

The most experienced supervisors at every fire will keep a clear view of the “big picture” rather than focus on only one aspect of the operation. They will assume during ongoing size-up that every room is surrounded by concealed spaces, that firestopping is absent, that all of the building’s concealed spaces are interconnected, that the concealed spaces are combustible, and that fire will soon break into the concealed spaces if it has not already done so. If some of these assumptions prove to be untrue at an incident, it is a good day for everybody.

 

There are two types of concealed spaces (also called voids) in buildings: combustible and noncombustible. Noncombustible concealed spaces are enclosed by materials that do not provide fuel to fires, such as concrete, masonry, gypsum, steel, and asbestos. Combustible concealed spaces are enclosed on at least one side by materials that burn, like wood and plastics.

 

The building and fire codes classify buildings based on the amount and location of combustible materials used. (For details on these types of construction, see National Fire Protection Association (NFPA) 1 Fire Code; NFPA Standard 5000 Building Construction and Safety Code; NFPA Standard 220 Types of Building Construction; or the International Building Code (IBC); or your building construction textbook.) The only type of structure without concealed spaces is Type IV (heavy timber or mill), because it is designed to have none.

 

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Combustible concealed spaces (photo 1) are found in Type V (wood-frame) and Type III (ordinary or brick-and-joist) construction. These spaces may have combustible surfaces on all sides, as in a floor-ceiling assembly of wood joists, wood subfloor, and lath-and-plaster ceiling. These spaces may have some noncombustible surfaces, as in a wall assembly of wood studs covered with gypsum drywall board (photo 1). They may sometimes be found in Type II (noncombustible) construction in nonload-bearing interior partitions with wood studs. Fire extending into a combustible concealed space will spread rapidly on the combustible surfaces and can break out into rooms at a distance from the room of fire origin. These spaces can also accumulate gases and vapors resulting from pyrolysis and combustion, which have too little oxygen to burn and which can burn explosively (backdraft) when oxygen is added.

 

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Noncombustible concealed spaces (photo 2) are usually found in buildings of Type I (fire resistive) and Type II (noncombustible) construction. These include spaces completely enclosed by concrete or masonry and spaces enclosed by other noncombustible materials like gypsum drywall board and steel studs (photo 3). Although these spaces do not burn, they can accumulate gases and vapors resulting from pyrolysis and combustion, which have too little oxygen to burn and which can burn explosively when oxygen is added. They can also act as ducts to carry flame and other products of combustion to other parts of the building. 

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Ineffective supervision may overlook the presence of concealed spaces and the potential for rapid fire spread and for rapid structural collapse if the building uses manufactured wood products and trusses.

The flip side of the ineffective supervision coin is complacency. Whether our supervision is effective or not, usually nothing bad happens to us at our incidents. We get away with so much so often that we believe that this way is the acceptable procedure. Yet, there is always one incident that makes the news—the one where concealed spaces and fire behavior inside them are the key to successful incident outcome—and those factors either were not included or were not given sufficient weight in size-up and in the incident action plan.