Construction Concerns: Interior Void Spaces

Article and photos by Gregory Havel

At many structure fires, we assume that the fire is limited to the contents of one room, unless smoke and flames are showing at many windows or through the roof on our arrival. Although the fire may in fact be in only one room at the time of our arrival, it may take less time for it to spread to several rooms and to the structure itself than it takes for us to lay out equipment and mobilize an offensive attack, especially in today’s lightweight construction.

Photo 1 shows decorative wood box beams in a large home built in 2008-2009. The exterior of each beam features oak boards, stained and varnished. None of these beams is load-bearing. Several are incomplete wood frames with only part of the trim boards attached, while those in the foreground have been completed using oak crown molding to close the space between the drywall board ceiling and the wood of the box beams. The crown molding is about ½-inch thick, and is beveled on the edges so that it fits into the corner at a 45-degree angle. The steel column is load-bearing, and supports a steel I-beam that carries the manufactured I-joists that support the ceiling and the floor above.


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Photo 2 shows another view of the same column and box beams, from a side that has not yet been completed. Several of these box beams are only decorative, and are attached to wood framing that was screwed to the I-joists after the drywall board was installed. However, the box beam running from lower right to upper left in the photo, with the opening for the steel column, conceals the steel I-beam that supports this part of the house.


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Photo 3 shows the same features earlier in construction. The steel column and I-beam that support the lightweight manufactured wood I-joists are exposed, with wood framing attached for the future box beam. The space between the flanges of the I-beam is used to conceal the pipes for the radiant-floor heating panel above. (Note the doubled I-joists that will carry the extra weight.) The steel column and others like it will be enclosed with decorative columns of real wood, manufactured wood, drywall board, cast gypsum, or fiberglass-reinforced plastic, depending on the owner’s preference and the budget for the job.


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The interior of the box beams is an interconnected combustible void space, which is also interconnected to the enclosure for the structural steel, and through it to the void space between ceiling and floor created by the I-joists. It will make little difference in a fire if the wood of the box beam burns through first, or if the drywall board and joint-taping system fails earlier. In either case, the fire will spread rapidly through the void spaces and into the upper floor, creating an immediate structural collapse hazard. As the fire spreads, it will heat the structural steel, causing it to expand and to become weak and unstable.

If an offensive attack succeeds in controlling a fire in this house, overhaul will be labor-intensive and hazardous. Opening combustible void spaces like hardwood box beams is strenuous work, even with power tools. Pulling ceilings will expose us to a local structural collapse hazard from manufactured wood members that weakened by fire. Both of these actions can expose us to fire that is still burning beyond the reach of our hose streams.

A single-family residence of this size and construction ought to have a prefire plan in place by the time it is occupied. This should include notes on the hybrid construction (i.e., manufactured wood or wood trusses supported by structural steel), the interconnected void spaces, the presence of radiant-floor heating panels, and any other features that can lead to rapid fire spread or structural collapse.

Although prefire planning residential buildings like this may seem to take too much time, it is an investment that we will wish that we had made if a fire at this location injures or kills one of our firefighters.

Gregory Havel is a member of the (WI) Fire Department; retired deputy chief and training officer; and a 30-year veteran of the fire service. He is a Wisconsin-certified fire instructor II and fire officer II, an adjunct instructor in fire service programs at Gateway Technical College, and safety director for Scherrer Construction Co., Inc. He has a bachelor’s degree from St. Norbert College. He has more than 30 years of experience in facilities management and building construction.

Subjects: Interior void spaces, building construction for firefigher operations, construction hazards

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