In the robust wood-frame construction, especially in the middle of the 20th century when wood-frame platform construction came into use, fires infrequently breached the interior compartments exposing the building's structure. If it did it generally stayed within one wall stud or ceiling/floor joist channel. Not only was there more mass to resist the heat and degradation to the building's support system, but the closed joist and wall stud areas of platform construction afforded natural fire stopping.
What we now refer to as lightweight construction does not offer the same inherent resistance to fire. We became so used to the paradigm of an aggressive interior attack and a quick stripping of the sheetrock or plaster that it became hard wired in our culture. This went on and has actually been in place now for as many as five generations of firefighters.
Let me also mention here and now that exterior finish on these buildings was highly resistant to fire exposure and wooden decks had not yet become popular, at least in the east where a Levittown link patio of concrete or stone was most common. But in the late 70's and more the early 1980s residential construction began to change.
New Materials Used In Construction
In the 1980s we began to see a shift in construction materials for residential buildings. As mentioned earlier there was a desire by developers and builders to increase efficiency and profit and a reduction in the number of the forests formerly used to supply the nation's lumber. With more imported wood, the cost was increasing and economy dictated that innovation be employed to save resources and reduce costs.
This in some ways was probably a sense of urgency that could provide a powerful shift to more engineered products and the increased use of plastics, fiber board, and the products that were specifically designed and manufactured to have just enough load carrying capacity to meet the needs of a structure.
We began seeing dimensional lumber less frequently to carry spans of open space. More often we began seeing trusses with webs of 2-by-3 lumber and I-beams that were nothing more then 2-by-3 flanges on top and bottom and a web of 3/8-inch or 1/2-inch in plywood and composite material. But here was the beginning of change. The construction methods using plywood I-beams that have cutouts for utilities and floor trusses made out of 2-by-3's are actually the complete inverse of the platform wood-frame construction with dimensional lumber. One characteristic that these structural members present is a large combustible void space which was nearly non-existent in platform frame construction.
The voids create effective burn chambers, with plenty of oxygen, maybe even some air currents, and combustible fuel surface area. Add to the inherent combustible nature of the voids, the adhesives for all sorts of joining and connecting of structural members and you can have a violent production of hot flammable gas that moves ahead of the fire providing two sides of a fire triangle on steroids only waiting for the introduction of enough oxygen to flashover in the space. Now I want to get more specific about the danger and need for different tactics.
We are starting to have clear evidence that fires in lightweight residential construction are presenting an even greater safety risk than originally envisioned. The fires in these structures are behaving more violently. The materials are burning fiercely and very efficiently putting off higher amounts of heat and flammable gas. To the well protected firefighter they not even detect that there is a problem because they are unaware of what is going on behind the walls. Then instantaneously, the conditions move from the structural voids to the compartment trapping a firefighter in pure hell.
Separate fires killed a firefighter in Prince William County, VA, and seriously injured a crew in Loudoun County, VA. In both cases, fires started on the exterior wooden deck of a single-family home and gained easy access as it exposed and destroyed the siding; entered into the structural voids of the house then burned violently on the readily available combustible materials.
As mentioned above, I am only looking at one scenario involving lightweight construction -- fires that either enter a void from a hostile exterior fire or one that otherwise is able to start directly in or enter voids. We can no longer count on the stability of structural members like we used to. With the significant heat output and unburned, ready to ignite gases produced by modern fuel types and fire loading we can expect a different kind of hostile fire that should be assumed to be in the structure and not contained by the passive fire protection of compartmentation.