Between 1920 and 1940, a popular floor material was gaining headway in many commercial buildings because of its many designs, ability to regulate heat/cool temps, limited maintenance, and sound insulating ability. You often see this flooring in old banks, hospitals, schools, drug stores from the past, museums, and more. Terrazzo floors were and still are prominent features in many buildings. Firefighters became aware of this floor material, as we usually do, when a catastrophic event brought it to our attention. The 23rd Street Fire in New York City on October 17th, 1966, 12 Fire Department of New York (FDNY) firefighters were killed when a floor collapsed into a burning cellar beneath the Wonder Drug Store. This building utilized a 5-inch concrete slab with terrazzo tile and wood floor joists. Fires like this are important to us because the brothers must not die in vain—we must use this event to educate future members. When it comes to building construction, the issues posed by terrazzo floors are very important. Specifically, when masonry materials are used in conjunction with lumber, we have be mindful of the deadly prescription for collapse. Ordinary (Type III) construction comes in many forms and is one of the most common buildings you will encounter. Today, the use of concrete slab on wood floor joists becoming more and more popular without much notice from the fire service.

First, let’s discuss the properties of concrete used in flooring systems. We know concrete floors are composite, meaning they use two different materials to sustain strength. Today, in Type I (Fire Resistive), Type II (Non-Combustible), and Type V (Combustible) used as a slab, concrete is poured with steel infused, usually in the form of rebar. Concrete has excellent compressive strength, however it lacks tensile strength. If concrete (Portland cement, sand, water, and aggregate) did not have steel rebar, pre- or poststressed cable, or grid work, it would crack and ultimately collapse from underneath. The steel within concrete gives it tensile strength. The result is a composite structural element called reinforced concrete. Many earlier buildings used wood to provide the tensile strength from underneath. (1).

As previously mentioned, the most notable fire involving the wood-concrete floor is New York City’s 23rd Street Fire. The floor of the Wonder drug store (ordinary construction) utilized 3 x 14-inch wood beams, 3/4-inch wood decking atop these beams that had five inches of concrete finished with terrazzo.  As you may know, concrete has excellent insulating properties, as does terrazzo. In the 23rd Street Fire, an advanced fire condition within the cellar burned for more than an hour. The heat condition was concealed by the advanced fire, which left firefighters unknowingly operating above a raging cellar. When the large floor joists burned through, this eliminated the tensile strength of the concrete and terrazzo tiles, leaving the weight and loads of the firefighters and all of the drug store’s contents acting upon it. The floor had no chance. Again, as previously mentioned, hospitals, schools, banks, drug stores, and museums constructed between 1900 and 1940 and use terrazzo tiles are likely to have wood tensile supporting elements. It is imperative that below grade is checked during ANY assignment at these locations. It has been observed that steel structural elements have been inserted during renovation or reconstruction.

Getting to present day, concrete slabs are becoming very popular again used in conjunction with wood floor joists and decking. Can you guess where? Private dwellings! Do you enjoy a toasty floor on cold mornings? Do you hate walking around your home on cold ceramic tile? Well do contractors have the solution for you! Radiant floor heating systems. The explosion of this renovation or improvement to Type V (Combustible) residences has brought a new element into an already dangerous atmosphere when fire is present. The introduction of an elevated concrete floor supported by dimensional wood floor joists and plywood is a hazard that many firefighters are not aware of. (2-5). There are no signs that a tile floor has radiant heat from the surface or via a warning decal as many districts have with truss systems. Firefighters should be aware that the addition of two to three inches of concrete or mud floors with tile above and water running within is a recipe for disaster should a below grade fire be present.  Fire and heat can go unnoticed for some period, burning away the supports from underneath. The addition of two or three firefighters stomping around on the floor above could mean collapse. Was your wood-frame home, built in 1925, designed to withstand that added load? A 10 x 10-foot room with 2 inches of concrete adds more than 2,200 lbs to the floor system. No matter which way you try to justify it, when the floor joists burn, global collapse without warning is a significant possibility. Firefighters may be unaware of the intense heat and damage caused by fires beneath them due to this. It is imperative to check below grade first, especially when tile floors are found throughout. The collapse of these floor systems can leave firefighters taking the express into a burning cellar.

The use of concrete and wood floor systems is not a new thing, but it is again becoming a popular thing. History tells us the use of these floor systems cause catastrophic failure and delay warning signs that are evident to fire firefighters. Whether it is the appeal of an Art Deco terrazzo floor or the ability to have radiant heated floors by the flip of a switch, the addition of concrete that relies on wood members solely for its support is troubling, to say the least. Firefighters must be aware of what they are standing on and recognize when there is potential for such systems being used. When fire is found to be beneath one of these floor systems, evacuation is a must. Communication must be made to the incident commanders and members should not be operating above.

SALVATORE ANCONA is a deputy chief fire instructor at the Nassau County (NY) Fire Service Academy; a lieutenant with the Seaford (NY) Fire Department; a fire inspector with the North Bellmore (NY) Fire Department; a former captain and training officer for the Bellmore (NY) Fire Department; and a former paramedic supervisor in New York City. He has a bachelor’s degree in fire and emergency services administration from John Jay College of Criminal Justice, an A.S. degree in fire science from Nassau Community College, and is attending Georgetown University for his masters in Emergency & Disaster Management. Ancona is the author of the building construction page “The Sons of Brannigan” on Facebook and was a recipient of the 2019 FDIC International Honeywell DuPont Scholarship.