BY DANIEL MADRZYKOWSKI, KEITH STAKES, NICHOLAS DOW, AND HOLLI KNIGHT
THE METHODS AND MATERIALS used in the construction industry are continually evolving. Over the years, we’ve seen some distinct and significant changes in residential construction. One of the most notable examples was the change from balloon-frame construction to platform construction. With regard to fire spread, balloon-frame construction provided vertical void spaces that extended from the basement to the attic. These vertical voids enable a fire in the basement to spread rapidly to the attic through the walls of the structure. In fact, from a size-up perspective, you may see smoke or fire from the attic, which might lead you to the mistaken conclusion that the origin of the fire was in the attic. If you knew that this was a neighborhood with balloon-frame construction, you would know to check the basement before heading up. This construction method was one of the reasons behind what became known as the lowest-level check during size-up and initial fireground operations.
The fire service knew that size-up cues could mislead firefighters to operate in areas of the structure above a potential unconfined basement fire. Platform construction, which replaced balloon-frame construction, adds a measure of fireprotection because each respective floor assembly provides a natural horizontal fire stop in the wall cavities. This blocks the floor-to-floor flow paths in the walls and prevents rapid vertical fire spread.

Another example of change in the construction industry was the transition from using solid, dimensional lumber to using lightweight, engineered lumber in the construction of the floor assemblies. Early platform construction used solid, dimensional lumber as the structural supports and solid, lumber planks for the subfloor. This evolved to the use of nominal dimensioned lumber to support the floor. In other words, a nominal dimensioned “2 × 12” actually measures 1½ inches by 11¼ inches. The wood member measures 2 inches × 12 inches prior to being dried and planed. Then it is made available for purchase and use in construction.
The next evolution in platform construction was to use engineered wood sheet material to replace solid nominal, dimensioned 1-inch × 6-inch wood boards for the subfloor. Two common materials currently in use are tongue-and-groove plywood and oriented strand board (OSB), typically 23⁄32 or 3⁄4 inch thick. OSB is generally more affordable than plywood, so it is widely used. In 1969, the lightweight, engineered lumber I-beam was invented to replace solid lumber joists in residential platform construction.1 Originally made from solid lumber flanges and plywood webs, now the flanges may be composed of laminated veneer lumber (LVL) and the webs may be composed of OSB. Homes built since the 1980s are likely to have floor assemblies constructed from lightweight, engineered lumber supports and plywood or OSB for the subfloor.

These changes in residential, woodframe building construction have led to changes in how fires grow and spread throughout these structures, so the firefighting tactics for size-up and fire attack must also evolve. The most recent data from Project Mayday indicates that 2,853 Maydays were reported as “Falls into Basement/Trapped.” Approximately 40% of these Mayday incidents were reported to have involved residential floor collapse.2
UL’s Fire Safety Research Institute (FSRI) searched online articles for current trends. Based on news reports of fires in the United States, the search showed that in just a three-month period, from October1-December 31, 2023, reports identified fire incidents where 22 firefighters fell through a floor and were injured.

The Research
FSRI has conducted several series of experiments where residential platform floor systems were exposed to fire.3, 4 FSRI has also studied the impact of ventilation on below-grade fires.5 The findings of these studies show how rapidly lightweight construction systems can lose their structural integrity when exposed to heat or fire, even for a short duration. Table 1 shows that the lightweight engineered lumber joists and trusses, with a 65% design load and a 20- foot span, collapsed within five minutes of the fire spreading to the components of the floor assembly. Under similar test conditions, the nominal dimensioned solid wood joists collapsed in less than 11 minutes.
Examination of floor collapse incidents combined with research results led to several tactical considerations, including the following:
- The high risk of operating on a floor over a fire in a building made with lightweight construction materials.
- The lack of reliability of techniques such as sounding the floor or using a thermal imaging camera (TIC) to determine the structural integrity of the floor.
Consider some of the methods commonly taught to firefighters for sounding the floor such as striking the floor with a tool to test for structural integrity or using the heel of your boot to look for holes. These methods were developed based on fireground experience with legacy floor construction that consisted of rough-cut or dimensional solid wood joists and solid lumber subfloors. Keep in mind that floor assemblies composed of solid wood components were traditionally built with solid lumber joists, at least 1½ inches thick, wooden subfloor boards at least ¾ inches thick, and then covered with a ¾-inch-thick tongueand- groove solid wood plank floor. This legacy construction provided additional time for firefighters to operate on it prior to it becoming structurally compromised or developing holes from fire damage. As the traditionally built floors aged, the subfloor boards would dry and shrink, resulting in gaps between the boards. Photo 3 shows the gaps between the subfloor in homes built in the 1950s.

The gaps between the subfloor boards provide a path for heat and fire to extend upward. This can result in the sides of the boards charring and exposing heat to the floor covering materials on top of the subfloor. With this type of construction, the initial fire spread would likely burn through the floor prior to the floor support joists, which are thicker and have a larger mass, losing their structural integrity. How the fire would continue to progress is then dependent, in part, on what materialthe flooring itself was composed of. Thick hardwood floors would slow down the progression of fire through the floor while lighter weight flooring would allow the fire to extend through the floor at a faster rate. With this type of floor construction, there is a higher potential for the fire to cause a localized soft spot or a hole in the floor, prior to the loss of the structural integrity of the floor itself. This type of localized surface damage could be identified with traditional sounding techniques or with a TIC.
Modern platform floors, which have been widely used since the 1980s, are composed of lightweight joists supporting a subfloor of plywood or OSB. In the lightweight design, components of the joist or truss can burn away or detach when exposed to flame in a much shorter time than is required to burn a hole through a solid sheet of subfloor material with no air gaps. The web of the joist is typically thinner than the thickness of the subfloor. With flames under the floor system, the web is being heated on both sides from the fire, while the subfloor is only being heated from one side. As a result, the floorsupport can lose its structural integrity before the subfloor or finished flooring shows signs of thermal degradation or high heat. This is what makes sounding and TICs unreliable for determining if a lightweight flooring system exposed to fire is safe to operate on. Once the structural integrity of the lightweight engineered lumber joist is compromised, there is the potential for sections of the floor to collapse when loaded by firefighters or even a complete wall-to-wall floor collapse.4, 6

Case Study
A fire experiment was conducted for the purpose of measuring the post-fire environment that fire investigators would be exposed to. The fire was started in an unfinished basement area with a single-door walkout on the basement level. The fire was allowed to burn post-flashover for several minutes. The basement fire was suppressed, and the first floor was then checked for extension. The TIC showed the contrast of the hotter subflooring between the areas of the subfloor that were protected by the floor supports (lightweight engineered lumber I-beams) and thus cooler from above. The TIC screen is shown in photo 5. In addition, the floor in this area flexed, or moved, as the firefighter approached the area shown in photo 5. This phenomenon and thermal image were significant because there were no additional layers of flooring above the subfloor. In finished construction, there will be additional layers, which will further impede a firefighter’s ability to determine any substantial clues from above. These additional layers can include hardwood flooring, tile, and carpet and padding.
Photo 6 shows the same section of the floor from the basement (fire) side. Notice that the entire web of the wood I-joist has been burned away and the charred remains of the top and bottom flanges are compressed together, indicating deflection in the entire assembly. The structural integrity and load-bearing capacity of the joist have been significantly reduced. Photo 7 provides a comparison of a sample of OSB cut out of the subfloor, shown in photo 6, next to a sample of full-thickness OSB. Notice that even though the entire 7⁄16-inch-thick wood flange burned away, very little of the 23⁄32-inch-thick OSB subfloor has burned away.



This scenario helps demonstrate that sounding and TICs cannot reliably determine the structural integrity of the floor system. Tapping or striking the floor system in this case provided a false indication that the load-bearing capability of the floor assembly was safe to walk on based on the apparent solidness of the subfloor. However, we can clearly see that the structural integrity of the joist and the floor assembly have been compromised. A TIC can be useful but cannot determine structural integrity. In this scenario, the thermal contrast provided by a TIC is a cue not to enter that area and to check the floor supports prior to starting operations on the floor.
Subfloor Fire Growth and Sounding Demonstration
FSRI conducted a demonstration to show how fire can spread differently between a legacy floor assembly using solid wood elements and a modern floor assembly using lightweight engineered wood elements. Each representative floor system was 12 feet × 12 feet and supported with a concrete foundation approximately one floor above grade.

The solid wood joist floor system was constructed of nominal dimension 2-inch × 12-inch joists and 1-inch × 6-inch wood boards as the subfloor. The subfloor boards were installed on a diagonal relative to the joists and maintained a spacing of 1⁄8 inch apart. This was representative of the floors shown previously in photo 3. The lightweight engineered floor was constructed of 12-inch-tall wood I-joists with a 7⁄16-inch-thick OSB web, as shown in photo 2 and a 23⁄32-inchthick OSB subfloor. The lightweight engineered floor assembly was finished on top with 3⁄8-inch-thick polyurethane padding and standard contractor-grade polyester carpet. Photo 8 shows the floor assemblies used in this demonstration.
The floor systems were exposed to a “contents” fire consisting of a stack of 10 pallets and one bale of straw centeredunder each floor assembly. The fires were ignited in the straw with a small flame near the base of each pallet stack.


Photo 9 shows the progression of each fire. Once the components of the floor systems ignited, the fire continued to burn for approximately five additional minutes and was then suppressed with water from a 150-gallon-per-minute hose stream.
The resulting fire damage for both floor systems is shown in photo 11. Notice how the edges of the solid wood subfloor boards have started to burn away, increasing the gaps between the boards. The increased gaps between the subfloor boards allow for increased burning due to additional air. With the lightweight engineered wood floor system, the OSB subfloor has not been burned through; however, the webs of the wood I-joists have been burned away and lost structural stability.
Photos 12 and 13 give a sense of the thickness and amount of unburned material remaining of each of the subfloor materials. The solid lumber of the legacy floor assembly is losing mass at a faster rate than the OSB subfloor material, because it is exposed to fire on three sides. Similarly, the lightweight engineered lumber I-joist webs have burned away significantly, reducing their load carrying capacity, while the OSB subfloor sheets with less mass loss are still sound.
After fire suppression, a firefighter standing on the concrete block perimeter (serving as the foundation) wall used a six-foot fire service hook to strike each flooring system. When he struck the solid wood members of the legacy subfloor with the hook, pieces would break away, yielding a hole and localized structural deficit. When he struck the carpet-covered OSB of the modern assembly with the hook, it bounced off the OSB, providing no indication of structural deficit. The OSB subflooring was solid and intact; however, the I-joist support under the entire floor was not. After several strikes with the hook, the firefighter penetrated the OSB. Each strike of the hook shook the floor assembly. It is unknown if this would serve as an indicator in the field or if the floor would collapse with firefighters standing (loading) on the floor while sounding.


Photo 14 shows an upper view of the side-by-side floor assemblies postfire. On the left, the legacy floor shows the increased gaps between the subflooring planks and the hole created by the firefighter attempting sounding procedures by striking with the hook. On the right, the modern floor shows substantial deformation of the entire floor assembly; however, there was zero indication in the center of the area that there was an unconfined fire burning beneath.
What Can You Do?
Preplan. Know where lightweight construction practices are used in your first-due response area. More specifically, know where the lightweight construction assemblies without additional protection measures (such as drywall covering the underside of the floor or automatic fire sprinklers) are located. During size-up of a structure, if you have a basement or first-floor fire in a two-story building, begin with assessing the extent of the fire and attempt to estimate how long the fire has been burning. Get effective water on the fire from the most advantageous position and attempt to determine the extent of the fire damage to the flooring assemblies before committing resources to the floor above.

Use a TIC to look for heat signatures on the floor from above or around the edges of the floor from a safe position. If a thermal contrast showing heat is coming from below, that is a sign to stop the advance and reevaluate until the fire is knocked down and the structural integrity of the floor can be determined from below. If you are advancing on a floor that seems to move, give with movement, or feel spongy, back out of the affected area.
It’s all about risk management. Conducting operations on a lightweight floor system over a well-involved or flashed-over compartment fire without confirmation of the structural integrity of the flooring assembly is a high-risk operation. Take the time to review line-of-duty-death incidents that show that, once a firefighter falls through the floor into an active fire below, the chances of survival are low.5 If effective water is on the fire, both the risks and the consequences of falling through the floor decrease, increasing the operational effectiveness of the overall response.


REFERENCES
- Fisette, Paul. “The Evolution of Engineered Wood I-Joists.” The University of Massachusetts Amherst, 2008, bit.ly/4hQEBph.
- “2021 Annual “Project Mayday” General Report.” Don Abbott’s Project Mayday, 2021, bit.ly/40QXoL7.
- Izydorek, Mark, et al. Structural Stability of Engineered Lumber in Fire Conditions. Underwriters Laboratories, 2008, bit.ly/3Cr4ITr.
- Kerber, Stephen, et al. “Improving Fire Safety by Understanding the Fire Performance of Engineered Floor Systems and Providing the Fire Service with Information for Tactical Decision Making.” UL Fire Safety Research Institute, 2012, bit.ly/3YQFRQe.
- Madrzykowski, Daniel, and Craig Weinschenk. “Understanding and Fighting Basement Fires.” UL Fire Safety Research Institute, 2018, bit.ly/3ADkjyI.
- Madrzykowski, Daniel, and Jonathan Kent. “Examination of the Thermal Conditions of a Wood Floor Assembly above a Compartment Fire.” National Institute of Standards and Technology, 2011, bit.ly/4ewh9e2.
DANIEL MADRZYKOWSKI, KEITH STAKES, NICHOLAS DOW, and HOLLI KNIGHT are researchers with the Fire Safety Research Institute, part of the UL Research Institutes.