New Dynamics of Basement Fires

Basement fires are among the most dangerous. UL plays a critical role in examining the hazards associated with various types of residential flooring systems to better understand this risk.


In the real world, the fire service will never respond to two fires that are exactly the same. During all of our recent basement fire experiments, where the variables were systematically controlled, there were no reliable and repeatable warning signs of flooring collapse. Because the consequences of falling through a floor into a basement fire can be catastrophic, it is critical for firefighters to understand the hazards associated with different types of flooring products and systems to enhance operational procedures, protection methods and overall firefighter safety.1

Under fire conditions the new, light-weight engineered floor systems led to greater risk of structural failure in a shorter time. (26)

Under fire conditions the new, light-weight engineered floor systems led to greater risk of structural failure in
a shorter time.3


Basement fires are one of the most challenging and dangerous types of fires that firefighters face. They can find themselves in a position where they’re operating above a fire — in some cases without knowing it. Often the fire service has no idea how long the fire has been burning, the type of floor system exposed to the fire conditions, or the structural stability of the floor system, and there are little, if any, warning signs of collapse.2


In particular, basement fires that involve an unprotected wood floor assembly can pose a number of challenges to Fire Safety. Light-weight engineered floor systems provide architectural, economic and productivity benefits to the homeowner and the construction industry. However, under fire conditions, these light-weight engineered floor systems lead to greater risk of structural failure in a shorter time as a consequence of the reduced cross-sectional dimensions of the engineered products as compared to traditional dimensional lumber floor systems. So, despite the superior structural performance of these new products to traditional lumber construction under “normal” conditions, the trend reverses in a fire environment. This is highlighted by the increasing number of firefighter fatalities due to collapse of these engineered systems under fire conditions. The National Institute for Occupational Safety and Health (NIOSH) issued a report, Preventing Injuries and Deaths of Fire Fighters Due to Truss System Failures, highlighting the risks of injury and death that can occur during firefighting operations involving engineered floor truss systems.3


UL partnered with several research organizations, product manufacturers and fire service representatives to examine hazards associated with various types of residential flooring systems. Funding for this project was provided through the National Institute of Standards and Technology’s American Recovery and Reinvestment Act Grant Program. The main objective of this first-of-its-kind research was to improve Fire Safety by better understanding the response of residential flooring systems to fire.4


UL conducted five types (or series) of experiments to examine basement fires and collapse hazards posed to the fire service:

  1. Floor Furnace Experiments: UL examined several different engineered floor systems and dimensional floor systems and different protection methods. We also altered the load to understand different stresses in the floor systems.
  2. Heat Release Rate Experiments: Through these experiments, UL developed a repeatable fuel load that would be representative of fires that firefighters would find in actual basements.
  3. Basement Field Experiments: In these field experiments, UL used fabricated basements to examine firefighter ventilation practices and how different types of floor systems performed under basement fire conditions.
  4. Basement Lab Experiments: In these controlled experiments, UL replicated the same basement configuration as in the previous field experiments, but was able to control variables such as weather conditions, and wind conditions and study different types of floor systems and protection methods to examine the code implications of what firefighters could see in the field.
  5. Full-Scale Field Experiments: UL was able to perform two full-scale collapse experiments, where we ignited the fire in the basement of an acquired residential structure and allowed it to spread naturally throughout the house.

There are 10 key tactical considerations that result from this comprehensive and innovative Fire Safety research that firefighters can use immediately to enable improved decision making at the fire scene:

UL developed 10 key tactical considerations that firefighters can use to improve decision making at the fire scene.

UL developed 10 key tactical considerations that firefighters can use to improve decision making at the fire scene.

  1. Collapse times of all unprotected wood floor systems are within the operational time frame of the fire service regardless of response time. Specifically, engineered floor systems can collapse as early as 2 – 3 minutes after the structure becomes involved in the fire.
  2. Size-up should include the location of the basement fire as well as the amount of ventilation. In these experiments, collapse always originated above the fire, and the more ventilation available the faster the time until floor collapse. When possible, the floor should be inspected from below prior to operating on top of it.
  3. Signs of collapse vary by floor system:

a. Dimensional lumber should be inspected for joist rupture or complete burn through.

b. Engineered I-joists should be inspected for web burn through and separation from subflooring.

c. Parallel Chord Trusses should be inspected for connection failure.

d. Metal C-joists should be inspected for deformation and subfloor connection failure.

  1. Sounding the floor for stability is not solely reliable and therefore should be combined with other precautionary tactics to increase safety.
  2. Thermal Imaging Cameras (TICs) may help indicate that there is a basement fire when the fire condition is visible to the camera’s field of vision, but TICs cannot be used to assess structural integrity from the floor above.
  3. Quickly descending the stairs to find relief at the bottom was not possible. Floor temperatures at the bottom of the basement stairs were often worse than temperatures at the top. This finding contradicts conventional wisdom about heat rising and the thought that cooler temperatures would be found at the bottom of the stairs.
  4. Coordinating ventilation during fire suppression is extremely important. Ventilating the basement created a flow path up the stairs and out through the front door of the structure, almost doubling the speed of the hot gases and increasing temperatures dramatically.
  5. Floor sag is a poor indicator of floor collapse. Any perceivable weakness is a late indicator that the floor system has already been damaged or completely compromised.
  6. First-floor temperatures can be a poor indicator of fire conditions that exist below, especially when remote from the top of the stairs or the initial fire condition.
  7. Hose lines should be available when opening up void spaces that contain areas of combustible construction.6

Well-designed fire tests not only help provide information on a particular scenario but should generate the detailed data needed to help validate computer models. The use of computer modeling in fire engineering is only growing, so it is vital that the limited expensive and large-scale fire tests that are conducted be designed with model validation in mind. For this research, UL generated a computer model of one particular full-scale basement fire test scenario involving fully ventilated conditions.7 This specific example concerned the fire dynamics within a basement with openings and an unprotected wood ceiling with geometric complexity. The results show that predicting the fire growth within basements with wood ceilings can be achieved reasonably well with FDS, a fire modeling software. With a validated model, then, it is much more economical to answer a whole range of questions on the sensitivity of the fire behavior to other parameters such as number of openings, size of openings, sequential openings, etc.8


The building and construction industry is continually introducing new engineered products that provide better structural stability, allow for faster construction and are more cost effective. Additionally, the increased market demand for environmentally sustainable products is driving engineered lumber products to further reduce material mass, which could potentially result in even greater concern for Fire Safety. The 10 key tactical considerations that resulted from UL’s comprehensive and innovative Fire Safety research enable firefighters to improve decision making on the scene when encountering these new materials.9 In addition, UL is helping advance the use of fire modeling tools that can have a significant impact on furthering insights into scenarios that firefighters must confront and on improving training materials for firefighters.


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