DEPARTMENT OF HEALTH AND HUMAN SERVICES

Centers for Disease Control and Prevention

National Institute for Occupational Safety and Health

Summary

Fire fighters are at risk of falling 

through fire-damaged floors. Fire 

burning underneath floors can 

significantly degrade the floor 

system with little indication 

to fire fighters working above. 

Floors can fail within minutes 

of fire exposure, and new construction technology such as engineered wood floor joists may 

fail sooner than traditional construction methods. NIOSH recommends that fire fighters use 

extreme caution when entering 

any structure that may have fire 

burning beneath the floor. 

Preventing Deaths and Injuries of Fire Fighters 

Working Above Fire-Damaged Floors

Description of

Exposure

Fire fighters are at risk of falling 

through fire-damaged floors. Floors 

may fail within minutes of flame 

contact. Carpet, ceramic tile, lightweight concrete, and similar floor 

coverings may increase the danger 

to firefighters because of the added 

weight being supported by the floor 

system and the insulation these materials provide that may cause the 

floor to not feel warm, despite the 

fire underneath. 

All wood-based construction materials are subject to failure when exposed to fire. Experimental studies 

and NIOSH investigations suggest 

that engineered wood floor systems may fail sooner than traditional sawn lumber floors. The difference in times to failure appears to be 

a matter of minutes, and fire fighters will seldom know how long a fire 

has been burning when they arrive 

on scene. Consequently, fire fighters 

must use extreme caution when operating on any flooring system potentially exposed to fire. 

Engineered wood I-joists represent 

a rising technology in the building 

sector; they offer several advantages 

over traditional construction methods. Engineered wood I-joists are 

typically prefabricated using sawn or 

structural composite lumber for the 

top and bottom flanges (usually 1 1⁄2 

to 3 1⁄2 inches wide) and plywood or 

oriented strand board (OSB) sheathing for the vertical web (3/8 to 7/16 

inches thick) (see Figure 1). Engineered wood I-joists are lighter, stiffer, and will not warp, twist, or shrink 

like traditional framing materials. 

Engineered wood I-joists also reduce 

total construction time and labor 

costs by their ease of installation. 

Engineered wood I-joists have grown 

in use since the early 1990s and by 

2005 were estimated to be used in 

more than half of all wood-frame 

construction [APA 2005]. Changes 

in the building construction industry driven by technological advancements and societal needs suggest that 

the use of engineered wood products 

will continue to grow.

Figure 1. Engineered wood I-joist. 

Photo courtesy of APA-Engineered 

Wood Association.
The engineered wood I-joist has a different cross-sectional profile than a standard solid sawn wood joist (see Figure 

2) and in testing, burned more quickly. Typically, the thinner web was consumed first (see Figure 3). Time-to-failure 

testing has been conducted by several groups, most recently Underwriters Laboratories (UL) [2008]; [Straseske and 

Weber 1988; Weyerhaeuser 1986]. The UL tests show that 

unprotected lightweight engineered floor joist (I-joists) assemblies can fail in as little as 6 minutes, and that traditional 

unprotected residential floor construction assemblies failed 

in less than 19 minutes. Previous studies that used different test methods suggested even shorter times-to-failure. 

These study results indicate that any floor system can fail 

quickly, and that unprotected engineered wood I-joists may 

fail sooner. Results from experiments conducted by the 

National Institute for Standards and Technology (NIST) 

are expected in the spring of 2009, and will be available at 

www.fire.gov/. The NIST experiments were conducted in 

ventilation limited conditions meant to represent an actual basement fire.

Fire fighters who operate on fire-damaged floors of all 

types have fallen through the weakened floor and been 

trapped in the fire below [NIOSH 2005]. Similar hazards 

exist when fire fighters work under fire-damaged floor systems that collapse onto them. The following is a NIOSH 

case investigation involving an unprotected engineered 

wood floor system. The weakened floor was undetectable 

from above, although outside conditions indicated the 

possibility of a basement fire. 

Case Study

On August 13, 2006, a 55-year-old male career engineer 

(the victim) died and his partner was injured after they 

fell through the floor at a residential structure fire. The 

house was built in 1999, and the first floor contained a 

heated flooring system consisting of a hot water piping 

system encased in lightweight concrete supported by engineered wood I-joists and trusses. The basement was 

unfinished and the bottom sides of the I-joists and floor 

trusses were exposed. An engine company was conducting a fast attack on a suspected basement fire, while a ladder company conducted horizontal ventilation. The victim and his partner were conducting a primary search 

on the ground floor. Smoke filled the ground floor and 

made visibility near zero, but little heat was detected as 

the victim and his partner conducted a left-hand search. 

They sounded the ceramic tile floor and took one crawling step forward on their knees when the floor collapsed. 

The partner fell on the other side of a basement door 

into a hallway and crawled out of a basement window. 

The victim fell into the room of fire origin and was recovered the next day. The floor collapsed about 11 minutes 

after the initial 911 dispatch [NIOSH 2006].

Controls

To minimize risk when working above fire-damaged 

floors, NIOSH recommends that fire departments and 

fire fighters take the measures identified below. Many of 

these prevention measures are from the NIOSH Alert: 

Preventing Injuries and Deaths of Fire Fighters due to 

Truss System Failures [2005]. Lightweight truss systems 

and engineered wood floor joists have similar fire degradation risks. 

 Conduct a thorough fire size-up and communicate the 

findings to all personnel on-scene before entering the 

building. Incident commanders and company officers 

Figure 2. Traditional floor joists. 

Figure 3. Fire-damaged I-joists where victim fell 

through floor. Note how the vertical web is almost completely consumed [NIOSH 2006a].
should be trained and experienced in structure fire 

size up to avoid putting fire fighters at unneeded risk 

of working above fire-damaged floors.

 Do not enter a structure, room, or area when fire 

is suspected to be directly beneath the floor or area 

where fire fighters would be operating, or if the location of the fire is unknown. 

 Never assume structural safety of any floor (regardless 

of the construction) having a significant fire under it.

 Conduct pre-incident planning inspections during the 

construction phase to identify the type of floor construction. If pre-planning is not conducted, assume residential 

construction and small commercial buildings built since 

the early 1990s may contain engineered wood I-joists.

 Report construction deficiencies noted during preplanning to local building code officials. For example, 

engineered wood floor joists should only be modified 

per manufacturer specifications―usually limited to 

cutting to length and removing pre-cut knockouts for 

utility access. Report damaged or cut chords or webs 

to building officials.

 Develop, enforce, and follow standard operating procedures (SOPs) on how to size up and combat fires safely in buildings of all construction types. Rapid intervention teams (RIT) should include a portable ladder with 

their RIT equipment when deployed at basement fires.

 Provide training on identifying signs of weakened floor 

systems (soft or spongy feel, heat transmitted through 

floor, downward bowing, etc.). Make fire fighters aware 

that all floor types can fail with little or no warning.

 Use a thermal imaging camera to help locate fires 

burning below or within floor systems, but recognize 

that the camera cannot be relied upon to assess the 

strength or safety of the floor. Fire fighters should be 

trained on the use of thermal imaging cameras, including limitations and difficulties in detecting fire burning 

below floor systems.

 Immediately evacuate and, if possible, use alternate 

exit routes when floor systems directly beneath the 

floor where fire fighters would be operating are weakened by fire.

 Use defensive overhaul procedures after fire extinguishment in structures containing fire-damaged floor systems of all types.

 Consider becoming active in the building code process 

and influence requirements for fire resistance of floor and 

ceiling systems to further fire fighter safety and health.

In addition, NIOSH recommends the following:

 Trade associations and building contractors should consider providing education and training to fire service 

organizations on the hazards fire fighters face when 

fighting fires that have weakened all types of structural members. An example of such training is available at 

www.woodaware.info.

 Builders, contractors, and owners should consider protecting all floor systems, including engineered wood 

I-joists, by covering the underside with fire-resistant 

materials [Underwriters Laboratories 2008]. 

 Builders, contractors, and owners should consider incorporating sprinkler systems into residential construction. Sprinkler use reduces the chances of both 

residential and fire fighter fatalities [USFA 2008]. 

Acknowledgments

The principal contributors to this publication were Tim Merinar and Jay Tarley, NIOSH, Fire Fighter Fatality Investigation and Prevention Program (FFFIPP) and Robert Koedam, 

formerly with NIOSH.

References

APA [2005]. Wood I-joist floors, fire fighters and fire. APA―

The Engineered Wood Association. Tacoma, WA. Form No. 

TT–015B.

NIOSH [2005]. NIOSH alert: preventing injuries and deaths 

of fire fighters due to truss system failures. Cincinnati, OH: 

U.S. Department of Health and Human Services, Public 

Health Service, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, 

DHHS (NIOSH) Publication No. 2005–132.

NIOSH [2006]. Career engineer dies after falling through floor 

while conducting a primary search at a residential structure 

fire—Wisconsin. Morgantown, WV: U.S. Department of 

Health and Human Services, Centers for Disease Control 

and Prevention, National Institute for Occupational Safety and Health, Fatality Assessment and Control Evaluation 

(FACE) Report F2006–26.

Straseske J, Weber C [1988]. Testing floor systems. Fire Command. June:47–48.

Weyerhaeuser [1986]. Flame penetration ratings according 

to ASTM test method E119 utilizing a small scale furnace. 

Longview, WA: Weyerhaeuser Company Fire Technology 

Laboratory, Report No. 665.

Underwriters Laboratories [2008]. Report on structural stability of engineered lumber in fire conditions. Northbrook, IL: 

Underwriters Laboratories, File No. NC9140.

USFA [2008]. USFA Position Paper—Residential fire sprinklers. United States Fire Administration, U.S. Department 

of Homeland Security. March 28, 2008. http://www.usfa.

dhs.gov/downloads/pdf/sprinkler_position_paper.pdf
DEPARTMENT OF HEALTH AND HUMAN SERVICES

Centers for Disease Control and Prevention

National Institute for Occupational Safety and Health

4676 Columbia Parkway

Cincinnati, OH 45226–1998

Official Business

Penalty for Private Use $300

For More Information

The NIOSH Alert: Preventing Injuries and Deaths of Fire Fighters due 

to Truss System Failures includes relevant information and prevention recommendations. Construction truss systems and engineered 

floor joists have similar collapse hazards associated with fire degradation. The NIOSH Alert is available at http://www.cdc.gov/niosh/

docs/2005-132/

The American Forest and Paper Association (AF&PA) and the U.S. 

Fire Administration have developed the following Web site with information for the fire service about traditional and engineered wood 

products: http://www.woodaware.info/. A CD entitled Awareness 

Level Firefighter Training for Modern Wood Products developed in 

cooperation with the Illinois Fire Service Institute is available from 

fire@woodaware.info.

Underwriters Laboratories, with funding from the Department of 

Homeland Security, has developed an on-line course for fire professionals, “Structural Stability of Engineered Lumber in Fire Conditions” available at http://www.uluniversity.us./

The National Institute of Standards and Technology (NIST), Building 

and Fire Research Laboratory maintains a Web site with links to publications on fire safety topics: http://www.fire.nist.gov/. Information 

on engineered wood I-joist research at NIST can be found at http://

www.nist.gov/public_affairs/techbeat/tb2007_0830.htm#firetest.

To receive more information about occupational safety and health 

topics, contact NIOSH at

Telephone: 1–800–CDC–INFO (1–800–232–4636)

TTY: 1–888–232–6348  E-mail: cdcinfo@cdc.gov

or visit the NIOSH Web site at www.cdc.gov/niosh.

For a monthly update on news at NIOSH, subscribe to NIOSH eNews 

by visiting www.cdc.gov/niosh/eNews.

Mention of any company or product does not constitute endorsement 

by NIOSH. In addition, citations to Web sites external to NIOSH do 

not constitute NIOSH endorsement of the sponsoring organizations 

or their programs or products. Furthermore, NIOSH is not responsible for the content of these Web sites.

This document is in the public domain and may be freely 

copied or reprinted. NIOSH encourages all readers of the 

Workplace Solutions to make them available to all interested employers and workers.

As part of the Centers for Disease Control and Prevention, NIOSH 

is the Federal agency responsible for conducting research and making recommendations to prevent work-related illnesses and injuries. 

All Workplace Solutions are based on research studies that show how 

worker exposures to hazardous agents or activities can be significantly reduced.

Preventing Deaths and Injuries of Fire Fighters Working 

Above Fire-Damaged Floors

DHHS (NIOSH) Publication No. 2009–114

Safer • Healthier • PeopleTM February 2009