Evaluating Building Materials for Firefighting Operations
How wood, masonry, steel, concrete and composites behave in fire, with verified temperatures and collapse times from NIST, UL FSRI and NIOSH.
Understanding the materials that make up a structure is crucial for making informed decisions on the fireground. Different materials react to fire in varying ways, affecting the fire’s behavior and the safety of everyone involved.
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Key Numbers at a Glance
Figures come from controlled tests and published codes. They are benchmarks for comparison, not predictions for any particular fire.
Understanding the materials that make up a structure is crucial for making informed decisions on the fireground. Different materials react to fire in varying ways, affecting the fire’s behavior and the safety of everyone involved.
How to read the numbers on this page
Many collapse times here come from furnace tests that follow the ASTM E119 standard fire. That fire reaches about 1,000°F at 5 minutes and 1,700°F at 60 minutes. [1]
UL notes that these test times are a benchmark for comparing assemblies. They are not meant to tell you how long a specific building will stand, because room size, fuel and ventilation change every fire. [1]
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1. Wood
Wood is one of the most common building materials, especially in residential structures. It is combustible, and its performance under fire conditions varies depending on factors like thickness, moisture content, and whether it has been treated with fire-retardant chemicals.
Emergency Scene Size-Up
During size-up, observe the extent of wood construction. In older buildings with heavy timber, the wood can char and provide some degree of fire resistance, but in lightweight wood-frame buildings, collapse can occur rapidly, requiring a defensive approach.
By the numbers
Why heavy timber holds on: the standard design char rate for exposed wood is 1.5 inches per hour. [4] The base of the char layer sits near 550°F, and the char insulates the wood core behind it. [4]
Lightweight vs. conventional (UL, 2008): in furnace tests with no ceiling, a 2x10 floor collapsed at 18:45 and a 12-inch wood I-joist floor collapsed at 6:03. [1]
Real basement-fire layout (UL/NIST): floors failed between 3:28 and 12:45 after ignition. Dimensional lumber averaged 11:57 and engineered floors averaged 7:00. The longest unprotected floor lasted 12:45, so the researchers concluded there is no guaranteed safe time to work on top of a basement fire. [1]
Sagging is not a reliable warning: dimensional lumber floors sagged less than 6 inches before collapse, while steel C-joist floors sagged more than 14 inches. UL/NIST found that floor sag did not work as a dependable collapse predictor. [1]
Roofs (NIST, Phoenix tests): in single-story wood-frame houses with manufactured roof trusses, peak temperatures reached 1,500–1,800°F, and the roofs collapsed roughly 16 to 17½ minutes after ignition. Thermal imaging did not prove to be a reliable collapse predictor. [1]
Protection matters: in Canadian (NRC) basement-fire tests, engineered floors failed 35–60% sooner than solid wood joists, but a gypsum board ceiling brought their failure times close to those of solid wood. Residential sprinklers prevented structural failure in the tested scenarios. [1]
Gypsum vs. plaster ceilings (UL, 2008): ½-inch gypsum ceilings began falling away at roughly 14 to 24 minutes. A ¾-inch plaster ceiling held for about 74 minutes. [1]
Frequency of close calls: NIST’s review of firefighter near-miss reports from January 2005 to March 2011 found 118 residential structural collapse incidents, 76 of which injured a total of 128 firefighters. [1]
Case study, Hackensack, NJ (July 1, 1988): five firefighters were killed when a wood bowstring truss roof over an auto dealership collapsed, a classic example of lightweight roof construction failing with little warning. [8]
See the Wood Frame Construction page for balloon and platform framing, void spaces, I-beams and hangers.
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2. Masonry
Masonry materials like brick, stone, and concrete are generally fire-resistant. However, high heat can cause spalling or collapse especially if the mortar has degraded over time.
Emergency Scene Size-Up
Identify the presence of masonry walls, especially load-bearing ones. In intense fires, these materials can fail suddenly, so be cautious of potential collapse zones.
By the numbers
General collapse zone (NIOSH): a collapse zone is often defined as a perimeter set at a distance from the building equal to 1.5 times the height of the structure. NIOSH also identifies free-standing parapets as a special risk that can become unstable or collapse without warning during a roof collapse. [6]
Parapet and masonry wall collapse zone (Fire Engineering): this is a separate recommendation, not part of the NIOSH document. Author Gary Bowker writes that once operations become defensive, command should establish and enforce a collapse zone of 1½ to 2 times the height of the wall. [7] That is equal to or larger than the general 1.5× figure, so when a parapet or masonry wall is the hazard, use the larger distance your department’s SOPs allow.
Parapets are freestanding: they may rise three or more feet above the roof deck and do not resist sideways loads well, and the taller the parapet, the more easily it can become unstable. [7]
Heated steel pushes walls: a 50-foot steel I-beam heated to 1,000°F expands roughly 4 to 5 inches in length, which is enough force to push a parapet outward. Steel anchoring plates on the front of a wall indicate steel rods tied back to the roof structure, and Fire Engineering calls them a warning sign. Steel I-beam purlins resting on the side walls are another danger sign. [7]
Fire at one end can drop a wall at the other: the same article notes that a working fire at one end of a building could produce a wall collapse at the opposite end, and it describes uncontrolled fire in the cockloft or between floors as a red flag for going defensive. [7]
Collapse can come late: at a July 28, 2002 FDNY fire in a Manhattan ordinary-construction building, the incident commander pulled crews out and the front parapet collapsed 40 minutes later, with no injuries. [7]
Historical example: the FDNY lost six firefighters when a parapet collapsed at a furniture store fire on April 4, 1956. [7]
Retrofit hazards and pre-1933 California masonry are covered on the Masonry Construction page.
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3. Steel
Steel is used in many modern buildings due to its strength. However, steel loses strength rapidly when exposed to high temperatures, leading to potential structural failure.
Emergency Scene Size-Up
When encountering steel structures, assess the intensity of the fire and the exposure time. If the steel is not adequately protected, consider defensive tactics to avoid entering areas where collapse is imminent.
By the numbers
Half its strength by about 1,100°F: NIST states that by 600°C (about 1,100°F) most structural steels have lost more than half their strength, which is why structural steel in buildings is insulated. [2]
How fast a fire gets there: the standard test fire passes 1,000°F at 5 minutes and reaches 1,700°F at 60 minutes. [1]
Light-gauge steel joists: in 1980 National Bureau of Standards room-burnout tests, unprotected 7¼-inch steel C-joist floors failed in under 4 minutes (3:47 and 3:59), while a 2x8 wood joist floor failed at 10:43. [1]
Lots of sag first: in UL/NIST full-scale tests, steel C-joist floors sagged more than 14 inches before collapse. [1]
Lightweight metal hangers in wood buildings are also unprotected steel. See Wood Frame Construction.
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4. Reinforced Concrete
Reinforced concrete combines the fire resistance of concrete with the strength of steel reinforcement. It performs well but can spall after long heat exposure.
Emergency Scene Size-Up
Reinforced concrete can often be approached more aggressively, but stay alert for signs of spalling or weakened sections.
By the numbers
Explosive spalling: in NIST laboratory tests on high-strength concrete, surface layers broke away explosively when the concrete reached roughly 200–325°C (about 390–615°F) at a depth of 25 mm. The cause is attributed to internal vapor pressure from heated moisture together with thermal gradients. [3]
Violent when it happens: test cylinders fragmented with a loud bang, throwing pieces outward at high speed. [3]
Why it matters: spalling can strip away the concrete cover that shields the reinforcing steel. [3]
Strength loss: between 100°C and 400°C, high-strength concrete can lose close to 40% of its room-temperature strength, about 20 to 30 percentage points more than normal-strength concrete. [3]
These results come from laboratory specimens. Spalling in a real building depends on the concrete mix, its moisture content and the fire.
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5. Composite Materials
Composite materials, such as those used in modern high-rise buildings, often combine several materials like plastics, metals, and fibers. Their behavior under fire varies widely and can be unpredictable.
Emergency Scene Size-Up
When dealing with composite materials, take a cautious approach. These materials can produce toxic smoke and may fail unexpectedly, requiring a more defensive strategy.
By the numbers
Modern contents burn faster: in repeated UL FSRI experiments, a room furnished with synthetic materials reached flashover in 3:20 to 4:50, while the same room with natural furnishings took 29:30 in the first test and more than 30 minutes in the other three. [5]
What flashover means: UL FSRI defines it as the point where fuel surfaces exposed to radiation from fire gases above 600°C (about 1,100°F) ignite almost at once. [5]
Composite assemblies differ too much from product to product for a single failure time. The numbers on this page cover wood, steel and concrete tested by UL, NIST and NRC. For a specific composite, check the manufacturer’s fire-test data.
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Firefighting Strategies Based on Material
Understanding the building materials present at a fire scene allows firefighters to choose the most appropriate strategy, whether aggressive or defensive. For example:
Lightweight wood-frame buildings: May necessitate a defensive approach to avoid entrapment.
Reinforced concrete structures: Might allow for a more aggressive interior attack if conditions permit.
Always consider the material’s fire behavior and its impact on structural integrity during your size-up to ensure the safety of your crew and the effectiveness of your firefighting operations.
By the numbers
In UL/NIST basement-fire floor tests, collapse came between 3:28 and 12:45 after ignition, and the researchers found no guaranteed safe time to operate above a fire. [1]
Visible floor sag was not a dependable collapse predictor, and thermal imaging did not prove reliable for predicting roof collapse in the NIST Phoenix tests, so size-up has to weigh materials, protection and time on the fire. [1]
Kerber, S., Backstrom, R., Dalton, J., Madrzykowski, D. Full-scale floor system fire experiments (UL for NIST, NIST ARRA project on engineered floor systems), 2012. The report also summarizes UL (2008) ASTM E119 tests, the National Bureau of Standards (1980) room-burnout tests, NRC Canada basement-fire tests, NIST Phoenix collapse tests (NISTIR 7094) and NIST near-miss data. technicalpanels.fsri.org
NIST Metallurgy Division. 2005 Annual Report, Safety and Reliability. metallurgy.nist.gov
Phan, L.T. High-Strength Concrete at High Temperature: An Overview. National Institute of Standards and Technology. PDF
American Wood Council. Technical Report 10: Calculating the Fire Resistance of Wood Members and Assemblies (char-layer temperature); and NDS Chapter 16 nominal char rate as summarized by WoodWorks. awc.org · woodworks.org
UL Fire Safety Research Institute. New Comparison of Natural and Synthetic Home Furnishings (published September 30, 2020). fsri.org
NIOSH. Workplace Solutions, DHHS (NIOSH) Publication 2014-120, on preventing fire fighter deaths and injuries from structural collapse. cdc.gov
Bowker, G. Parapet Collapse: An Old Danger for Today’s Firefighters. Fire Engineering, June 1, 2018 (figure adapted from Hill, H., Failure Point: How to Determine Burning Building Stability, Fire Engineering Books, 2012). fireengineering.com
Five Firefighters Killed in Truss Roof Collapse at Auto Dealership Fire, Hackensack, New Jersey, July 1, 1988 (summary published by Firefighter Close Calls); and Fire Engineering, “Five Fall in Hackensack.” firefighterclosecalls.com · fireengineering.com
Figures cited on this page come from the sources listed above. Times and temperatures from laboratory and training-facility tests are benchmarks and do not predict how any specific building will perform.
