Reading a Commercial Structure With a Thermal Imager

As the fire service brotherhood, and even the nation, still comes to grips with the tragedy, the disaster in Charleston, SC, reminded me how dangerous commercial fires can be. The buildings are large, the fire loads can be massive, and the construction usually works against us. This month's article examines how to read commercial fires with your thermal imager. This is not, in any way, an assessment, of the tactics used that fateful day in June; frankly, I know only what has been reported in the media, and could not even pretend to offer insight into the events of that horrible day.

This column, rather, is a refresher to firefighters everywhere on using a TI to evaluate the inside and outside of a commercial building.

Reading the Outside

We often forget that the thermal imager can give us information prior to committing firefighters inside a building. The amount of information, and the importance of the information, will vary greatly depending on a number of factors. Building construction is probably the most vital factor, as it determines how much heat is retained by the building.

Most new commercial construction follows the same pattern across the U.S.: cinderblock or concrete slabs are stacked around the perimeter for walls; holes are left for windows and doors; metal web trusses are strung across the top of the walls and a flat deck is installed on top. There may be regional variations, but most new commercial construction is a lightweight masonry wall system with a lightweight metal roofing system. In other words, it's common Type III construction.

The lightweight nature of this construction is a boon to builders; it is easy to erect and the materials are relatively inexpensive. These buildings are a mixed blessing to firefighters: they cannot absorb as much heat as heavier construction, which means heat escapes to a thermal imager. Since they cannot absorb as much heat, they are at greater risk for collapse after a fire.

Photo 1 is a thermal image of a Type III building on fire, where the right occupancy is a carpet store and the left is a restaurant. The image demonstrates how quickly the fire "tells its story" to the thermal imager. This image was taken approximately 10 minutes after the initial alarm went out. The thermal image shows dramatically how the fire has spread across the roof line, indicating that the fire has spread underneath the roof across most of the carpet store. The windows show heavy heat build up, as does the door.

The thermal image tells us that this fire is advanced and building up heat rapidly. Knowing that the roof is likely support by metal trusses, and that there is no life risk in the store, an officer may re-evaluate interior attack plans in this situation.

Reading the Inside

In these commercial buildings, one of our biggest concerns as firefighters is the stability of the truss roof system. Again, trusses are a boon to builders as they create low-cost, lightweight methods of spanning larger areas and bearing more weight. However, trusses are a bust for firefighters. Once the truss is damaged by heat, the strength of the truss decreases exponentially, eventually leading to a collapse.

When firefighters operate underneath a truss roof, they need to be keenly aware of how far the fire has advanced in the truss system. Once the truss is exposed to the fire, firefighters know that the clock has started ticking, and time is running out underneath the truss. Photo 2 shows a truss roof under normal conditions. A close examination shows that the left portion of the roof is supported by open metal web trusses; the right side is supported by a more traditional metal post and beam system. In high heat conditions, we would expect the right side to endure longer than the left, as the lighter weight truss system will be damaged by heat faster.

Photo 3 demonstrates what truss impingement could look like on a thermal imager. Notice how the trusses have disappeared into the heat of the fire coming from the left side. If the roof support system is at or near the same temperature as the super-heated gases of the fire, then the roof system is being aggressively attacked by the fire. A heavy support system, such as post and beam, should endure this heat impingement longer than a light support system, such as open trusses. Notice that the TI does not predict the time to collapse; it merely helps us identify that the countdown to collapse has probably begun.


The thermal imager is not a magic tool; but it can be a very effective tool in helping company officers, as well as incident commanders, make better decisions. As building construction gets lighter in the pursuit of cost reduction, firefighters will have less time to operate in those structures. The advantage of this lightweight construction is that heat transmits quickly to the outside surfaces, allowing us to use a TI to interpret what is happening inside the building. We can also use a TI to verify the integrity of the roofing system above as we advance in a structure.

For more insight on the effects of construction on using a TI for size up, visit the Technology section of And keep the Charleston Nine, their families, their friends and their coworkers in your prayers.

September's Firehouse article discusses how a TI can help at a commercial building, especially in size up. The usefulness of a TI in size up can be affected by the type of construction. Online, we'll examine how different types of construction can mask what is happening inside the structure. Type III is address in the print article; the remainder will be addressed here.

Type V

This wood frame construction is probably the lightest weight construction. The fact that it is wood probably helps keep us, as firefighters, from getting overly confident inside these buildings. We know they will burn…it's just a matter of time. Because there is usually little mass in the overall construction, the effects of a fire are transmitted rapidly to the exterior walls. As a result, the outside surfaces of the structure heat up quickly in response to where the fire is inside the building.

Type IV

The heavy timber construction of days past is probably the most durable. The thick timbers, as well as the millions of bricks, have the ability to absorb a tremendous amount of fire and heat. In fact, I once worked in such a building that had sustained significant fire damage in 1944…some of the timbers still had charring on them, but had been painted over. These buildings, frequently mills or old warehouses, will generally hide much of the fire from a TI on the outside. Because the fire must heat so much mass, it generally does not impact that outside surface of the building. As such, a TI will rarely show the advancement of a fire inside a Type IV structure.

Type II and I

These structures tend to be high rises, although they are not exclusively high rises. These types of buildings may be coated in materials that are very reflective of infrared energy, such as polished steel or mirrored glass. As such, reading the effects of a fire can be nearly impossible. Many of these buildings, while heavier than a Type III, are still much lighter than a Type IV. As such, if the exterior shell allows, an active fire may transfer heat to the exterior of the building, allowing a TI to "see" what the heat signature looks like and where the fire may be headed. Keep in mind that the exterior material will probably be the biggest hindrance to reading what is happening inside the building.


Lighter weight construction will be more likely to show the impact and travel of an active fire. The exterior material will impact the "readability" of the heat signature, as certain reflective materials may mask the interior conditions. By bringing your TI with you on fire planning exercises, as well as inspections, you can gain practice evaluating and interpreting thermal images from the various types of construction. As always, when working outside, don't discount the effects of the sun on your thermal image.

Be safe.