One of the most critical parts of any fireground size-up is determining the construction type of the fire building. Different structural components behave in different manners when exposed to flame and heat from fires. By identifying the type of construction, the incident commander can start to formulate an attack plan based on the expected behavior of the fire building.

When we look at the construction of the fire building, we are trying to determine several different pieces of information:

• Will the building's structural components themselves burn?
• Does this construction type tend to compartmentalize or limit fire spread, or is it composed of interconnected void spaces that encourage fire spread?
• How long can frefighters safely stay inside this burning building, based on construction type, the amount of fire, and if the fire has passed the protective envelope of the ceiling and walls to extend into the void spaces?
• What is the collapse potential for this type construction?

The collection of all this information, or as much as possible, should be at the start of every size-up.

Size-up systems

There are several different types or styles of size-up. One of the more extensive is the 13-point COAL WAS WEALTH, which stands for Construction; Occupancy; Apparatus and personnel; Life; Water supply; Auxiliary appliances; Street conditions; Weather; Exposures; Area; Location; and Extent, Time and Height of fire.

Of course, we can't forget the LOVERS U for ladder companies (Laddering, Overhaul, Ventilation, Entry, Rescue and search, Salvage, and Utilities) or the five-point BELOW (Building, Extent and location of fire, Life hazard, Occupancy, and Water supply) for first-arriving company officers and firefighters.

No matter which size-up system a fire department uses, true size-up starts whenever its members are out in their district. They should take notice of what's being built or renovated, and they should try to gain access to buildings before the finish work, including the installation of ceilings and walls, takes place.

Key things to look for include lightweight truss assemblies, C-rails and wooden I-beams. These are but a few of the “newer” types of construction that are prone to early failure under fire conditions. Commanding officers should start a database of these buildings so that when new firefighters join the district there will be some documentation of what's out there in terms of building construction.

Construction types

There are five generally accepted construction types, but some people think there should be a sixth for all the varieties of lightweight construction. The major construction types are:

• Fire resistive,
• Non-combustible,
• Heavy timber,
• Ordinary construction and
• Wood frame.

1. Fire resistive. In fire-resistive construction, all components of the building are non-combustible. For example, think of a burn building at the local fire academy — everything in the building is concrete, or it's protected by either concrete or a concrete-type fireproofing.

   Because the building components provide no additional fuel, fires in this type of construction are confined to the fire area contents. For apartment buildings, unless the fire auto-exposes to the floor above or extends via unprotected utility chases, the fire usually will be confined to the original fire apartment.

   That doesn't mean that these are easy fires to fight. Just the opposite is true. These are extremely hot and punishing fires because the concrete absorbs the heat generated by the fire. To attack the fire and move in on it, the engine company must flow enough water to cool the concrete. In many cases, by the time the companies are able to advance into the fire apartment, most of the fuel has been consumed.

   The real problem with this type of fire is when the fire area is very large, as in an open-plan office area or commercial occupancy. Large quantities of water will be required to darken down and cool the fire area, requiring the use of several large-flow hand lines or master-stream applications.

   On a positive note, collapse is usually not a concern for most blazes in a building of fire-resistive construction. Unless the fire area is so large or the fire burns unchecked for an extended period of time, the building should not be prone to collapse.

2. Non-combustible. The most common style of non-combustible construction is rolling-bar joist trusses with a metal deck roof. Stores and commercial buildings frequently are built using these components.

   Unlike fire-resistive construction, the non-combustible structural members usually aren't protected. Instead, they're installed unprotected, and a ceiling and roof covering are added to give the entire assembly a fire rating. In other words, an unprotected bar-joist roof assembly has no fire rating without its ceiling tiles. Once the assembly is exposed to heat and fire, it starts to soften and weaken almost immediately. Because of this, these buildings are one of the worst in which to fight fires. Once the fire reaches the truss assembly, failure of the roof system is imminent.

   Roof operations on non-combustible buildings should be avoided when there's a serious fire within. If there's enough fire inside the building that roof ventilation is required, it's probably unsafe to be on the roof. Similarly, if the fire is small enough to allow firefighters to access the roof, then vertical ventilation probably isn't required.

   There are times when roof operations on a non-combustible building may be considered. One time is if the fire is a roof deck fire, when the fire burns between the metal decking and the underside of the roof covering. Q-decking, one of the most commonly used forms of metal roof decking, has corrugated troughs where the melting roof covering collects and burns.

   Because the fire is burning above the roof supports, the risk of collapse is lessened. These fires usually are extinguished with a combination of opening up from above the fire and cooling the underside of the roof with large quantities of water, which helps return the liquid tar to a more solid state.

   The other time that roof operations may be considered is when the fire building is very large in area. In this case, roof operations may be initiated well in advance of the fire area in an attempt to isolate the fire to one end of the building. The roof openings should be cut but not pulled. As the fire advances toward the unburned portion of the building, the openings should be pulled. If the roof operations are coordination with hand lines beneath the openings, the fire may be confined to just one end of the building.

3. Heavy timber. These buildings, usually old manufacturing mills or factories, have masonry outer walls and a supporting interior framework of heavy timbers. The timbers, depending on the definition used, can be anywhere from 6 by 6 inches or larger.

   Fires in these buildings can be extremely tough to fight if the fire has a decent head-start. The large open areas, coupled with heavy fuel loads, usually will result in a fast-moving fire that generates tremendous radiant heat. Collapse also is to be expected, so collapse zones need to be established. These fires usually go out when they run out of fuel; the fire department's job is to protect the exposures — keeping the fire to the building or block of origin is the goal.

   Construction types, continued

   After heavy timber, the next class of construction is one of the two that most of our fire duty occurs in.

4. Ordinary construction has exterior walls of either brick or masonry, with everything else — the interior walls, floors, and roof supports and decking — made of wood. Because of the masonry exterior walls, heat and smoke have fewer places to escape to the exterior. This situation makes for hot, smoky fires.

   A fire in ordinary construction will attack the structural components even as they add to the fuel load. Luckily for the fire service, the combustible building components usually are protected by walls and ceilings. This protection offers a window of opportunity to attack the fire before it attacks the structural members. Exceptions to this rule are basement and attic fires, where there may not be any ceilings installed. Fires in such areas will attack the supporting floor beams or attic rafters immediately. For fires in areas where there's some protection of the supports, an interior attack, rapidly followed by opening up walls and ceilings, is the prescribed firefighting method.

   Fires in ordinary-construction buildings are usually ladder company incidents. These fires require a lot of ventilation, and once the engine company has darkened the fire area down, the truck company will be needed to open up the voids to check for hidden fire. Fire that has entered the void area above the ceilings to attack the supports could lead to a localized collapse.

   Other areas of fire spread include open interior staircases, pipe chases and other utility shafts. Fire that has entered these void spaces will spread from floor to floor and eventually enter the cockloft or attic. Once the entire building becomes involved, a total collapse of the fire building is a possibility.

5. Wood frame. Wood-frame buildings come in many shapes and varieties, but they share one common factor: everything in the building will burn. Not only do firefighters have to contend with the fuel load of the building's contents, they also have to contend with the additional fuel load of the wooden structural members and the often-flammable exterior siding. If the fire department does nothing on arrival, the fire building eventually will become fully involved and then fall down.

Older types of wood-frame construction include mortise-and-tenon joint, balloon frame and platform. Newer styles include gang-nailed trusses, wooden I-beams and several other varieties. The one thing that both old and new wood-frame construction have in common is that they have hidden void areas where fire can burn unchecked.

In the older frame buidlings that use conventional lumber, fire that runs into the void spaces is usually contained between the floor joists or wall studs at first. A room-and-contents fire that extends through the ceiling and into a floor bay will burn horizontally along that bay but be contained between the two floor joists. In newer lightweight-truss buildings, however, there's nothing in the truss loft to limit horizontal fire spread except the four outside walls. A fire that gets above or through the sheetrock protective envelope will spread unchecked throughout the loft.

Because of these conditions, fighting fires in wood-frame buildings requires that the fire be hit hard and fast. Enough water should be applied to the fire to limit fire growth as quickly as possible. Sometimes this will require the use of either a 2-inch hand line or a master stream. Once the heavy fire is darkened down, crews may switch to the smaller, more maneuverable 1ⁱ-inch hand lines to finish extinguishment.

More importantly, interior crews must drop the ceilings rapidly to expose any fire that may have extended into the void areas. Once fire extends into the voids, it starts to attack the support members. In older construction, there's still time to operate and continue to open up ceilings and walls to expose the hidden fire. If the fire burns unchecked, there eventually will be some type of collapse, usually a localized collapse of floor joists followed by a more complete collapse of the entire building. In the newer, lightweight-construction buildings, a more serious collapse, possibly of an entire floor or roof assembly, should be anticipated. This collapse may happen with little or no warning, so it's important to know what type of wooden-frame building you are dealing with.

Identifying the fire building by type of construction and understanding how fire reacts with that class of construction are the most important tasks for a first-arriving officer. This information should be used to establish the type of attack that the first-alarm assignment will employ. Of course, size-up should continue until the last piece of apparatus leaves the scene. The fireground is an always-changing environment, and it needs to be monitored continually for changes.

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Bob Pressler, a 23-year veteran of the fire service, is a retired lieutenant from the Fire Department of New York. He created and produced the videos “Peaked-Roof Ventilation” and “SCBA Safety and Emergency Procedures” for the video series Bread and Butter Operations. Pressler has an associate's degree in fire protection engineering from Oklahoma State University.