Commercial Building Fire Size-Up (What Changes When The Box Is Bigger Than A House)

A first-due officer pulling up to a commercial structure fire is looking at a problem set that does not exist on residential calls. The building is larger by an order of magnitude. The occupant load swings from zero at 0300 to several hundred at lunch on a Tuesday. The hose stretch is no longer thirty feet through a front door. It is three hundred feet through a maze of cubicles, racks, or assembly lines. Air consumption is no longer a function of crew fitness. It is a function of how far the crew has to travel inside before they can even start working.

Most departments train residential fires hard and treat commercial as the call you handle when it comes. That is a problem, because commercial fires kill more firefighters per incident than residential, and the failures that lead to those deaths are almost always tied to size-up failures in the first three to five minutes. The captain who treated the warehouse fire like a big house fire is the captain who put the crew in a position where the stretch took twice as long as the air supply, the building geometry trapped them, and the eventual mayday was the inevitable result of decisions made before any water moved.

The good news is that commercial size-up is teachable, and the patterns are consistent across occupancy types. The captain who has done the preplans, who knows the construction features of the local commercial building stock, and who can run the size-up checklist that is genuinely different from the residential one is the captain who keeps the crew out of trouble. Frank Brannigan's Building Construction for the Fire Service treats commercial occupancies as their own category, and IFSTA Essentials 8e dedicates a separate chapter to it for good reason. The principles transfer. The numbers do not.

The single biggest size-up question on a commercial structure is who is inside right now. That number is not on the dispatch ticket. It depends on the time of day, the day of the week, and the type of occupancy. A retail store at 0300 has zero occupants except possibly a stocking crew. The same store at noon on Saturday has two hundred. An office building at 1400 Tuesday has a full staff. The same building at 1900 Friday has a janitorial crew. An assembly occupancy, restaurant, theater, church, is governed by the occupancy load posted at the door, but the actual count varies wildly based on the event.

The first-due officer should be reading time-of-day and day-of-week against occupancy type before they even arrive on scene. Dispatch at 1130 on a weekday to a strip mall restaurant means a lunch crowd inside. Dispatch at 0200 to the same restaurant means kitchen crew at most. The size-up declaration on the radio should reflect that read. "Engine 12 on scene, two-story commercial, heavy smoke from the second floor, occupancy is a Class B office, time of day suggests partial occupancy, primary search will be conducted simultaneously with attack." That declaration tells everyone arriving what the search picture looks like.

The fire alarm panel matters here. If the building has a monitored fire alarm system and the panel is accessible, it tells you which zones are activated, often which devices, and sometimes which floor or wing the fire is on. Spending thirty seconds at the panel before stretching the line saves the company from stretching the wrong direction. This is a basic step that gets skipped on routine response because nobody trained on it.

A residential hose stretch is typically thirty to seventy feet from the engine to the seat of the fire. A commercial stretch can be anywhere from one hundred to five hundred feet, sometimes more in larger occupancies, big box retail, warehouse, manufacturing. That difference is not just longer hose. It is friction loss, air consumption, supply line management, and crew fatigue compounding at every fifty feet.

The pre-connected attack line that solves every residential fire is wrong on most commercials. The crew needs either a longer pre-connect dedicated to commercial response, two-and-a-half-inch line for the volume and reach commercials require, or a standpipe operation if the building is equipped. Pulling a two-hundred-foot pre-connect on a warehouse fire and finding out at the door that you are still two hundred feet short of the seat is the kind of error that turns a working fire into a mayday call.

Stretch estimation is a preplan exercise. The captain should walk every commercial occupancy in the first-due area at least once a year, identify the furthest interior point from the most likely apparatus position, and write down the hose load required to reach it. That number goes on the preplan card. When dispatch hits at 0300 on that building, the captain pulls the right load on the first try.

Any commercial occupancy of three stories or more, and many smaller ones, has a standpipe system. The standpipe is the building's water supply for upper floors and for the deep interior of large footprint buildings. The crew that does not know how to operate it loses the fight before it starts.

The basics that every company officer should know cold. Class I standpipes are designed for fire department use, providing two-and-a-half-inch outlets at each landing or floor. Class II standpipes are designed for occupant use, providing one-and-a-half-inch hose and small nozzles, and are generally not appropriate for fire department attack. Class III standpipes combine both. The fire department connection on the exterior, the FDC, is how the engine supplements the building's pressure. Knowing whether the standpipe is wet, dry, or combined affects how fast water arrives at the outlet.

The hose pack that goes up the stair is typically one-hundred-fifty feet of one-and-three-quarter or two-and-a-half hose, a nozzle, an inline gauge, and an adapter that fits the building's outlet thread. The crew pre-connects below the fire floor, typically two floors down, hooks to the standpipe on the floor below the fire, and stretches up the stair to the fire floor. The reasons for two floors down rather than fire floor or one below are about smoke conditions and survivability at the standpipe location.

The training mandate on standpipes is clear. Every company that responds to commercial occupancies needs to drill standpipe operations regularly. It is not residential firefighting. It is a different skill set, and it has its own air management profile because the crew has already burned ten or fifteen minutes of cylinder time getting to the floor before water moves.

Commercial construction varies enormously, and the construction type tells you what the fire is going to do and how long the structure will hold. Frank Brannigan's five-type classification is the foundation. Type I, fire-resistive concrete and steel high-rise. Type II, non-combustible steel frame with masonry or metal siding. Type III, ordinary, masonry exterior with wood interior structural elements. Type IV, heavy timber. Type V, wood frame.

The collapse profiles are different for each. Type I buildings rarely collapse but trap fire and heat in compartments that can build to extreme conditions. Type II buildings have unprotected steel that loses strength rapidly at temperatures above one thousand degrees, leading to roof and frame deformation under fire. Type III buildings have hidden voids in the wall and floor assemblies that allow vertical and horizontal fire spread. Type IV buildings burn longer than expected before failing because of the mass of the timbers. Type V buildings, common in newer commercial light frame, fail fast for the same reasons residential lightweight construction fails fast.

The size-up on construction starts on the exterior. Parapet walls suggest masonry, often Type III ordinary. Visible steel frame with metal siding is Type II. Exposed heavy timbers, often historic conversions, are Type IV. Wood frame with stucco or wood siding is Type V. The captain who can name the construction type in the initial size-up has set the rest of the operation up for the right tactics.

The void spaces are where commercials kill firefighters. Suspended ceilings hide a plenum that runs the entire footprint of the building. That plenum is the path for fire and smoke spread horizontally across uninvolved compartments. Drop ceiling fires, where the involvement is above the tiles and not visible from below, are particularly deceptive because the crew sees clear conditions below the ceiling and fire raging above them. Opening one ceiling tile early is a basic move that catches this before the crew is in trouble.

A sprinkler-protected commercial occupancy is a fundamentally different fire than an unprotected one. The system, when operating, controls fire growth and often holds the fire in check until the fire department arrives. The first-due officer's job changes from primary attack to support of the system. Supply the FDC immediately. Confirm that water is moving in the system. Investigate to find the affected sprinkler and the fire it is controlling. Once extinguishment is confirmed, shut the system down to control water damage.

The wrong move on a sprinklered building is shutting the system down on arrival because of nuisance leakage or visible water flowing from the building. Until you have eyes on the fire and confirmation it is out, the system stays on. Shutting it down prematurely turns a controlled fire into a free-burning one, and the crew that was relying on the sprinkler to hold the line while they stretched is suddenly fighting a fire that has tripled in size.

Confirming the system is operating, supplying the FDC, and using the system to your advantage is a size-up element that should be part of every commercial response. The captain who treats the sprinkler as an obstacle rather than an asset is fighting the wrong battle.

Everything in this article points to the same conclusion. Commercial fires reward preparation and punish improvisation. The departments that run commercial fires cleanly are the departments that preplan every significant commercial occupancy in their first-due area, that train regularly on standpipe operations and long stretches, and that build the company officer's mental library of construction types, occupancy loads, and system features for the buildings they will actually respond to.

If your department needs reps on the decision-making piece of commercial size-up between live training opportunities, StruckBox offers a free tactical fire simulator that scores size-ups across residential and commercial scenarios with smoke and flame visible from Side Alpha, dispatch information including occupancy type and time of day, and AI feedback on tactical thinking and radio communication. It is not a substitute for a preplan walkthrough but it is the highest-volume way to build the recognition patterns that turn a commercial response from a problem into a coordinated attack.