Building Construction: Duplexes

It is imperative that you begin to
study building layouts

By Will Thalheimer 

You arrive on scene of a duplex home for a reported residential alarm activation on the left side. On investigation, you find that the home has a carbon monoxide detector activated. You’ve already fresh air calibrated your gas meter, and it’s showing 35 parts per million (ppm) for carbon monoxide. While investigating for the source, you check the gas stove, but it is off. You also check the propane boiler and water heater, but no elevated carbon monoxide levels are present at these sources. You find nothing.

After some thought, you decide to check the right side of the duplex. The resident does not answer the front door, and you suspect the right side to be occupied. The decision to force entry is made, and your carbon monoxide meter begins to light up, beep, and vibrate. The meter immediately begins to rise, and you now have 500 ppm. 

Duplexes present a unique challenge for firefighters. These buildings require forethought at the company level to ensure that incidents are handled safely and effectively. Acquiring knowledge of building construction, their shared utilities, and care when serving the customer will make incidents in duplexes safer.  

Building Construction 

Firefighters must understand the construction variables associated with duplex buildings. Duplexes often have incomplete firewalls, shared attic spaces, and shared basements and are riddled with void spaces. By understanding these key building traits, you can understand how to better mitigate hazards in theses residences.   

One of the most difficult things in a duplex home response is determining the size of the firewall. Often, the firewall will vary from building to building. By knowing the firewall size, it would help determine other key factors, such as shared spaces. Identifying any shared spaces early in fire suppression operations can become crucial to determining predicted fire spread. Unfortunately, it is difficult to easily identify shared spaces from the exterior of the structure. Clues will be in the details when conducting a 360. Some ways that a firefighter could identify shared spaces from the exterior is by looking for separate or single bulkheads/basement access, oil fill pipes, and roof penetrating firewalls.  

Because of the design of the building’s firewall, many duplex residences have share spaces, such as basements and attics. This can make fireground operations tricky, as it contributes to fire spread. Knowledge of shared spaces can be used in creating strategy in fire attack. In some cases, the duplex residents may be renters who do not have unlocked access to the shared basements or attics. Access to a shared attic could also pose an issue with only a single access located in the adjacent unit.  

In most fire prevention jurisdictions, duplexes are considered one- and two-family homes. This allows the duplex home builders to avoid many of the restrictions and regulations that a multifamily home would have to comply with. Because of the classification, the home will go unregulated once built. The homeowners are free to do with the home as they please despite the fact it has more than a single family occupying the structure. Because of the classification in NFIRS of a 1-2 family, there is little data about these structures available. 

Just like any building in your response district, it is imperative that you begin to study building layouts. A typical duplex’s floor plan will mirror opposite its adjoining unit. Often, you may find an entire neighborhood built by the same developer. This could help fire companies be aware of the building’s floor plan or common issues. Any common trait you can identify will help you when operating in a low-visibility environment.  


During a fire, identifying and shutting down utilities becomes a priority. Shutting down utilities can become tricky in duplexes because of the many the possible configurations a duplex may present. Some of the questions a firefighter must ask are the following: Is the heating shared or separate? Is the propane or natural gas share or separate? Are there two panels or a main with subpanels, and how many meters go into the building? Utilities pose a significant issue because an issue in one occupancy might affect the other occupancy.  

While conducting a 360, a fire company may answer many of the questions they might have about the utilities. Look for and identify gas connections into the buildings. Some buildings might share a tank and have separate meters for each side. Look at the electrical meter(s).  

Customer Service 

Some hazards that a company may have to mitigate will require the firefighters to gain access to both sides of the duplex. The fire officer must be prepared to explain to both residents the need to access the adjoining side. Naturally, with any hazard mitigation that affects both sides, both occupants must then be notified. When operating in a duplex, the fire company must be extra diligent of how any hazards may be affecting both sides of the building. Just like with a multifamily home, ownership of the duplex can add a layer of difficulty while documenting your actions and narrative in your NFIRS. Obtaining both occupant information as well as owner information may be required.  

As in any aspect of the job, experience will play a major factor in overcoming any obstacles faced while operating in a duplex. Preplanning, incident visualization, and coffee table discussions can all help a fire company prepare for most incidents. Before your next call at a duplex, take the time to think about the challenges you might face as a responder. How can your knowledge of these building make your community and crew safer? Take the time to discuss these buildings with your crew and make the next incident a success.  

William Thalheimer is a firefighter/AEMT with the Derry (NH) Fire Department. He has a degree in fire science, is an Everyone Goes Home Advocate with the National Fallen Firefighters Foundation, and is a New Hampshire state fire instructor. 

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