Jonathan Hall: The Hazards of Knee Walls

By Jonathan Hall

You have just finished a nice firehouse dinner and are enjoying some camaraderie around the table when your company is dispatched to a reported dwelling fire. As the box number is given and the assignment read, you know your engine company will be first due. While en route, dispatch advises that it is receiving multiple calls and is filling out the assignment.

On arrival, you find a 1½-story, wood-framed, single-family dwelling with heavy smoke coming from the top floor. As your captain is giving a size-up and performing his 360° survey, you and your partner begin stretching a preconnect to the front door. As your partner finishes flaking out the line, your captain returns and advises that everything appears to be on the top floor.

Your partner quickly forces the front door as you bleed the line. Your company begins advancing the line into the dwelling, where the entire first floor is clear. As you make the stairs and begin your ascent, smoke is visible at the top. On the top floor, there is heavy smoke and high heat, but no visible fire. Your company searches in vain for the fire.  Finally, the decision is made to open up the walls in search of any hidden fire.

Your partner crawls over to the wall to open it up with his halligan. With his first blow, the entire room erupts with immense heat. You open the nozzle to cool the atmosphere, but fire seems to be coming from everywhere. You can feel the extreme heat burning you through your protective gear. Your company struggles to fight its way back to the stairs and down to safety.

What happened? How did the entire room light up so quickly with no warning?  Unbeknownst to you, the wall your partner opened up was a knee wall that had a large void space behind it that concealed a tremendous volume of fire. As soon as your partner opened a small hole, it provided the proper mixture of fuel and oxygen to ignite the entire top floor. As you are taken to the hospital for evaluation of your burns, you wonder how this incident could have been avoided.

What is a Knee Wall?  What are the Hazards?

Knee walls are often found in the occupiable half stories of dwellings (photo 1). A knee wall is a short stud wall that extends from the floor to approximately chest height where it attaches directly to the underside of the pitched roof (photo 2).  These walls enclose the lower portion of the roof rafters, or truss system, along the eave line of the structure.

Knee wall construction is not only found in older style homes. New premanufactured attic truss systems have knee walls built into the truss. These trusses are built like any other truss system and are delivered to the job site preassembled.

The biggest hazard with knee walls is that they create a tremendous void space that extends the entire length or width of the structure (photo 3).  Fire can get into this void space from the inside or the outside of the dwelling. Once in the void space, the fire and products of combustion can spread out without restriction.

In addition, the fire load in a knee wall void space can be significant. The flooring, walls, rafters, roof decking, and insulation provide a substantial fuel load. Often, small doors are placed in the knee wall to allow for storage in the void space. Homeowners can place considerable items in the void, which contribute to the fire load.

Because these walls attach directly to the underside of the roof, the fire is able to extend through the roofline and into the void space created by the knee wall on the other side of the room. Frequently, there is an additional void at the peak of the roof created by a collar tie, a short joist that connects the two rafters (photo 4). These three void spaces allow for fire to be concealed all around members operating on the top floor.


It is vital that members take time to ensure they perform an accurate size-up of the structure.  What are the height, size, and construction of the fire building?  Is smoke or fire visible? Where is the smoke or fire coming from? What is the extent of the fire? Are any structural components compromised? Offensive or defensive?

As mentioned earlier, knee walls are often found in the occupiable one-half-stories of dwellings.  On arrival, members should establish that a half-story is present and then look for clues to determine the presence of knee walls. Several things may indicate that the floor is occupiable: dormers, windows on the gable ends, window coverings, and window-mounted air-conditioner units. None of these indications definitively mean that knee walls are present; they just suggest that the odds that they may be encountered are greater…

Once it is determined that knee walls may be present, ascertain whether the fire involves the void spaces. Look for smoke or fire coming from the soffits, the lower roof vents, the corners of the gable ends, and from the lower edges of the roofline (photo 5). These signs suggest the likelihood of void space involvement.

Fires on the floor below the top floor or in dwellings built using balloon-frame construction may easily extend to the void space. It is critical that knee wall-created void space fires are identified as quickly as possible.

Thermal Imaging Cameras

Use of thermal imaging cameras (TICS) can significantly aid in determining whether the fire involves the knee wall void space.  Scan the building from the outside on arrival to determine if the camera indicates high heat near the eaves. The higher heat signature on a typical roof would be near the peak of the roof. If the camera shows it is significantly hotter near the eaves of a half-story home, there is a high probability that the fire involves void spaces.

Once you make entry into the structure, scan the ceiling toward the exterior walls on the floor below the top floor.  High heat near the exterior walls indicates the possibility of a knee wall fire.

As crews advance to the top floor, use the camera again. Scan the floor to determine if the fire basically involves the room and contents or whether it has extended into the void space created by the knee wall. Remember, knee walls are located on multiple sides.

Fire Attack

Fires involving knee walls on the top floor can take on two forms. Fires that were room-and-contents fires initially may extend into the void space. As members extinguish the room-and-contents fire, they may experience little relief from the heat. Horizontal ventilation will provide little relief because the heat is contained in the unventilated void space. If the void spaces are not quickly opened up and checked, fire extension may continue to develop unseen until it exposes itself and potentially traps operating members.

The other form that knee wall fires can take involves fire concealment in the void space.  This type of fire may originate in the void space or extend from the floor below, which is very common in balloon-frame construction. In this type of incident, members will make it to the top floor and find very smoky conditions with extreme heat. Firefighters will often search in vain for the fire until they are driven off the floor by heat or the fire suddenly exposes itself in an extreme fire event.

Crews should ensure they have a charged attack line with them and a backup line close behind. As soon as you make it to the top of the stairs, use the TIC, as mentioned above.  Make small inspection holes in a knee wall before venturing far out onto the floor. Have the line in place, and be ready to aggressively attack any fire you find. Placing inspection holes close to the stairs allows for rapid egress if an extreme fire condition results. These stairwells are frequently very narrow; ensure there are not too many members operating on the floor so that safe and rapid egress is possible…

As the attack line advances out onto the fire floor, position the backup line at the top of the stairs; this line ensures that the attack line is not cut off from making egress down the stairs if something should go wrong. Remember that the void spaces are connected, meaning that fire conditions may erupt on the opposite side of the dwelling. The backup line should be in position to attack any fire that may be exposed remote from the attack line. It is not uncommon to have multiple lines operating on the top floor when knee walls are involved.

If the fire involves the knee wall-created void space, crews must be very aggressive when opening up the walls and ceiling. A charged hoseline must be in place anytime a wall or ceiling is being opened. Always remember that the fire can easily travel the entire length or width of the house and can extend through the roofline to the opposite side.

If conditions deteriorate and members are driven off the top floor, they should proceed to the floor below and open up the ceiling near the exterior walls so hose streams can be directed up into the void space. A majority of the fire can then be extinguished from the floor below.


Fires involving the top floor can initially be ventilated using horizontal means. Horizontal ventilation will relieve some of the smoke and heat conditions. This may be sufficient if it is solely a room-and-contents fire. If the fire involves the void space, horizontal ventilation will be inadequate.

Use vertical ventilation if the fire is concealed in the void spaces. Proceed to the peak of the roof using an aerial or a roof ladder to ventilate. A hole placed at the peak may alleviate some of the heat and smoke conditions on the fire floor. In addition, a hole placed at the peak will ventilate any void space created by a collar tie running between the rafters. A hole placed at the peak will not ventilate the tremendous volume of fire gases trapped in the void space created by a knee wall.

If the fire is concealed in the void spaces, ventilate the lower edges of the roofline (photo 6). Because of the extreme pitch of the roof, use an aerial device or a roof ladder to safely achieve this. Cut holes near the edge to vent the void space. You may have to make multiple holes since the fire and gases can spread out to the entire length or width of the house. It may be necessary to cut holes on the other side of the roof near the edge of the roofline to vent that void space as well.


Fires involving knee walls are different from most fires. The void spaces created by these walls place members in an extremely dangerous environment with hidden fire on all sides of them. Take the proper steps to recognize this type of fire as quickly as possible.

These fires are very personnel intensive. Multiple hoselines often have to be used, and the entire top floor must be overhauled to properly extinguish the fire. In addition, multiple ventilation holes must be placed in the roof to allow conditions to become tenable enough to extinguish the fire.

If members are not properly prepared to attack a fire involving knee walls, extreme fire conditions can result and trap members on the top floor.  These fires have caused numerous Maydays to be transmitted and have tragically resulted in line-of-duty deaths.  Employing a thorough size-up as well as using proper techniques will hopefully reduce the risk to operating members.


JONATHAN HALL has been in the fire service since 2000. He is a firefighter in St. Paul, Minnesota, assigned to Engine Company 8. In addition, he is a lead instructor for the department’s training division and teaches a multitude of disciplines to members of all ranks.



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