Hidden dangers: Fires in knee walls

How to safely mitigate incidents with half-stories


By Brandon Pfaff

When the first-arriving unit gives its size-up report and you hear, “I have a 2½-story single-family dwelling, fire showing on the second floor,” your brain immediately starts to paint a picture in your mind about the tactics, equipment and next steps. But what do you picture for the half story – an empty attic space, a converted bedroom, a cluttered storage space?

Not only are half stories often occupied, but these spaces can create considerable danger for responding firefighters. After all, it’s essentially a void space that can accumulate super-heated gases, fire and smoke, all of which can easily go unnoticed in the absence of basic building construction and fire behavior knowledge, along with basic firefighting equipment, such as hand tools and a thermal-imaging camera (TIC).

Note the window at the peak of the roof, which, along with the steep pitched roof, can be easy indicators of knee walls being present.
Note the window at the peak of the roof, which, along with the steep pitched roof, can be easy indicators of knee walls being present. (Photo/Brandon Pfaff)

Many such void spaces are created by the construction of short wall sections referred to as knee walls. Here we'll explore the impact of knee walls and what firefighters need to know to safety mitigate incidents with half stories.

What is a knee wall?

A knee wall is sometimes confused with a pony wall, but there is a considerable difference between the two features.

  • Knee wall: A knee wall is a vertical wall that varies in height between 2-4 feet and is used to support the rafters on the roof. This wall assists in creating a more occupiable space within the half story (typically attic space). This effectively creates a void space, often used as storage, between the inside of the knee wall and the eaves of the rafters.
  • Pony wall: A pony wall is a half-wall typically found in areas of the dwelling to divide space up without losing the visual continuity to another room space within the same floor.
Diagram showing knee-wall used in timber roof construction that has no structural function.
Diagram showing knee-wall used in timber roof construction that has no structural function. (Photo/Wikipedia)

Identifying knee walls

While knowing the exact location of a knee wall can be difficult at first, identifying the basic presence of knee walls can be simple to a trained eye. Here’s how to do it.

When performing a size-up, first look for a steep pitched roof with a small window or even a standard-size egress window in the center of the rafter lines. This is a good indication of a knee wall. Additionally, look at the roof. You should already be evaluating the pitch of the roof and the covering to consider ventilation tactics. The use of steep rafter angles usually produces a greater ceiling height to make the space more occupiable for residents. Along with this, knee walls are often used to make the space feel more like a traditional bedroom, as they can square off the room to allow a bed or other furniture to fit against the wall.

Knee walls are found more often in conventional-style construction with the use of traditional rafter and ceiling joist construction. Some knee walls may have been created to simply provide more out-of-sight storage areas, and in many cases, we will find these areas full of highly combustible storage.

A knee wall is a vertical wall that varies in height between 2-4 feet and is used to support the rafters on the roof. This wall assists in creating a more occupiable space within the half story (typically attic space). This effectively creates a void space, often used as storage, between the inside of the knee wall and the eaves of the rafters.
A knee wall is a vertical wall that varies in height between 2-4 feet and is used to support the rafters on the roof. This wall assists in creating a more occupiable space within the half story (typically attic space). This effectively creates a void space, often used as storage, between the inside of the knee wall and the eaves of the rafters. (Photo/Home Energy Rx)

Looking to see if dormers are present on the roof is another useful way to spot knee walls, especially in older homes. Bear in mind, in newer constructed homes, dormers are commonly used for aesthetics and may not be indicative of or contiguous to other roof areas and voids.

Being aware of the general age of the home is vital, not only for ventilation tactics but also the hidden dangers of other construction features. Dormers are used to expand the space within the attic space, giving the occupant more space to store items and allow more sunlight into the space.

Knee walls and fire dynamics

Knee walls present a unique situation for fire spread. The large void space that the knee wall creates allows for a horizontal spread of flames and super-heated smoke. There are many cases where the rafters within the void space are not protected with drywall/plaster or lathe material, allowing fire to spread not only horizontally but also vertically between the rafters to the ridge line. As a result, fire can free burn above the companies operating below.

All this hidden fire and gas will create an environment of extreme radiant heat, low to zero visibility, and a high potential for flashover. The incident commander needs to be aware of the smoke conditions from the exterior and understand that if smoke conditions start to change in a negative way, the interior companies operating will most likely not see those changes.

Putting it all together

Now that you have identified the possible presence of knee walls, how does this affect tactics?

If a fire is impinging on the attic space, even if from flame spread from a window below into the soffit, then ventilation and suppression must occur quickly. If you believe that there is a high chance of a fire occurring in this attic space, an engine company working in coordination with a truck company must make every effort to prioritize fire attack in that space. The truck company must split their staffing to ventilate the roof and open the interior walls to expose potential hidden fire within the large void spaces in the knee walls.

Again, working in coordination with the truck company, the engine company must have a handline in place, flowing water when the truck company is opening the ceilings and/or walls in the attic space. The other half of the truck company should be on the roof preparing to start vertical ventilation.

The best area to cut is where a dormer is present. The sawyer will start by cutting adjacent to the intersecting line where the peak of ridge pole and the main roof meet. Cutting downward, the sawyer should cut to below the ending wall of the dormer. This cut will be approximately 4-6 feet long. When cutting the horizontal lines, make the cut approximately 4-6 feet wide.

This photo depicts the two side knee walls creating a void space at the eaves of the roof. Right above the finished ceiling are collar ties, tying the rafters together, that assist in outward spread of the roof. The attic space is a void space concern for fire operations as the fire can be free-burning above the operating company’s head.
This photo depicts the two side knee walls creating a void space at the eaves of the roof. Right above the finished ceiling are collar ties, tying the rafters together, that assist in outward spread of the roof. The attic space is a void space concern for fire operations as the fire can be free-burning above the operating company’s head. (Photo/Nordic Home Inspections)

The hook member can then begin to punch through the roof decking into the void space below. You should see the top plate of the knee wall appear at the top third of your cut. When cutting into a roof without dormers present, the sawyer should cut from 1-2 feet below the ridge pole of the roof downward 4-6 feet, then repeat the same size for the horizontal cut.

Cutting small inspection holes can also be handled prior to the main ventilation to determine placement of the hole. Look for high-velocity pressurized smoke that’s dark in color. If flames are coming through the inspection hole, move horizontally away and continue to cut the inspection holes until smoke only is found.

Communication is key in these scenarios, as well as using tactics that are coordinated among the companies operating and the incident commander.

About the Author

Brandon Pfaff is a 13-year student of the fire service, currently serving as a firefighter in a suburban volunteer department not far from Philadelphia. Pfaff is also employed at an engineering firm based out of Bucks County, Pennsylvania, where he has performed building code enforcement/fire inspector duties for almost 5 years. He has a background in building construction, as a graduate of Upper Bucks County Technical School Carpentry program. Pfaff holds certification in ProBoard Fire 1, 2; Inspector 1, 2, 3; Fire Instructor 1, 2; Fire Investigator and Officer 3, to name a few certifications. Pfaff previously served as training officer at another department.

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