DON’T IGNORE THE KNEES…. THE DANGER OF THE KNEE WALL VOID SPACE
Recently, the Harrisburg Bureau of Fire conducted our in-house truck academy for our apprentice firefighters that are completing their first year of service.
This intensive program covers all aspects of truck company operations, as well as aerial apparatus placement, setup, and usage. At the end of the program, successful candidates are then able to be assigned as the second firefighter on a truck company.
We currently have the luxury of having many vacant structures with which to work, which makes the truck academy training very realistic. During the class, we get to look at many of the types of buildings that we have in the city.
For those occupancies that we can enter and operate inside, we’re able to cover building construction and fire behavior in the various construction types.
One of the areas on which we spend time is knee walls and the inherent issues that come along with them. We trained in a 2-1⁄2-story ordinary (Type III) row home and a 2-1⁄2 story wood frame (Type V) row home that both contained significant knee wall construction and voids.
In this series of images from the two different styles of building construction, you can quickly see the availability of fuel and pathways for fire to spread with in the knee wall voids.
The first home in the series of images (the ordinary construction row) will show before and after images from overhaul with no fire conditions. The second part of the series (the wood frame row) shows some of the areas after significant fire damage and overhaul.
There are 25 images related two these buildings and the knee walls within them. Each image will have a caption to explain what we found and how it effects fire ground operations.
1. 2 1⁄2 story ordinary constructed (Type III) row of three homes. Note the different style dormers and consider the changes they have on the size of the knee wall voids in each section of the home.
2.When opening up the roof we were careful to leave the ceilings in tact to show how the void spaces are interconnected from the Knee Wall area over the ceiling and into the attic space. In this image you can see the side c knee wall space and how it is connected to the several inch void leading on an angle to the cockloft area. There is plenty of fuel to burn in this area.
3. Another view of the side c knee wall void area. Notice the open area along the brick wall at the bottom of the photo. This is open to the second floor ceiling void as well as the side b wall void. Hopefully you're starting to see how these voids can quickly communicate fire throughout the structure.
4.Closer to the peak of the roof above the 1/2 story attic area ceiling you can see a large triangular void space. The chimney passes through this area and once again you can see a void space open to the areas below.
5.This view looking form side C towards the A side shows the entire void area opened up. You can see the area inside that has been finished off for living space and how it is almost completely surrounded by void spaces. Also you can see how they are interconnected and can communicate fire rapidly upward. Keep this image in mind when considering where you’re going to have to vent when fire enters these spaces.
6.Here is an interior view of the side C Knee Wall void area from the inside. The entire area behind this wall is void and wide open.
7. Opening this area is going to be critical in controlling fire. This area will likely have to be vented from the outside before being able to make significant progress from the interior. If you suspect there is fire in the knee wall void a charged line must be in place prior to opening up these voids.
8.Once completely opened you can see the large amount of space that was concealed here as well as the upward fire spread opportunity provided by the rafter bays that are interconnected.
9.Another view of the rafter bays. This area is connected to the Knee Wall and the cockloft. It is also completely constructed of wood and can quickly become involved in fire and communicate it rapidly toward the cockloft.
10. This much larger Knee Wall area was located on the side C wall of one of the homes. Obviously the larger these areas area the more fire they can contain. Often you will see areas like this with a small door in the wall allowing the occupant to utilize the area for storage.
11. The size of the dormer can give you some clues as to the size of the knee wall. Obviously the smaller the dormer the larger the area that will be covered by the knee wall.
12. This is a shot looking up at the area where the rafter bay connects to the cockloft. As you can see it opens up into a fairly sizable space. A significant volume of fire can take hold of this area.
13. Another view of the knee wall area after it has been opened up. Also take note of the various layers of ceiling material that have been added through the years. From plaster and lath to furring strips and fiberboard, each layer will make it more and more difficult to effectively open the space above.
14. Inside the knee wall space there is plenty of opportunity for lateral fire spread between attached structures. In this image you can see a brick fire wall was constructed to prevent lateral fire spread. However, you can see that time has taken its toll on the wall and there are gaps in the mortar which could easily and quietly communicate fire to the adjoining space.
15. In the same row you can see a knee wall void space where an attempt at a fire wall was started but was not completed. The discovery of a brick fire wall in these homes is always a welcome site, however you can not trust that they will be 100 percent effective. All of the void spaces must be opened up and checked.
16. Many row homes have a rear bump out often of two stories. The roof of the two story bump out has its own cockloft and is usually tied directly into the voids of the main portion of the building. Here you can see the second story bump out cockloft wide open into the 1/2 story side C knee wall void. So again, a fire on a lower floor can quickly communicate via these paths and spread vertically throughout the entire structure.
17. In this image we can see the Side A Knee Wall before and after opening up.
18. This image shows the connection point between the roof rafter bays and the cockloft a the top of the building. Notice there is no type of fire stopping allowing for quick vertical fire spread.
19. This is an image of the 2-1/2 story wood frame row. If you look closely you can see that the dormers are quite small which creates a potential for a large knee wall void space. Another point to consider here is exterior fire spread toward the soffit. Whether it is fire extending out a window or up the exterior of the home once it reaches the soffit it has access to the knee wall and eventually the common cockloft of the building.
20. Again you can see that the dormers are small and set back creating a large area for void spaces. These dormers are also interconnected to the knee wall voids and have their own mini cockloft which of course is directly tied into the main cockloft of the structure.
21. At the bottom right of this dormer you can see the open space between the knee wall void and the wall of the dormer which intern is open to the mini cockloft of the dormer and the main cockloft of the building. This area must be effectively overhauled.
22. Here is a view of side C and what remains of the knee wall on that side of the building. Also if you look closely you can see where a dormer was at one point but was removed and covered with plywood. Also note that there was no significant or any for that matter support for this area of the roof. Remember to always sound the roof. This is an area that could quickly fail under the fire conditions that were found in this room.
23. Fire can travel from the knee wall up through the rafter bays to the cockloft which it did here. You can tell by the deep charring that there was a significant volume of fire in this area. You can also see by the “clean” wood and walls that this fire was contained within the void spaces.
24. Back at the front window you can see that the area below the window is open to the side A knee wall space. This is a sure way for fires to spread vertically within the dormer.
25. A tighter view of the side A Knee Wall void space shows a significant area for fire spread.