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In architectural terms, a knee wall is the flat plane formed when a sloping surface, typically the roof plane, intersects with the wall at a point that is lower than the normal ceiling height. In other words it's usually a short wall in an attic that connects to the sloping roof. It got it's name because it usually came up to about knee height. Often, they are referred to by their height, for example: "a 6-foot (1.8-meter) knee wall" or "a 5-foot (1.5-meter) knee wall."
Though most often found in attics, knee walls can be built in other places as well. Sometimes people build these short walls to function as room dividers, for example. They're also sometimes installed in entryways as a decorative element.
At least two practical reasons can be given for the existence of knee walls. The first is that they can be part of the building’s structural integrity. To clarify this concept, knee walls can also be bearing walls. Bearing walls help support the weight of the structure above them. They are also called load bearing walls. Attic knee walls can even be added to a building that has already been constructed if the building exhibits structural problems, such as a sagging roof.
The second practical reason has more to do with the comfort and livability of the space in which the knee wall is found. Imagining a finished attic space will help illustrate this. If there were no knee walls in an attic, it would be very difficult to furnish the space in a comfortable manner because there would be tiny angles where the ceiling and wall joined, instead of a flat wall surface to furnish against. The attic space would have a strange feel and scale to it. Knee walls improve the livability of finished attic spaces, creating a welcoming and relaxing room.
In addition, the space formed by the presence of a knee wall often makes great storage space. Often, there is quite a bit of room between the short wall and the vertex formed by the connection between the floor and the ceiling. Building a knee wall door or hatch into the face of the knee wall can access this space. Making access to knee wall space for storage is a good option to consider if an attic is being finished, but the building owner does not want to entirely give up the storage opportunity that an attic typically provides.
My parents' cape house had knee walls along the front of the roofline. The spaces they created were horrible. They were too small to easily use for storage; they were nothing but a conduit for drafts, bugs, fire and rodents. Fortunately for us, we never had problems with the latter two, but if, say, a fire had started in one of the bedrooms, it would have engulfed the whole second 3/4 story because of those spaces. There was even a tunnel over the stairs connecting the front crawlspaces to the one above the bedroom ceilings, as if an arsonist planned it that way.
If I could have designed that house, I would have lined the whole triangular area with drywall
, then fill in the peaks with cabinets and shelving units to make use of the space; the peak above the bedroom ceilings could have been used for finished, insulated sleeping lofts. That would have made it a much safer, more efficient, less bug-prone house.
I always called knee walls half walls. I didn't know the official name was knee walls. In one of the houses we lived in, we had a knee wall between the living room and the dining room. It was made of brick and looked very attractive. It divided the living and dining rooms, but gave the area an open feeling. I liked it.
On the TV show HGTV I've seen some large attic areas with knee walls. They were intended to be a playroom or a bonus room. The walls gave the room a very pleasing feel. They had doors or just an open area to access to a large storage area. Behind the knee walls is also a great place for kids to hide when playing hide n'seek!