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In architectural terms, a knee wall is the flat plane formed when a sloping surface, typically the roof plane of a building, intersects with the wall at a point that is lower than the normal ceiling height. In other words it's usually a short wall standing perpendicular to a sloped roof, and is usually added as support. Its name is related to its height; in most cases it comes up to about a person’s knee. These sorts of walls are most commonly found in attics, but can be found anywhere there are sloping panels. Older homes with eaves on multiple levels are one example, and many starkly modern designs also make use of sharp angles that require these sorts of walls for support. While support is perhaps the most important reason these walls are built, they can also improve the functionality of a space, and are sometimes even added as a design element or for decorative purposes.
Building construction is often a precise science, one that depends as much on artistic vision as on the proper mathematics of angles, weights, and supports. This is perhaps nowhere as obvious as it is where a structure’s roof is concerned. Unless properly supported, the whole building may cave in on itself. Shorter walls at knee height are one way of providing additional support to sloped or crested ceilings, which is to say, those that come to a point or triangular peak at the top of the structure. Sometimes simply anchoring the sloping panels to the main skeleton of the house is enough, but adding additional beaming can provide greater protection. An entire finished wall is often the strongest option, and has the added benefit of creating at least some sort of useful space beneath the rafters. This is how many finished attics are formed.
Though most often found in attics as support, these sorts of walls can be built in other places as well. Sometimes people build short walls to function as room dividers, for example. They're also sometimes installed in entryways as a decorative element. In these cases the walls aren't providing support as much as a desired look or feel.
Excluding purely aesthetic structures, there are usually at least two practical reasons for erecting knee walls. The first usually has to do with structural integrity and support, in large part because, when properly constructed, knee walls can also be bearing walls. Bearing walls help support the weight of the structure above them. They are sometimes also called load-bearing walls. In some instances they 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 wall is found. Imagining a finished attic space will help illustrate this. If there were no walls, however short, 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. Walls improve the livability of finished attic spaces, and make cleaning, storage, and even living more practical.
The space between the angle of the roof and the wall doesn’t have to be abandoned, and in some cases is actually developed as an intentional 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 door or hatch into the face of the knee wall can access this space. Making access to storage is a good option to consider if an attic is being finished for more everyday use but the building owner does not want to entirely give up the storage opportunity that an attic would typically provide.
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!