Knee walls, or partition walls, are usually found inside the attic to offer a vertical wall surface within the overall area of the attic floor, reducing the living space from where the roof rafters behind it keep extending their slope right down to the floorboards and offering extremely restricted low space at that side of the room.
Knee walls are considered load-bearing partition walls. They are not to be confused with half walls or pony walls, however, as that open concept type of walls is mostly used as room dividers, adding interior design features to separate spaces.
How Tall Do Knee Walls Need to Be?
The exact height of a knee wall or partition wall can vary quite a bit. Generally speaking, though, three feet tall and five feet tall are fairly common heights for a knee wall for a number of reasons.
Before deciding what height is the best for your knee wall, think about why you are adding one to your home. If you are adding one to have more vertical space in an area to hang things like art or to store items, you may want to build the knee wall that is on the taller side.
If, however, you are using the knee wall to divide a kid's play area from a family room, you may want a shorter knee wall so you can still see the kids when they are playing in the area but have a greater sense of divisions between the area designated for kids and for adults.
Make sure not to build the knee wall too short, or it can become a tripping hazard as people may not see it until it is too late. It will also look silly if you have a barely there knee wall in your home.
Figure out the height of your knee wall before you go any further. This will affect how many materials you need, how long the project will take, and its total cost so it is good to figure out sooner than later.
It is also a lot harder to add an extra two feet to the knee wall when you are far along in the process than it is in the beginning, so it makes sense to figure out the height before you even start on the project.
How Do I Build a Knee Wall?
Adding a knee wall to your space is a project you can do if you just follow the steps below. Make sure you take safety precautions when working with the tools necessary to build the knee wall, keep the area clean of debris, and make sure pets and small children are not in the area where you are building the wall.
Step 1 - Identify the Most Practical Place for the Knee Wall
Look around the attic space and decide on a balance between the height of the wall to be built and the square footage still acceptable for live-in space.
Try to figure out what bookshelf, chair, or other accessories and furniture you might want to have against that particular wall in the future—it would be sad to complete the project just to find out that your bookshelf or headboard is just a couple of inches from fitting perfectly against the wall.
So since trial and error is easier done with a pencil, a two-foot level, and a straight edge, this is where the decisions are made.
Since you obviously want to make this new space as roomy as possible, decide on the height your wall needs to be, cut yourself a length of a stick to that height, and tape the stick to your 2-foot level.
Place your stick vertically against the gable wall of the attic and touch the slope of the ceiling so that it stands vertically between ceiling and floor tracing marks on the wall, the ceiling, and the floor at that exact location.
Measure the distance from the floor mark to where the ceiling meets the floor, and trace another mark at that same distance at the opposite wall of the attic.
You can now get your chalk-line and draw a perfectly straight line from the gable wall to the opposite wall on those two marks.
With that line laid out, bring your stick-level assembly at various spots along the chalk-line to make certain that the wall height is constant and not submitted to irregularities such as a sagging rafter or something. If you find variances in the height, move the chalk-line over towards the inside to compensate and maintain a minimum height.
Trace the final location lines on the end walls, the floor, and the ceiling to provide a guide when erecting the knee wall.
Step 2 - Prepare the Pieces
The soleplate is normally made of one solid straight length of lumber as long as the distance between the two end walls. However, if it has to be made from two or more lengths of lumber, mark on the floor where the studs will be placed so that your soleplate sections of no less than four feet can be cut to leave the splice about centered between two studs and evenly reinforced with a 14-1/2 inch long piece between both studs.
Cut your soleplate on the short side of your mark to prevent the frame from getting jammed between the two end walls and damaging them too seriously.
As much as possible, the top plate should be one full length. since the edge facing inwards should be beveled, the angle of the bevel should now be figured out. One of the easiest methods without getting into complicated mathematics is to use a speed square.
The 90° point of the square is placed with the “hook” side up against the ceiling (or rafter) with the 90° on the lower side of the pitch. This is your pivot point as indicated on the square. Place one end of your level at the pivot point and set it perfectly horizontal against the face of the speed square.
The longer side that is, in fact, opposite to the 90° angle (called hypotenuse) has a series of angles inscribed all along its outside edge. All you have to do is read the angle wherever the level lands when perfectly horizontal.
That angle subtracted from the 90° right angle gives you the amount of angle to tilt the circular saw in order to rip the inside edge of the top plate to the required bevel cut.
As an example- If the level landed at the 27° mark on the speed square, using 90° as a constant:
90° - 27° = 63°, therefore you need to set the saw bevel at 63°.
Secure the top plate across two sawhorses and make sure your saw is equipped with a ripping blade. Then adjust your portable circular saw for the proper bevel angle.
You can now put on your safety glasses and ear protectors and plug your circular saw into an extension cord that permits a reach with the saw from one end of the top plate to the other and start your saw.
Put the wide part of the saw table on the piece of lumber and start cutting with the long point on top, maintaining a straight line by carefully following the top edge of the top plate against the blade.
Lay both the soleplate and the top plate side by side with the inside up, and mark the positioning of each stud across both pieces beginning at one end and marking them every 16-inches all the way to the other end.
With the long pointed corner of the top plate’s bevel edge aligned with the ceiling marking, temporarily tack the plate in place with a screw at both ends.
Screw down the soleplate along with the floor markings and measure the long point height of the studs inside the two plates.
Using the pitch angle of the ceiling, adjust the angle of your miter saw and proceed with cutting enough “cripple studs” to frame your wall.
Placing both plates on their inside edge with the beveled edge of the top plate flat on the floor, proceed at nailing a cripple stud at each end to help hold the rectangle together, making the assembling procedure easier. You can then finish nailing the rest of the components in.
Stand the wall up and bring it in place along with the ceiling markings first, where you can screw in place with #10 or 12 wood screws 3 to 3-1/2 inches long, making sure the screws line up with the roof rafters for a more secure hold. Secure the top plate all along with screws holding to every rafter.
Working next with the soleplate, check it all along to see if it is fitted tightly along the chalk-line and against the floor. If it’s too tight of a fit, place a scrap piece of lumber on the floor against it, and hit on it with your hammer until the soleplate lines up with the floor markings.
It should not, however, be any more than a 1/8” over-fit in the height. If the soleplate doesn’t sit tightly against the floor, use some wood shims to fill underneath where it will be screwed or nailed in, and trim the ends once secured into place.
If at all possible, secure the wall at both ends to the adjacent walls. If the walls are hollow, you could resort to using mechanical anchors.
Before covering the wall with drywall, make sure nothing shifted and went out of level by doing spot checks with your 2-foot level.
Step 3 - Add the Wall Treatment and Sheeting
If this knee wall is erected in the attic, it will probably be a good idea to insulate it with batten insulation and a vapor barrier if the space is to be lived in.
Once your wall is insulated, and the vapor barrier applied on top, you can measure the surface area of the wall and cut your drywall or paneling to fit. The drywall can be cut using a drywall square, straight edge, drywall saw, or utility knife. Drywall can then be secured to the new wall frame with the appropriate drywall screws or drywall nails.
In order to protect the flooring from plaster and paint drippings, cover the floor with protective drop cloths. All the seams can then be covered with drywall tape and joint compound. Just make sure the plaster is applied in multiple successive thin coats rather than thicker layers which would be more difficult to sand to a smooth finish. After the compound is completely dry, use fine sandpaper to further smooth out the wall surface.
Once the sanded drywall finish is perfectly smooth and even, the trimmings, moldings, and baseboards can be cut to length and nailed in place to add a more finished look to the room.
Finally, you can paint your wall starting with a coat of quality primer followed by one or two coats of paint. If you chose paneling as a wall covering, you may opt to coat the wall with stain.
It’s now time to pick up the drop cloths and tools and give the room a good cleaning to get rid of all the remaining dust. And there you have it, a newly renovated room with a nicer look provided by vertical walls all around.
Building a knee wall is an easy project for some DIYers, but for people who have never built a large project before, can be quite difficult. If you are new to this type of project, you should consult professionals.
Make sure you fully understand how to correctly use all of the materials necessary for this project and be careful to prevent any errors from occurring.
If you have followed all the above steps, you have successfully built knee partition walls. While these are most commonly found in attics, the same process outlined above can be used in other areas of your home as well.
For more on this topic, check out our piece on Making Your Own Curtain Wall Partition.