I'm looking to add a knee wall into my basement. Any guides I've found online have been for wood floors, but mine will be going into a concrete floor. What's the best way to do this to reduce wobbliness?
The left wall will be framed up from the floor to make a flat wall as well.
You can just use construction adhesive and "cut" or fluted concrete nails or concrete screws and it'll be rock solid. Your problem isn't in anchoring the plate, though. The bigger problem is above the plate, where the studs attach. This is where movement occurs if the plate is mounted well.
There are many ways you can deal with this.
Use screws to connect the bottom plate to the studs. End-grain nails don't hold very well and will quickly loosen with any side force on the wall top. 3-1/2" construction screws are less likely to loosen.
Staple or screw heavy-gauge sheet metal along the bottom of both sides of the wall, gusseting the plate and studs. 20 ga. or heavier would hold screws well. Lighter gauges could work if stapled at close intervals.
Overlay plywood on both sides of the wall, creating a much more rigid structure than you'd have with drywall alone, which resists twist. Even 1/4" would do. Then anchor the end stud to the foundation well near the top.
Embed a heavy steel bent T plate into the concrete, fastened to the concrete and the end stud. This can reside under the end stud to conceal it and be fastened to a second, interior stud. It should be at least 3/16" thick and extend up the wall 10-12" minimum and connect at several points. You can mortise the concrete for a wider bottom flange to keep it flush, or use a 4½" bottom flange, which will be hidden under your drywall.
Cut a hole through the slab and embed a double stud with anchoring cement. This eliminates all stress on the stud connections, transferring load to the rigid post. This lumber would need to be pressure-treated and should be fairly dry before use so it doesn't shrink and get loose.
Feel free to ask for clarification on any of these. I've used variations of all of them in real-world applications. My family's home construction business took knee walls and newel posts very seriously, as floppy ones quickly destroy the perception of quality.
Simply attach the knee wall to the concrete floor and the concrete/block stub wall, exactly the same as if you were attaching them to wooden framing. The only difference is the kind of fasteners you'll be using.
To attach to concrete, you'll use a concrete screw or anchor bolt (Tapcon™ is a well-known brand of concrete screw and RedHead™ is a well known version of anchor bolt no affiliation with either brand, no recommendation intended or implied). These will require drilling appropriate sized holes in the concrete and precisely lining up the holes in your lumber to match. Alternatively, you could use a powder actuated nailer to directly drive nails through the wood and into the floor.
To drill the holes for screws or anchor bolts, you'll really want to use a rotary hammer. If you don't have one, they're commonly available for rent at dedicated tool rental locations. Many big-box retail stores also offer tool rentals. If you choose to go this way, lining up the holes really isn't that difficult. Lay out your bottom plate (use pressure treated lumber since it will be indirect contact with concrete), lay out your stud locations, then mark and drill through the bottom plate with a regular wood drill bit. When you hit the concrete, you can push a little more to make a mark in the concrete of precisely where the hole is. If you're not leaving a mark or worried about destroying a (cheap) drill bit, you could use a quick shot of spray paint directly on the wood and into the hole to make a mark on the concrete. After you've drilled and marked all your holes, move the bottom plate and drill the holes with the proper size bit using the rotary hammer.
Do the same for the stud (use pressure treated here as well) that will attach to the wall.
Note that you only need PT lumber for the boards that will be in direct contact with concrete/masonry. The rest of the wall can be built with non-PT lumber.
If you have hydronic (in-floor radiant) heating, you either need to know exactly where the in-floor piping is (and avoid it), or use construction adhesive to attach things to the floor. Drilling willy-nilly into your floor is an easy way to puncture the in-floor tubing, flooding the basement and requiring an expensive repair.
Thanks for the reminder Mazura!
A good way to do this is to buy some concrete anchor bolts (sometimes called expansion anchors). You'll need a masonry bit of sufficient size and a hammer drill as well.
Mark where your 2x4s will go on the wall and then drill the holes in the concrete. Drill corresponding holes in the 2x4s. Now push/hammer the fasteners through the 2x4s to the concrete and tighten. Add washers if you need more leverage. Properly tightened, the boards shouldn't move.
You might want to make your bottom plate out of pressure treat as well, since water can wick through concrete on occasion.
I just did this exact project and was amazed at how much rigidity adding the plywood provided! I attached my studs to the bottom plate first, then stood it upright and drilled the pilot holes for the concrete anchors where I wanted them,including attached the metal plate at the end of the knee wall. Even when full framed with the t plate, the wall felt pretty wobbly. Adding plywood sheets stiffened it up considerably! Once we installed the drywall it was rock solid. This was my first time building a low wall like this and these comments were incredibly helpful. The only thing I wish I had done differently is make sure I accounted for the thickness of the t plate one the end studs. It threw the final stud off plumb but used shims and an end piece of plywood and it was all good in the end.
