When building a knee wall over a concrete pad, the bottom frame plate needs to be mounted to the concrete itself. You cannot simply screw it in place. Instead, lag screws are required. First, select a wood drill bit that is slightly smaller than the lag bolts and drill holes every 12 to 16 inches across the bottom of the frame, in between the vertical studs. Place it where you want it, copy those marks onto the concrete slab, switch over to a concrete drill bit, drill down into the concrete, and then mount your knee wall with lag bolts into the concrete pad.
Through the Subfloor
The typical subfloor is around 5/8 inch in thickness but can vary all the way up to 1 1/2 inches if there are double layers of plywood or oriented strand board in place. The subfloor itself is not a suitable mounting platform for the knee wall. It must be mounted to the floor joists themselves. Use a stud finder to locate the floor joists and then use wood screws to mount the bottom plate to the subfloor and floor joists. For best results, use screws that are at least 4 inches in length, long enough to go through the two-by-four material, through the subfloor and into the floor joist. Set them at least every 12 inches across the bottom plate.
Into the Wall Studs
Further anchoring at the wall studs only serves to strengthen the wall. If the wall material hasn’t been installed yet, screw the vertical section of the knee wall into place where it butts up against the full wall. Use at least 4-inch screws to sink through the width of the two-by-four material and into the wall stud; place the screws at least every 12 inches up the wall. If you have existing wall material in place, use a stud finder first to locate the studs and then draw a level line up the wall to represent where the stud is, then use that line to guide your screw mounts.
If you cannot place your knee wall directly overlapping a wall stud, use a nail board to secure it in place. This is a horizontal two-by-four cut down to 16 inches to fit between two wall studs and then screwed and mounted into place, allowing you an anchor point. Set these at 12 inch intervals from the bottom of the knee wall to the top so you have plenty of room to nail. The same rule applies to the floor, since you need to be nailing into something solid, not the subfloor. If doing this after the fact, you’ll either have to remove drywall for the walls or work from the underside of the home in the crawlspace to access the area between floor joists. For second-story areas, this is accessed through the ceiling from the first floor.