Here in Colorado, if you want to finish your basement, you have to construct a floating wall. The reason you have to build a floating wall is due to the soil. We have what are called expansive soils, in that the clay will expand when, or if it gets wet. This can cause the concrete floor of the basement to heave an inch or two. While that doesn’t sound like much, it’s enough to cause some serious problems if you don’t take it into consideration. Imagine your concrete floor rising two inches, then also lifting the first floor of your house along with it. Bad news.
Building a floating wall isn’t too hard, but it’s a little hard to visualize at first.
- Lay a 2×4 on the ground where you want to build a wall. This is called the bottom base plate. Attach it to the concrete floor using liquid nails (or some equivalent industrial strength adhesive) and concrete nails. Concrete nails are driven through the 2×4 into the concrete floor using a .22 caliber ammunition.
- Lay another 2×4 on top of the base plate. This is called the top base plate.
- Cut 4 or 5 pieces of 2×4 that are a few inches long. Lay them flat, between the top and bottom base plates every foot. This is the floating wall gap distance your floor can heave. These will be temporary spacers.
- Square the bottom and top base plates. Nail in a 2×4 header across the ceiling joist directly above the base plates and spacers below. Make sure it’s directly above the base plates so your wall will be vertical and not leaning.
- Toe-nail 2×4 studs to the ceiling header board and to the top base plate every 16 inches on center. Leave the spacers between the two plates until you finish building up the studs. This will stop the top base plate from pushing down.
- Drill a pilot hole every 36 inches along the top base plate. Then drive a big 60d nail through the pilot hole, through the top base plate, and all the way through the bottom base plate until you hit concrete. Make sure you get long enough nails such that there nail head is about an inch above the top base plate.
- Once you finish building the wall, remove the temporary spacers.
You now have a floating wall (you can also think of it as a hanging wall). As you will see, nothing is attached to the bottom base plate except the 60d nail. The nail will act as a guide rod should the concrete basement floor start to rise. However, it will not lift the top base plate because the 2×4 studs running to the ceiling are holding the top base plate in place.
It’s not a hard project. The toughest part is just making sure you have the bottom base plate directly under the top base plate, and making sure the stud is vertically level so your wall is level. And remember, the floating wall is meant to move an inch or two at most. It’s extremely unlikely the basement floor will heave that much, even in expansive soils. If it does, then your whole foundation is probably having issues and you better move!
For basement doors, apply the same concepts. Just make sure there is enough space between the bottom of the door, the door jamb, and the flooring you put down to accommodate any heaving that may occur. The picture to the left shows what my pre-hung door looked like when first hung (don’t do this). It went all the way to the floor. If the floor where to heave even a little, it would push up on the door jamb causing issues. I had to take it down and cut about 1/2″ of the bottom to give it some play.
Again, the floor isn’t going to move that much, if any at all. The only reason the building authority makes us build floating walls is to safeguard that the house won’t be lifted off its foundation should the floor heave a considerable amount. It’s totally a worse case scenario thing.