Since you’re reading this, you probably already suspect you have a problem with your sub floor. In my experience, water damage is the most common of sub floor problems. Whatever the cause, you need to resolve the problem before you begin with the sub floor project itself. So, you need to inspect the suspect area to determine the cause and extent of damage.
The first thing you need to do is remove the floor covering from the area and expose the wood underneath. Sub floor is generally installed using 4’ x 8’ sheets of particleboard or plywood. Sub floor that is spongy or gives when you push or step on it or sags between the floor joists, are signs of water damage. Warped or bowed sub floor can have a variety of causes from years of exposure to the hot sun, to underlying structural issues. Again, make sure you resolve what caused the sub floor to deteriorate before starting the sub floor project.
If the damaged area is a large area or an entire room, the installation is fairly straightforward. First you must remove the old sub floor. Next make sure any nail or screw heads are not sticking up anywhere on the floor joists. This is a good time to inspect the floor joists and assess if there is any underlying damage with them as well. If there is, make sure to repair the problem before proceeding with the sub floor installation.
You can replace your old sub floor with either particleboard or plywood. I personally prefer plywood, but either will work. Make sure that the thickness of the wood is the same as what you took out. If you’re not sure, take a small piece with you to the wood yard. Generally, ¾ inch is pretty common and I wouldn’t recommend anything thinner.
Finally you simply lay the new sub floor panels onto the floor joists and screw them into place. I recommend screws rather than nails as they hold the sub floor more tightly to the floor joists and help prevent squeaky floors.
Now, if you have a smaller area that simply needs patching rather than complete replacement, the first thing you will need to do after exposing the area is to cut out the damaged section. Use a piece of chalk and mark a square area around the damaged sub floor. Try to locate the floor joists by looking for the nail or screw heads in the old sub floor. Make your chalk marks on those two sides along that nail/screw line.
With a circular saw set to a depth a ¾ inch depth, cut along the chalk line. This will cut the sub floor without cutting into the joists underneath. When cutting along the nail/screw line, you should cut slightly to the inside of that line to avoid hitting the nails/screws. With the sub floor cut, pry it up with a crow bar. Remove any nail or screw heads that are sticking up. Use the piece you just cut out as a template for the new piece if you can. If the old piece comes up in pieces, as often happens, you will need to measure the opening. If you have to measure the space, make sure to do so from all 4 corners as your opening will most certainly not be square.
When putting the new piece in place, make certain that both ends rest securely over the floor joist. Now screw it in place, making sure that the edges are particularly tight and secure. If there is a small gap between the old sub floor and the replacement piece, you can seal that with some expanding foam, shaving it smooth when it dries.
Now you’re ready for your new floor covering, but that’s another story.
