You are pulling up the flooring in the house you’re renovating, and you discover an unwelcome surprise: a damaged subfloor. Suddenly your flooring project becomes more complicated. What can you do to fix the problem? Replacing a subfloor is a big project, but one you can take on with confidence with a little know-how, a few tools, and some determination. Subfloor repair may seem daunting, but it is actually fairly simple. Continue reading to learn the steps to replacing a subfloor in your home.
What is Subfloor?
The subfloor is the wooden base upon which your floor covering (tile, hardwood, LVP, carpet, etc.) sits. Typical subfloor materials include particle board, OSB, or untreated plywood. The subfloor is installed on top of the floor joists: the thick, horizontal beams that hold up the subfloor and the floor.
How Do I Know if I Need to Replace My Subfloor?
You will know when you pull up the old floor covering if the subfloor needs replacing. Signs of water damage include rotten, spongy, damp, or moldy subfloor. If you find any of these signs, it is likely time to replace the damaged subfloor. Other times, you may notice the floor sagging, squeaking excessively, giving way when you step on it, or smelling bad. You might also see cracked or popped tiles that show that the subfloor is swelling or water discoloration in the ceiling below the floor. A squeaky floor or any of these other signs can indicate that the subfloor may need to be replaced. A significantly water-damaged subfloor should definitely be replaced.
If you are replacing the entire floor covering, such as retiling the kitchen or removing LVP, and installing hardwood, you will likely want to go ahead and replace the subfloor. This gives you a clean slate and a fresh surface for your new floor.
Discover the Reason for the Damage
If you find rot in your subfloor, it is important to determine the source of the problem. If you do not identify and correct the problem, it may happen again. Subfloors rot primarily due to water damage, and there are several possible causes of water damage. Below are some possible causes:
- If the flooring is rotten near a window or an exterior door, water could be entering from the outside
- If the damage is located near a sink or toilet, there may be a leak somewhere nearby that needs to be repaired
- There may be moisture getting in from under the subfloor. If your home is over a crawl space, moisture could be coming up from the ground and damaging the wood. You can combat this problem by adding a moisture barrier in the crawl space, installing a crawl space fan, or diverting water from the foundation of the house with gutters and downspouts.
Decide How Much Subflooring to Replace
If you are replacing the subfloor due to damage, you could cut out the rotten wood (and a few extra inches on each side), and only replace that section. The trick is to make sure the new subfloor patch is level with the subfloor around it. If the damage is widespread, you want to ensure the subfloor is solid, or it will be challenging to make the flooring level. You may determine that you want to replace the subfloor in the entire room.
Once you have a plan, it’s time to get started!
Step One: Remove the Old Subfloor
Use a circular saw to remove the damaged section of the subfloor. You can set the blade depth to match the depth of the subflooring so you don’t cut floor joists accidentally. Use a pry bar to pry up each section (or if it’s really rotten, carefully use a sledgehammer to get rid of the old subfloor).
Remove all the nails or screws that secured the old plywood subfloor to the floor joists. This part is painstaking, but it is necessary in order to give you a new level subfloor. Use a reciprocating saw to clean up the edges near the walls.
Step Two: Prepare for the New Subfloor
Measure how much plywood you need with a measuring tape or a laser measuring tool. Plywood comes in 4-foot by 8-foot sheets; make sure you get a little more than you need. It is also important to measure the thickness of the existing subfloor. You want to purchase new subflooring that is the same thickness.
Attach a piece of lumber between the floor joists, to give you more surface area for the subfloor to attach to. Check to make sure the floor joists are level. If they aren’t, you may need to add wood shims to raise a low section or screw a new piece of lumber onto the side of a low floor joist.
Step Three: Installing Subfloor
This part is a little like doing a puzzle. You want to use as many whole pieces of plywood as you can. Decide on the best configuration to allow this, and get ready for the subfloor to go down. It is helpful to sand the edges of each piece of plywood before you install it, to help keep the joints smooth and give you a level surface for your flooring.
Apply a bead of construction adhesive to each floor joist before you lay the plywood down. This attaches the plywood securely to the floor joists and keeps the floor from squeaking. Use long screws and a drill or driver to screw the plywood down onto the floor joists. Make sure you leave a ⅛-inch gap between each piece of plywood so the wood has room to expand and contract with the changing temperatures–especially if the subfloor is over a crawl space or unheated basement.
When you have placed as many whole plywood pieces as you can, it’s time to finish the puzzle. Measure the rest of the sections and cut pieces of plywood to fit by using a chalk line and a circular saw. Remember to measure twice and cut once!
Step Four: Install Your New Flooring
You can now rest assured that your subfloor is free of mold, water damage, rot, or squeaks. Place the underlayment if you’re using one, and install your new flooring. Enjoy your new space and the great feeling that comes from completing a renovation project on your home!
