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How to Fix Water-Damaged Subfloor: A Step-by-step Guide

How to Fix Water-Damaged Subfloor: A Step-by-step Guide

One of your home’s most essential architectural pieces is one you will rarely see: the subfloor (the flooring lying underneath the floor you walk on every day). It is typically a layer of ¾ inch plywood underneath your carpet, vinyl, wood, or laminate flooring. This layer is quite strong until water seeps in. Even a small leak can have disastrous results if left untended.

A water-damaged subfloor will quickly become a problem for the structure of your house. Unfortunately, it is also a common problem plaguing homeowners. A leaky refrigerator or invasive rainwater can seep and settle into the subfloor and cause rot in just a couple of days. While damage begins quickly, it often takes months or even years to uncover the destruction. If you have damage to your subfloor, you should assume it has been there for a while.

Damage to the subfloor needs to be handled quickly and efficiently. Our team at All Dry USA can help with this repair and restoration process. However, if you are particularly good at DIY, here is how to try it yourself.

Read more: How to fix water damage on laminate flooring

Gather the Tools

Before you get started, you will need the right tools and materials.

For tools, you will need:

  • Tape measure
  • Circular saw
  • Hammer
  • Pry bar

For materials, you will need:

  • 5/8” plywood
  • 2×6 lumber
  • Galvanized nails

Find the Leak

The first step to fixing the subfloor is locating the water source causing the issue. Find the leak and fix it or else your repair will be in vain as water continues to infiltrate. If rainwater is wreaking havoc, look for exterior cracks and re-seal them. You can also place a gutter or awning over these exterior areas to direct water elsewhere.

If the culprit is an interior leak, start by investigating your plumbing. Approach all plumbing issues with caution, and consider bringing in an expert before you begin loosening pipes. You will find plumbing problems require expert assistance for a permanent solution, even if you handle the rest of the repair yourself.

Remove Flooring

So, you cannot tackle subfloor issues if you cannot see the subfloor. Take rubber gloves and safety goggles for protection and begin to remove the top layer of flooring. Set aside the floorboards carefully so you can replace them later.

If your flooring is carpet, pull it back to the nearest wall. You may need to remove baseboards to get to the carpet edges.

If the flooring is vinyl, cut around the repair area. Damaged vinyl can be replaced with a piece cut to the specifications after the repair is complete.

Tile flooring requires a special touch. Tile floors often adhere to a cement board layered between the subfloor and floor. You can check for dampness by pulling up tile, but a contractor can handle this with equipment designed to limit tile damage in the removal process.

Read more: How to repair water damage on baseboards

Assess Damage Amount

Now that the flooring is removed, you can assess the extent of the subfloor water damage. Rotted areas will be buckled, discolored, and eaten away. Mark the damaged areas with chalk to identify what needs to be removed. Assess any underlying floor joists location, which will determine how to remove and reinstall subflooring.

Cartoon Depiction Of Water Coming Through A Doorway

Remove Damaged Areas

Any area exposed to water damage and rot needs to be removed. Even if the damage is not extensive in a particular area yet, it needs to go. Anything damp will eventually produce mold and start to rot. If you want to have a permanent solution and preserve your home’s structural integrity, the repair must be complete and thorough.

Take the circular saw, set it to one-inch depth, and cut the rotting subfloor areas you marked with chalk. Cut precisely along those lines and cut as close to the joists as possible. Use the pry bar to remove damaged sections carefully. Make sure to remove loose nails as well.

Read more: How to fix water damaged wood floors

Floor Joists Reinforcement

Floor joists cannot be cut out without compromising the structural integrity and foundation of your home. If the joists show signs of water damage, it is essential to dry them out and reinforce them for extra support. Cut the 2×6 boards and nail them under the newly exposed and cleared subfloor area. When the joists are dry, nail the boards next to the joists. The new boards will compensate for any original joists that suffered water damage.

Replacement Installation

Next, measure the area where you removed damaged subfloor material. Cut replacement pieces for the areas where you just removed subflooring. The replacement plywood should be 5/8” thickness and made from exterior-grade plywood. This grade can withstand a moderate amount of moisture before the deterioration in the event water damage trickles in again.

This step should be done slowly and precisely to ensure the measurement and cut is entirely accurate. There should be a 1/8th inch gap between the old and new subfloor to account for the natural expansion of the new wood. The replacement subfloor should be attached using new nails. Hammer directly into the center of where the damage was removed. Vacuum any debris and clear the area before replacing the flooring.

Now that the subfloor has been installed and the damage removed, all that is left is returning the top layer of flooring to how it was before. Dispose of damaged subflooring and clear the area, especially any loose nails that can be wandering around. With that, the installation is complete!

How Much Does a Subfloor Repair Cost?

Finding unexpected damage is often cause for consternation as you quickly start running numbers to see how badly this will hit your wallet. The cost of a subfloor repair really will depend on several factors. Costs largely depend on the size of the room and surface area that has been damaged. Is it just the corner of a room or the entire room?

Most subfloors are plywood, but some use oriented strand board. The type of materials needed for repair can also influence the cost.

Read more: Water line to fridge leaking


One of the joys of being a homeowner is the satisfaction of a DIY project done well. While we know you can handle these projects, just know that All Dry USA is on hand to help. If you are facing subfloor water damage, let us help. Contact our team of experts to see how we can address water damage repairs and restoration, including subfloor repair.


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