How to protect subfloor from water damage?

Author: Ford Jenkins  |  Last update: Thursday, October 9, 2025

One solution to help protect the sub-floor from rainwater damage during construction is to use LP® TopNotch® 350 Durable Sub-Flooring. What is LP TopNotch 350 Durable Sub-Flooring? LP TopNotch 350 Durable Sub-Flooring is an OSB sub-flooring designed to reduce the impact of moisture during construction.

How do you protect a subfloor from water?

The modern solution is Polyethylene Film laid over the ground to act as a barrier to prevent moisture from rising from the ground into the subfloor.

Is the subfloor ruined if it gets wet?

In most cases, damaged subflooring doesn't need to be replaced. However, in the event of extensive water damage from flooding or extended periods of exposure to moisture, damaged sections may need to be removed to avoid mold and ensure structural integrity.

How do you waterproof a subfloor?

You can seal a subfloor using a sealant specifically designed for subfloors, such as a penetrating sealer or a moisture barrier. Penetrating sealers are typically made from a wax or oil-based formula that soaks into the wood, protecting it from moisture and rot.

How do I keep my subfloor from rotting?

How to protect your crawl space from wood rot
  1. Use a commercial subfloor sealing product like LumberKote to make exposed wood water resistant.
  2. Keep your rain gutters and downspouts in good repair.
  3. Inspect your crawl space at least once a year for signs of excess moisture.

Subfloor Moisture Fix

How do you fix a water damaged subfloor?

How to Repair Water Damaged Sub-Floor
  1. Stop the Water. If water damage was caused by a leaking appliance, the source of the water has to be resolved before taking any steps toward fixing the floor. ...
  2. Expose the Sub-floor and Examine. ...
  3. Remove Damaged Sub-flooring. ...
  4. Reinforce Floor Joists and Install New Sub-flooring.

How much does it cost to replace a rotted subfloor?

The national cost range for replacing a subfloor is between $600 and $2,500. Most homeowners pay around $1,600 for a 200 sq. ft. area in a kitchen or living room using OSB board.

What is the best sealer for plywood subfloor?

Epoxy. Epoxy sealers are usually the preferred method of sealing and waterproofing plywood. Epoxy doubles as an adhesive, which gives an additional layer of strength plus protection for the wood, helping it to last even longer. It can double as a primer too, if the wood is to be painted after sealing.

How do you stop subfloor moisture?

What steps should you take to prevent subfloor moisture?
  1. Take proper measurements before, during, and after installation of the moisture level. ...
  2. Add a flooring underlayment with a vapor retarder barrier, also known as a moisture barrier. ...
  3. Make sure that your contractors don't penetrate the vapor barrier.

How do you attach moisture barrier to a subfloor?

The barrier must be installed directly onto the surface of the subfloor. Use a sealing tape to seal the rows securely together. The installation should be in the opposite direction you plan to install the underlayment to reduce seams, in the same direction of the final floor.

How long does it take for water to damage subfloor?

If a leak or flood sits on your floor for 24 hours or more and/or reaches the edges of your floor and seeps underneath your flooring, it likely will need to be dried out completely. Usually, this will mean removing the flooring on-top of your subfloor.

Do I need to replace moldy subfloor?

When Should You Replace Subfloors? There are many signs that your damaged subfloor should be replaced with a new subfloor. These include squeaks and other noises, cupping or warping, cracked or popped tiles, sagging or unevenness, the smell of mold or mildew, or bubbling tile.

Does the subfloor need to be replaced if it gets wet?

A wet subfloor doesn't always need to be replaced, but it must be thoroughly dried and assessed for damage. Use fans and dehumidifiers to dry it out. If the subfloor is swollen, warped, or has mold growth, replacement is necessary.

What can I spray on subfloor for mold?

Treat exposed molded subflooring with a mixture of 1 cup of borax and 1 gallon of water or an EPA-registered mold removal product. Spray the solution on the moldy plywood.

How do you fix water damage under wood floors?

Step-by-Step Restoration Guide
  1. Step 1: Remove excess water. Using a wet vacuum, remove as much water as possible from the surface. ...
  2. Step 2: Dry the area. ...
  3. Step 3: Sanding the Floor. ...
  4. Step 4: Filling Gaps and Cracks. ...
  5. Step 5: Staining the Floor. ...
  6. Step 6: Applying Finish. ...
  7. Step 7: Polishing.

Do you need a vapor barrier on a plywood subfloor?

Yes. Hardwood flooring is quite sensitive to moisture and it's always worth adding a moisture barrier during installation. It is a necessity with concrete subfloors and damp areas, but water-resistive underlayments should be considered with other subfloors as well.

How do you get moisture out from under flooring?

Use fans, dehumidifiers, or even a wet-dry vacuum to dry water under laminate flooring and the subfloor.

What is the cheapest way to waterproof plywood?

The cheapest way to waterproof plywood is by applying a waterproof sealant or paint. Using water-based polyurethane or oil-based paint/varnish offers effective moisture protection at a low cost.

How do you protect plywood subfloor?

Applying a coat of epoxy sealant or water-based polyurethane provides effective protection against moisture. Another budget-friendly option is to use oil-based paint or varnish, which can also seal the wood. For outdoor use, consider marine-grade sealants for extra durability.

How long can plywood be exposed to rain?

Plywood can typically withstand rain exposure for a day or two without significant damage, depending on the type of plywood and the severity of the rain. However, long-term exposure can lead to serious damage such as warping, cracking, or rotting.

Is subfloor damage covered by homeowners insurance?

Does homeowners insurance cover subfloor? In general, homeowners insurance covers subfloor water damage. However, it must be caused by one of the perils listed in your insurance policy.

What is the life expectancy of a subfloor?

How Long Does Subfloor Last? Subfloor typically lasts 30 years or more, but if it is consistently exposed to moisture you may need to replace it sooner.

How do you fix a rotted subfloor with water damage?

How to Fix a Water-Damaged Subfloor: Your 8-Step DIY Guide
  1. Start by Stopping the Water. ...
  2. Expose the Subflooring. ...
  3. Make Your Mark. ...
  4. Carefully Cut Subflooring. ...
  5. Remove Damaged Materials. ...
  6. Speed Up the Drying Process. ...
  7. Reinforce Floor Joists. ...
  8. Install Your New Subflooring.

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