Water Damage Black Mold on Bathroom Subfloor (Easy Fixes)

fixing mold on bathroom subfloor

Mold under flooring can be dangerous if not taken care of. They can cause severe allergic reactions and even respiratory problems.

Plus, if they are not fixed and allowed to grow, they can cause the structure of your home to weaken.

If you have found black mold on your bathroom subfloor, the quickest way to kill them is by using a mixture of 1 cup of borax and 1 gallon of water.

However, depending on the conditions, there may be other ways to get rid of and prevent black mold. Below in this article, we will explore some of these conditions and solutions in greater detail.

So, let’s dive into some basics like…

What is Subflooring?

Subflooring is the material that lies beneath your flooring. It can be made of different materials like plywood, OSB, hardwood, or concrete.

The purpose of subflooring in your bathroom is to act as a support system for your flooring and to keep the flooring level.

Subfloors in the bathroom also help to insulate against noise and temperature changes.

When it comes to bathroom subfloors, there are two main types:

a) Plywood: Plywood subfloors are made up of layers of thin wood that are glued together. Plywood is a popular choice for bathroom subfloors because it is strong and relatively inexpensive.

b) OSB: OSB (oriented strand board) subfloors are made up of strands of wood that are glued and pressed together. OSB is also a strong and inexpensive option for bathroom subfloors.

How Does Mold Develop on Your Subfloor?

Mold and mildew need three things to grow inside the bathroom. These are moisture, darkness, and organic matter.

Subfloors are the perfect environment for white and black mold because they are often dark and moist.

Plus, if there is a water leak, the organic matter (wood) can provide food for the mold to grow inside the layers of flooring for a long without getting detected.

So, if you have a water leak, it is important to dry the area as quickly as possible and fix the leak to prevent mold from appearing.

Mold is also more likely to appear in warm buildings because the combination of high temperatures, water, and construction materials like plywood or OSB creates an ideal environment for mold infestation.

What Does Subfloor Mold Look Like – Signs You Need to Look at?

If your bathroom is prone to water leaks, it is important to inspect the subfloor for mold periodically.

Signs of mold under hardwood floors are generally easy to spot. The most common sign is visible mold growth on the surface of the wood. Most of the time, these molds appear to be cotton gray, white, yellow, or black in color.

Plus, if you see any of the following signs under your laminate or linoleum floors, it is likely that you have a mold problem:

  • Discoloration of the wood
  • The floor feels spongy
  • Musty smell
  • Carpet and pad roll back easily
  • Visible water damage, such as warp or rot

These signs are a strong indication that you have mold and you need to take action.

how to remove mold on bathroom subfloors

Mold Removal on Bathroom Floors and Subfloors – Steps to Follow

When you need to fix the moldy subfloor in the bathroom, the first step is to identify the source of the water. Because if it is a water leak, you must fix it before proceeding with mold removal.

You never want to start a mold removal project without fixing the water damage first, as this will only lead to the problem coming back.

Once you have fixed the source of the water leakage, you can start the mold removal process.

Step 1 – Ventilation

The first step is to ventilate the area by opening all the windows and doors.

You also want to turn on any fans that you have to help circulate the air.

Step 2 – Protective Gear

The next step is to put on some personal protective equipment (PPE). This includes gloves, a mask, and eye protection.

You want to make sure that you are protected from the mold spores that will be released into the air when you start the removal process.

Step 3 – Subfloor Mold Removal

There are a few different ways that you can remove mold from your bathroom subfloor.

So let’s take a look at some of my favorite methods you can try…

a) Borax & water

The most common method is to use a solution of borax and water.

To do this, you want to mix 1 cup of borax with 1 gallon of water. Once you have mixed the solution, pour it onto the affected area and scrub it into the moldy areas.

Let the solution sit for a few hours before rinsing it off with clean water.

b) Vinegar & water

Another method that you can use is a vinegar and water solution.

To make this solution, mix 1 cup of vinegar with 1 gallon of water. Once you have mixed the solution, pour it onto the affected area and scrub it into the moldy areas.

Let the solution sit for a few hours before rinsing it off with clean water.

c) Soap & water

If the bathroom floor area you need to treat is small and not stained heavily, you can scrub it out with soap and water.

This method is not as effective as the other two, but it can still help to remove some of the molds.

Once you have scrubbed the area, make sure to rinse it off with clean water.

d) Bleach & water

For removing molds on the subfloor, you can also try using a bleach and water solution.

Mix 1 cup of bleach with 1 gallon of water to create this mixture. Pour the solution over the afflicted region and scrub it into the moldy places.

Allow this solution to soak for a few hours before rinsing it off with clean water.

Remember, killing the molds with chlorine bleach on wood does not always work, especially if the surface is porous.

Because it has a high surface tension, it does not get soaked into wood or other porous surfaces, such as concrete, tile grout, etc., to kill the roots of the growing mold.

Bleach may be used to eliminate mold that grows on non-porous surfaces, such as glass, kitchen worktops, and ceramic tiles.

e) Professional mold removing

If the area is large enough and is heavily stained with molds, you may want to consider hiring a professional mold removal company rather than trying homemade DIY methods.

They will have the experience and knowledge to get the job done quickly.

And with them, you can be sure that every bit of mold is killed and removed around the edges, grooves, or under the bathroom floor.

This also decreases the chances of it coming back.

Sealing and Waterproofing a Bathroom Plywood Subfloor to Avoid Mold Return

Once you have removed all the mold from your bathroom subfloor, around the tub or toilet, or under the kitchen sink areas – it doesn’t guarantee that the mold will not return.

To make sure that it doesn’t, you need to take some preventative measures, such as sealing and waterproofing the treated wooden areas of the subfloor.

Doing this will help to create a barrier between the subfloor and the surrounding environment, which will make it more difficult for mold spores to take hold and start growing again.

There are a few different ways to seal and waterproof your bathroom subfloor…

a) Use a sealant

You can use a sealant such as polyurethane, epoxy, or latex to create a barrier between the subfloor and the surrounding environment.

Make sure to follow the instructions on the packaging and apply the sealant in a well-ventilated area.

b) Use a waterproofing paint

Another option is to use waterproofing epoxy-based paint on the subfloor.

This will create a barrier that will help to prevent water and moisture from seeping through and causing mold to grow.

c) Use a vapor barrier

Another effective remedy to use is a vapor barrier, such as polyethylene sheeting, to create a barrier between the subfloor and the surrounding environment.

This will help to prevent water vapor from condensing on the surface of the subfloor and causing mold to grow.

d) Use a dehumidifier

You can also use a dehumidifier in the area to help reduce the amount of moisture in the air.

This will make it more difficult for mold spores to take hold and start growing.

e) Use an anti-mold spray

Using an anti-mold spray on the subfloor can also help prevent mold from growing.

Make sure to follow the instructions on the packaging and apply the spray in a well-ventilated area.

f) Use a fungicide

Finally, you can also use a fungicide on the subfloor to help prevent mold from growing.

Again, make sure that you apply the fungicide in a well-ventilated place and follow the directions on the packaging.


Are all types of mold toxic?

While not all types of mold are dangerous for humans and pets, the ones that feed on cellulose are usually the culprits.

These include black mold, white mold, and green mold.

If mold spores or the toxic chemicals present in them get released into the air, they can be breathed in, causing a host of health problems.

This is why it is so important to take care of mold spore growth as soon as you notice them.

Can mildew and mold grow under vinyl planks?

Though vinyl plank flooring is recognized to be waterproof, it doesn’t imply that liquids cannot seep through the gaps, grooves, or edges.

If the water, spilled liquors, or other liquids get a chance to reach underneath your floors, it gets trapped beneath the planks and will allow the mold to develop slowly.

Do you need to replace the moldy subfloor or drywall?

If the integrity of your bathroom subfloor or the drywall has been compromised by water damage or mold, it will need to be replaced.

You should also consider replacing it if the mold is difficult to kill/remove or if it has been growing for a long time.

The cost of doing this can be higher than simply repairing the damage, but it is worth it to ensure that your home is safe from mold.

How often should you check for mold growth in the bathroom?

It is a good idea to check for mold growth every few months, especially if you live in an area with high humidity or if your home is prone to flooding.

You should also check for mold after any major water event, such as a leaky pipe or a flood.

This will help you to catch any mold growth early so that you can take care of it before it becomes a bigger problem.

Final Thoughts

Water damage in modern bathrooms and under kitchen sinks is a common problem because it can lead to the growth of mold and mildew.

Mold on the bathroom subfloor, if left unattended, can cause the wood to change color, warp, or rot. If you see any of these signs, it is essential to kill and remove them at the earliest.

The good news is that there are many ways that you can do this yourself with common household items.

If the mold problem is too big or you don’t feel confident in your ability to remove it, you can always hire a professional company to do the job for you.

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