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Waterproofing Subfloor: Best Methods You Can Use



V Susan
Hi! I'm Susan. I am passionate about woodworking, general DIY and home improvement. If you'd like to connect with me or talk about something you like at mellowpine, drop me a mail at susan@mellowpine.com


Waterproofing Subfloor

To ensure the longevity and protection of your subfloor, it is essential to waterproof it.

Waterproofing subfloors shield it from moisture, reduce mold and mildew growth, improve air quality, reduce maintenance costs, and increase property value. Sealants, paints, polyurethane, epoxy, chemical treatments, plastic sheeting, liquid latex, and cement backers can all be used to waterproof plywood subfloors. Concrete sealers are used to waterproof concrete subfloors.

Waterproofing a Plywood Subfloor

Although plywood is an excellent subfloor option, a lot of moisture can leak into the layers and cause them to separate or crack.

Fortunately, waterproofing plywood is a relatively simple process.

Below is a list of the methods that work the best.

Subfloor SealantsWaterproofing your SubfloorSetting up Waterproof Cement Backer Board
Contains water-repelling bondingPlywood can be waterproofed using paint, polyurethane, epoxy, chemical treatment, plastic sheeting, liquid latex, and oil drying techniqueHighly water-resistant
Plywood Subfloor Waterproofing Techniques

Subfloor Sealants

Subfloor Sealant
Subfloor Sealant

Waterproofing plywood using subfloor sealant is a straightforward process.

The sealant typically comes in colorless, paint-like containers and contains bonding agents that repel water, thereby adding a layer of protection that makes plywood waterproof.

To apply the waterproof sealant, first, clean the plywood floors with a brush and fill a paint tray with two inches of floor sealant.

Next, use a paint roller to spread a coating of subfloor sealant over the board and allow it to dry for six to eight hours.

Repeat the process two more times to create a strong seal.

Access Floor Systems is one of the top subfloor sealant producers in the USA.

Their Seal Bond Subfloor Sealant reduces dust, chemically hardens, fortifies the substrate, and protects against moisture.

Waterproofing Your Subfloor

Most studies show that no plywood is completely waterproof.

You can waterproof plywood by using the following methods.

Painting to Waterproof Plywood


One quick and simple way to waterproof plywood is to paint it.

To achieve this, apply multiple layers of paint or stain to your subfloor, using a bonding primer to ensure that the paint adheres effectively to the plywood.

If you need extra protection in case the paint fails, consider applying a topcoat.

Polyurethane Waterproofing for Plywood


Using polyurethane is another effective way to make a plywood subfloor waterproof.

You can apply this sealer using either a brush or a sprayer.

It’s worth noting that oil-based polyurethanes are generally more durable and provide better protection than water-based ones.

Epoxy Waterproofing for Plywood


Epoxy is also an excellent option for waterproofing plywood as it not only makes it waterproof but also strengthens it.

Apply it as a paint or spray, but the finish it creates is quite thick, making it challenging to apply to non-flat surfaces.

Additionally, using epoxy is a bit trickier than a regular finish since you must mix two parts of most epoxy resins in a specific ratio before using them.

Nevertheless, because the epoxy goes on clear and penetrates the surface of the plywood, the plywood retains its original color.

It wouldn’t be a good idea to use epoxy for the first time when you’re still learning how to use it.

Treating the Plywood Chemically

To make wood hydrophobic, manufacturers alter the plywood’s chemical characteristics and make it hydrophobic.

However, this method can be expensive.

Plastic Sheeting for Plywood Waterproofing

Plastic Sheeting can also be used to shield plywood from moisture damage.

To create the desired size, measure your plywood boards and cut out the appropriate sections of plastic sheeting.

Liquid Latex for Plywood Waterproofing

Liquid Latex
Liquid Latex

Applying a few coats of liquid latex spray is also a simple way to waterproof the plywood, but it is not economical on a larger scale.

It could also be challenging to determine how much latex is on the plywood, which could cause you to apply too little or too much.

Oil Drying Technique

Drying oil is a non-toxic finishing technique that can render a plywood sheet water-resistant.

This implies that it will not completely “waterproof” the wood in the conventional sense but will significantly increase its lifespan.

Compared to polyurethanes which are used for protection, oils don’t offer as much.

Setting up Waterproof Cement Backer Board

Cement Backer Board
Cement Backer Board

A cement backer board is incredibly water-resistant and won’t be harmed by exposure to water for an extended period.

However, installing a cement board requires more expertise than using a subfloor sealer.

To install a cement backer board, begin by using sandpaper to smooth out any unevenness and fasten the plywood to the joists.

Then, trim the cement board to size and use a trowel to distribute the boards with the thin-set mortar.

Comb it out with a scraping knife to prevent the mortar from coming up uneven or blotchy.

Then, use drywall screws and mesh tape to attach the boards to the plywood and tape the seams.

Finally, let everything dry before installing your flooring.

Concrete Subfloor Waterproofing

Concrete Subfloor Waterproofing
Concrete Subfloor Waterproofing

It’s critical to waterproof concrete subfloors to prevent moisture penetration and mold growth.

You can use concrete sealers to prevent long-term deterioration and chemical intrusions since they create a durable barrier that prolongs the life of the concrete.

Wide varieties of concrete sealers are available, and some function better than others.

If your concrete is fresh, allow the concrete to cure and wait a month before applying the concrete sealer, as damp concrete will prevent the sealer from sticking.

For proper adhesion, clean the cement surface and use specialized acid solutions to roughen the concrete surface.

Next, thoroughly combine the sealant and make sure it is homogeneous.

Pour some sealer onto a painting tray, and then use a paint roller to apply a small amount of the concrete sealer uniformly across the flooring.

Allow at least 12 hours for the sealant to dry completely, and use dehumidifiers to speed up the drying process.

Maintain a temperature of at least 50 degrees Fahrenheit during application and curing, which can last up to three days.

Is Waterproofing an OSB Subfloor Necessary?

OSB is a popular choice for basement subfloor due to its high water resistance.

This is because during the manufacturing process, all the wood fibers are coated with water-resistant resin adhesives.

However, it is not completely waterproof.

Painting them first before waterproofing is the best course of action.

Afterward ,use a paintbrush to apply the waterproofing solution, making sure to cover the entire board.

Subfloor Waterproofing: Advantages and Disadvantages

Advantages of Subfloor WaterproofingDisadvantages of Subfloor Waterproofing
Increase durabilityExpensive
Reduce mold and mildew growthReduce the concrete breathing capacity
Improves air qualityDemands frequently reapplying coats
Allows for simple floor installationMore leaking occurs due to inadequate waterproofing
Reduce maintenance costLimited pot life
Increases the value of a propertyHealth hazards because of the chemicals involved
Advantages and Disadvantages of Subfloor Waterproofing

Advantages of Subfloor Waterproofing

The primary advantages of waterproofing are as follows:


Your subfloor’s longevity will be increased by waterproofing it, which will aid in protecting it from wear and tear, and water damage.

Reduced Mold and Mildew Growth

If left unchecked, mold and mildew can lead to health issues.

Waterproofing your subfloor can assist in minimizing its growth.

Improved Air Quality

Your home’s air quality can be improved by waterproofing your subfloor by lowering the quantity of moisture in the air.

Easy Installation of Floors

Waterproof subfloors make it simple to lay floors.

It lowers the cost of installation and provides excellent DIY project opportunities.

Reduces Maintenance Costs

The waterproof factor makes essential maintenance much more manageable,thus reducing its cost.

Boosts the Value of Properties

Making a subfloor waterproof can enhance its marketability since most buyers avoid properties with water-related problems.

Disadvantages of Subfloor Waterproofing

While waterproofing subfloors provides many benefits, there are a few drawbacks as well.

Additional Cost

The high cost of the materials makes waterproofing expensive.

You need to factor in the worth of your time in addition to paying for the necessary supplies.

Limits the Capacity of Concrete Breathing

A protective covering decreases a concrete’s capacity to breathe.

Frequent Reapplication of Coats

To ensure that your concrete remains waterproof, it’s necessary to apply concrete sealers at least once a year.

It is also important to apply multiple layers of waterproofing coatings.

Moreover, applying the second layer of the coating takes longer than the initial application.

Failure to Fix the Cracks

Inadequate waterproofing to keep the cracks sealed off results in more leaking.

Restricted Pot Life

The pot life of a waterproof coating is confined.

Health Risks

The use of strong chemicals in the waterproofing process causes various health hazards if you are not adequately protected.

Therefore, it is preferable to leave the waterproofing task to experts with the proper skills and training to complete the task securely.

How Can a Wet Subfloor Be Dried Out?


The subfloor must be dried out as quickly as possible if flooding occurs and water seeps into the subflooring.

Begin by soaking up any surface water.

Then, set a dehumidifier to its highest setting to remove any remaining moisture.

To ensure that the entire room has circulating air, install as many fans as possible.

Place fans at high settings, aimed at the damaged section, until the subfloor is dry.

Finally, use a moisture meter to verify the subfloor is entirely dry.

However ,if the water damage is severe, it may necessitate the need for subfloor replacement.

Frequently Asked Questions(FAQ)

Does my Hardwood Floor Need Plastic Sheeting Underneath it?

Hardwood floors can weaken, bend, crack, and decay over time if they are exposed to moisture and humidity.

To shield your hardwood flooring from water seeping up from underneath, you must install plastic sheeting.

Is it Acceptable to Leave the Subfloor Exposed?

Using subfloors as regular flooring can damage their surface and reduce their lifespan, making it necessary to consider long-term repercussions.

While a concrete subfloor can be used as regular flooring in dry areas, plywood subfloors can become brittle and deteriorate when exposed to moisture and sunlight over time.

Which Waterproof Subfloor Material is the Best?

When you need to cover areas where moisture exposure can be a problem, OSB subfloors are an excellent solution.

V Susan
Hi! I'm Susan. I am passionate about woodworking, general DIY and home improvement. If you'd like to connect with me or talk about something you like at mellowpine, drop me a mail at susan@mellowpine.com