When you’re planning a home renovation, choosing the right bathroom subfloor is as important as a roof replacement or home insulation. The subfloor material you choose for your bathroom should be able to withstand the high moisture levels. 

The right material will help prevent water from seeping into areas around the bathroom sink, toilet, and bathtub. And, your bathroom will be able to combat any long-term damage. 

Keep in mind that excess moisture in the subfloor can mess up your entire bathroom flooring – resulting in an expensive, sudden bathroom remodel. Something that’s best avoided!

What is a subfloor?

bathroom subfloor materials

The lowest structural layer beneath the finished flooring is referred to as a subfloor. The major function of a subfloor, which is installed directly over the joists, is to produce a flat firm surface for the final flooring to rest on.

Generally, bathroom subfloors include an underlayment of the concrete backer board as well, for added protection. A concrete backer board further helps to protect the subfloor plywood or OSB (oriented strand board) from water damage and prevents cracked tiles or uneven flooring, especially during tile installation.

Which material is best for a bathroom subfloor?

The subflooring material is critical in your bathroom renovation. After all, your bathroom has to face extremely high levels of moisture on a daily basis. It’s important that you choose a material that’s resistant to moisture. 

The two most common subfloor materials are plywood and OSB. Plywood is a sheet made from wood layers. The wood is oriented in opposite directions in every layer.

OSB aka chipboard is a sheet material that’s composed of layers of flat flakes of wood sealed together with phenolic resins.

Most builders and renovation contractors insist that the best choice for a bathroom subfloor is plywood. And that too a thicker plywood to ensure that moisture doesn’t absorb into it.

However, there are also some builders who think that OSB is a more structurally consistent option. To make the selection easier for you, we’ll explore the pros and cons of both types of subflooring materials in the next segment.

A good subflooring option is a ¾” tongue-and-groove plywood – interlocking along the edges. Keep in mind that the exact thickness of the subfloor depends on the spacing of the joists. Also, both varieties come with their strengthened versions too. That is to say, you can have enhanced plywood or an enhanced OSB. 

According to some experts, 15/32-inch plywood should be used as a standard if the underlying floor joists are apart 16 inches or less, but thicker 3/4-inch plywood should be used if the joists are placed further apart. It’s always a good idea to check with your local building code authorities.

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Pros and cons

While plywood has been used as a subfloor material since the 1950s, OSB came into use in the 70s. Both come with their own set of advantages as well as disadvantages. 

Plywood
ProsCons
Available in different finishesIt May have weak spots, resulting in panel voids
Moisture resistant and fast drying More expensive than OSB
Strong and durableNot environmentally friendly
Good underlayment choiceLimitation of sizes
OSB  (Oriented Strand Board)
ProsCons
Dense and solid surfaceLower moisture tolerance
Less expensiveHeavy panels
Heavier, thicker, versatile materialProne to swelling around the edges
Good shear strengthCannot be painted

Bathroom subfloor cost

bathroom subfloor views

The average material cost to install a subfloor ranges between $2.85 and $3.25 per square foot. The total labor cost, as well as material cost per square foot, runs between $8.27 and $16.12.

On average, the cost to replace an entire subfloor in a 300-square-foot room ranges between $2,481.75 and $4,836.34. 

The cost depends on the size of the project and the material used. For example, a 4 x 8-foot sheet of 3/4-inch-thick subfloor-grade plywood costs about $21.50 while a 4 x 8-foot OSB sheet of 23/32-inch costs approximately $16.50. That’s $3 to $5 cheaper per panel as compared to plywood. 

How do you waterproof a bathroom subfloor?

It’s extremely important to waterproof the bathroom subfloor before installing the finished flooring. Otherwise, moisture will seep under the subfloor and cause mold growth or other damage.

Your wood subfloor requires an insulative, protective layer to prevent leaks and discourage mold growth. Good waterproofing measures will also get you a better resale value for your home.

You could use a clear polyurethane caulk to seal the potential leak points – any area that may be a point for water to seep in.

Last thoughts

Your bathroom is undoubtedly one of the most high-moisture areas of your home – primarily due to the vapor concentration from the toilet and the shower. That’s why using proper flooring material is important. A good subfloor will help keep your bathroom flooring healthy and strong – while keeping mold and mildew at bay.

What is a bathroom subfloor made of? was last modified: April 30th, 2022 by Ramona Sinha
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