What to Use for a Bathroom Subfloor

As you know, the bathroom is one of the most high-moisture areas in the home. This primarily is because there is so much vapor concentrated in one area from the toilet and the shower. Since this is a moisture-rich area, you must make sure that you use the proper flooring material in the bathroom to keep mold and mildew at bay.

What to use for bathroom subfloor

If you are planning to remodel your bathroom, which subfloor should you use? The best choice in the bathroom is plywood. However, you must make sure that you choose thicker plywood to ensure that water doesn’t readily absorb into it.

Aside from choosing the correct subfloor, there are some other things that you can do to make sure that the subfloor is sturdy before adding your flooring, whether it be tile, natural stone, or hardwood. You can also add a vapor barrier to help protect your floor from water damage, mildew, and mold.


Why You Should Be Careful in Choosing the Right Subfloor For A Bathroom

As mentioned before, the bathroom is a very high-traffic, moisture-rich area of the house. It is probably the area of the house that gives off the most moisture, even more than the kitchen or the laundry room. There is no way to get around the high level of moisture that builds up in the bathroom. So, the best thing to do is to accommodate the moisture issues by using building materials that can withstand the excess moisture.

One essential building material that you should choose carefully is the subfloor. The subfloor in this room should be very solid and should be as moisture and water-resistant as possible. The type of flooring that you use in the bathroom may be beneficial at protecting the subfloor from water damage. For instance, vinyl and porcelain tile may be relatively water-resistant, but they still may allow water to seep in.

Additionally, a solid subfloor can help prevent the joist underneath the bathroom from rotting. If the joist begins deteriorating, the floor can collapse. Floor collapse will likely be the worst-case scenario and often comes because of severe neglect.

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Signs of damaged floors that you will most likely experience before total collapse is as follows:

  • musty odors within the bathroom
  • visible mildew and mold growth
  • gaps in the flooring
  • areas of the floor that feel spongy
  • an unstable toilet

If you start to notice these problems, you should address them as soon as possible. Otherwise, you put yourself at risk for the bathroom floor collapsing. In such severe cases, you may have to abandon your DIY floor repair plans and allow the experts to come in and make sure that your bathroom floor is structurally sound.

What Types of Subfloor Can Be Used In a Bathroom?

There are several types of subfloors that can be used in the bathroom. They are as follows:


Plywood is a type of wood made by gluing several layers of wood veneer together. It is very water-resistant but is still subject to water damage if exposed to water over a long period.

Enhanced Plywood

This is the same as regular plywood. However, what makes it “enhanced” is that it is coated with a special water-resistant coating that can assist with making it even more and moisture water-resistant.


OSB, or oriented strand board, is a type of subfloor similar to plywood but tends to be less expensive. It is made by gluing 3″ – 4″ pieces of wood together.

Enhanced OSB

Like enhanced plywood, enhanced OSB is virtually like OSB. However, it is made more water-resistant by coating it with a resin.

These are typically the types of subfloors that are used in bathrooms. Although OSB is a more cost-effective solution, it can swell around the edges when exposed to water. This swelling can cause problems with your flooring, particularly if you have tile installed. Although OSB and enhanced OSB are relatively sound subfloors and do have their strengths, they may not hold up as well as plywood when it comes to bathrooms.

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What’s the Best Subfloor To Use In A Bathroom?

Yep, you guessed it! Plywood or enhanced plywood is going to be the best option for your bathroom subfloor. Although they are more expensive than OSB, they still hold up better in high-moisture areas. However, you shouldn’t just pick any type of plywood to use in the bathroom.

When it comes to installing plywood in your bathroom, you will need to consider a few things. For instance, is your bathroom on the first floor of the home? Also, is this bathroom a small half-bath that only has a sink and a toilet? If the answer to these questions are yes, you may be able to get away with a ½ inch sheet of plywood. Also, if you know that this bathroom isn’t used as often and doesn’t produce a high level of humidity or moisture, you may even be able to get away with using OSB.

If you are remodeling a bathroom with a high moisture level on the second floor, you will have to use stronger plywood. A CDX rated AC plywood that’s ¾ inches would probably work best. This is because it can withstand moisture much better. And will be better at withstanding heavy items on it, including tile, heavy tubs filled with water, and even you. You can buy the plywood in sheets.

However, in a bathroom, particularly one on a higher level of the house, installing tongue and groove plywood may be best. This will reduce the squeaking.

Installing a Vapor Barrier Can Help Protect the Subfloor and Flooring

As an additional precaution against mold and mildew growing between your flooring and your subfloor, you can opt to place a vapor barrier down. A vapor barrier is a plastic usually made of polyethylene that keeps moisture from damaging your subfloor and flooring. This piece of plastic will go a long way in helping to keep your subfloor safe from water damage.

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This is especially important if you install the tile. If the grout from the tile happens to wither away and exposes the floor beneath, having the moisture barrier can keep the water from damaging the subfloor until you replace the missing grout.

Can You Use New Subfloor Over Old Subfloor?

When working on a DIY project, you may be wondering if it is necessary to remove the old subfloor when remodeling your bathroom. It depends on whether the flooring manufacturer allows it. If you leave the old floors intact despite the manufacturer’s suggestion not to do so, you may void your warranty. However, if the manufacturer mentions that it is ok or does not indicate it at all, it’s your call.

Keep in mind that if you decide to lay a new subfloor over the old one, you will have to make sure that the new subfloor isn’t too thick. A thick subfloor over an existing one may make the floor too high, which can cause other problems, particularly where the floor meets the wall.

If the existing bathroom subfloor is severely damaged and rotted, it would be a good idea to replace it entirely. However, if it is still in relatively good condition but the only problem it was causing was squeaking, it may be best to keep the old subfloor and cover it with a new subfloor. You will have to carefully inspect the existing floor and ensure it is nailed down to prevent further squeaking.

Also, if there are any loose boards, you may want to replace them with new boards. Finally, if your floor is slightly uneven, you will have to sand it down or fill in the voids so that you won’t have unlevel areas within your flooring.