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Can You Use OSB For a Bathroom Subfloor? (How To)

You always want to start from the bottom up when doing any remodeling job. This includes when redoing the bathroom floor, and one of the questions involves using OSB.

OSB can be used as a subfloor for a bathroom floor, but it is not the best option. You should consider the use of plywood or, at the very least, using enhanced OSB because of the moisture possible in that room.

Man working on the OSB

This is one of the most common questions I was asked as a contractor. People would want to save as much money on the project as possible, but they would panic when they saw OSB going down.

The truth of the matter is there is not that much difference between OSB and plywood. In fact, OSB has benefits that plywood does not have, and it can be used in every location if the proper care is taken in advance.

We will discuss how to care for the floor in advance so you can save money and use OSB as well.

Before we break into the subject, talking about safety is essential. Safety is paramount whenever you are doing a project, regardless of whether it is a bathroom floor or any other DIY project.

Plan everything out in advance and look for any potential hazards and how to avoid them. Always use basic PPE, gloves, safety glasses, and proper footwear.

Other factors may need to be considered, such as the proper use of power tools. In the end, however, it is about being safe and enjoying the project after it you finish.

Should You Use OSB For a Bathroom Subfloor?

Man working on the OSB

As we discussed, it is possible and, in some cases, even preferable to use OSB as a subfloor in the bathroom. The real question is, however, is it a good idea to do it?

In this section, we will look at some of the potential problems that may come your way with OSB. Along with those problems, we will consider some solutions so you can use OSB for the project.

Perception – One of the most important things to consider when using OSB is perception. At one time, OSB was an inferior product, but these days, it can be used in any area where plywood is used if the proper care goes into it.

Of course, you can argue until you are blue in the face with the homeowner, but they will not always agree. Sometimes, you just need to use plywood because you want them to perceive that you are giving them a higher-quality product.

Moisture – One of the most significant issues that need to be considered in the bathroom is moisture. It doesn’t matter if it is the shower or an overflowing toilet. Water is going to be a problem sooner or later.

Both plywood and OSB will pull in water when they are exposed to it. The real key is to avoid that problem if possible, but that is not always possible.

If you use OSB and put it down dry, you can have a proper subfloor in the bathroom. Seal the gaps and ensure you are not allowing water to infiltrate the tile or other flooring and get to the OSB.

If water does get to the OSB, that is when you may have some problems. Like plywood, OSB will soak in the water and eventually release it. The problem is OSB will swell on the edges, including the cut edges.

When OSB swells, it will retain that shape and will not go back to its original shape, even when it dries. This may not be a problem with some types of flooring, but with tile, it could cause cracking in the tile and grout in some cases.

The bottom line is, you can use OSB in the bathroom but if it gets wet excessively, you are going to have problems.

PRO TIP: Allow at least 48 hours for OSB to acclimate to the indoor environment. Doing so will help it to dry if needed and avoid swelling due to temperature and moisture changes.

Weight – One often overlooked issue with using OSB is its weight. Although it is not a lot of weight per square foot more than plywood, it can be substantial when spread over the entire floor.

If you aren’t getting close to stressing the floor joists, you can use OSB without any problem. It is unlikely that you will be putting so much subfloor in the bathroom that you have to worry about an additional few pounds or hauling it around.

That said, if you are not working with the floor joists but are installing OSB over another subfloor, you are adding substantial weight. This can be a problem sometimes, so get an engineer involved if cutting it close to the mark.

What Preparations Do You Need To Use OSB For Bathroom Subfloor?

Man working on the OSB

Now that we have discussed the potential problems with using OSB, it’s important to know how to use it properly. It’s a great way to save money and works well in any environment if you take the following precautions.

Level – One of the most critical factors for installing any floor is the fact that it is level. Some floors, such as carpeting, are going to be forgiving when it comes to high points and low points, but that isn’t true with tile or vinyl.

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You must ensure that the floor is level before you begin the installation. This may include working with the floor joists and shaving some off if they are higher than others.

Even after the OSB has been installed, you may have to work with some low points and high points. The last thing you want is an unlevel floor to cause problems with the finished floor.

Clean – Another important factor for installing any floor is the cleanliness of the area. This should not be taken for granted, as it can make a difference in how the finished product will look.

You have to clean the area before installing the OSB, and after installing it, you have to clean it again before installing the floor.

Many people think that a minor issue will not cause big problems, but that is not the case. If you have a small bump under the OSB, it looks like a big bump in the finished floor.

Squeaks – One of the biggest problems I have seen homeowners make when installing a subfloor is not fixing the squeaks and creaks before they do so.

Admittedly, you don’t often notice these squeaks before you install the floor, but once the subfloor is down, a squeak will be a part of it.

Fix any squeaks ahead of time by adding additional nails or screws as necessary. If you don’t fix them now, they will be a problem forever.

Plan – One other suggestion is to plan everything out in advance. If you don’t plan, you’re going to end up with problems.

Ensure the OSB is inside, acclimated to the temperature and dry as a bone. Storing it outside and bringing it in immediately before installing it will be a problem.

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In addition, have enough supplies on hand so that you can finish the project. Not only is it frustrating to have an unfinished project because of a lack of supplies, but it could also end up causing problems in the long run.

How To Use OSB For Bathroom Subfloor

Man working on the OSB

Now that the preparation is out of the way, it’s time to install the OSB. This is a straightforward process, but you must do it properly.

1. Construction Adhesive – One important factor that should be considered for any subfloor is construction adhesive. It should be applied to the floor joists under the sheet being installed.

Construction adhesive helps the OSB stay in position but also keeps it from eventually separating and causing problems, such as cracked tiles or a squeaky area.

2. Work Perpendicular – The first sheet of OSB is laid perpendicular to the joist. Begin to work on either side of the first sheet of plywood, working your way to the edges and, eventually, installing the edges.

Continue to apply the construction adhesive under every sheet of OSB. It will do more than act as liquid nails in the long run.

3. Stagger – Even though you won’t be able to see the edges of the OSB under the floor, it will still be a problem if you don’t stagger the sheets.

Staggering the edges makes the entire floor stronger. It keeps it from shifting and moving along a particular line, resulting in cracked tiles or other problems.

4. Finish Strong – Finally, make sure that you check the OSB thoroughly for any issues before working on the tile or other flooring. You may find high spots along the edges or pieces of OSB that have become loose.

Fix all the problems in advance and start with a strong, smooth subfloor so your finished floor will look great.

You can use OSB as a bathroom subfloor, but you have to be cautious it doesn’t get overly wet. OSB can swell on the edges, resulting in the cracking of tile and other problems. As long as the OSB is dry, it can be a very solid and substantial subfloor.

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ReadyToDIY is the owner of this article. This post was published on April 18, 2023.

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