Skip to Content

Can You Put Cement Board Over Plywood? (How To)

There are many times when you may want to work directly on plywood but there are also times when using cement board is appropriate. If you have a plywood subfloor, can you install a cement board over it?

Cement board is an excellent choice to be installed over plywood if you are working in a bathroom or another area that is prone to damp conditions. As long as you prepare things properly, it will provide the appropriate subfloor in such a location.

fixing the cement board

Believe it or not, trying to install tile directly on plywood is never a good idea. It is often done, but the plywood can shift from side to side and shrink/expand, so you may end up with cracked tile eventually.

One of the best things that you can do is to prepare the plywood properly and install some cement board. It works very well in such a location and can be used in any part of the home, not just in a damp area.

Can You Put Cement Board Over Plywood? (How To)

One of the most important things to consider when installing cement board is the fact that you’re actually working with concrete. As a result, you need to consider your safety very carefully.

When you cut any type of cement board, you are going to release some crystalline silica dust into the air. If that dust is small enough to get inhaled into the lungs, it can cause some severe problems.

Silica does not leave the lungs but rather, it embeds in the lungs and causes them to harden. Over time, you could end up with silicosis, which is an incurable and often deadly disease.

Other safety factors also need to be considered when using cement board. This includes protecting your eyes with safety glasses and protecting your hands with all-purpose gloves. Some people even wear cut-resistant gloves, because they work with utility knives frequently.

Keep your utility knife blade sharp at all times and swap them out frequently. Make sure that you put the blade in an appropriate container so you don’t get cut when reaching in the trash can.

Let’s consider some of the factors associated with putting cement board over plywood. This includes benefits, preparation, and the process of getting the job done.

Should You Put Cement Board Over Plywood?

Confused man standing

The fact that you can put cement board over plywood is well-established. The reasons why you should or should not do it, however, can help you to make the choice on your own.

Consider the following benefits when deciding whether you should install cement board over plywood.

Longevity – Cement board will be there for the long term. It doesn’t break down, but the same cannot be said for plywood. If you want something that is going to last for a very long time, cement board is the way to go.

Warping – Another benefit of using cement board is the fact that it doesn’t work when it gets wet. Applying it in an area of the home that gets wet frequently, such as a basement or the bathroom has many benefits.

Expansion/Contraction – One of the biggest problems of putting tile directly over plywood is the fact that the plywood will expand and contract. Cement board does not expand or contract, so it makes a very sturdy and suitable surface to install tile and other flooring.

Tool Needed To Install Cement Board Over Plywood

Confused man standing

As is the case with any job, you want to make sure that you have all of your tools available before you begin working. Here are the tools needed for installing cement board over plywood:

5 Gallon Buckets

Rags

Notched Trowel

Screw Gun

Straight Edge

Carpenters Square

Utility Knife W/ Extra Blades

Safety Glasses

N95 Disposable Respirator

Knee Pads

All-Purpose Gloves

Of course, you may have certain tools that you like using more than others. There’s nothing wrong with substituting where appropriate, just make sure that you have it available when the job gets started.

What Preparation Is Needed To Install Cement Board Over Plywood

fixing the cement board

Preparation is one of the most important factors for installing cement board over plywood. Without the proper preparation, the job is not going to last very long.

Copywrite article owner is Readytodiy.com for this article. This post was first published on 2022-09-09..

The most important part of preparing a subfloor is to make sure that it doesn’t squeak or move. If the subfloor moves under the cement board, then the movement is going to transfer and it could end up cracking the tile.

Work your way around the plywood and try to step on almost every area that is possible. If you hear a squeak, then use screws to attach the plywood to the floor joists solidly.

Screwing down the floor joists will help most of the squeaks to go away. Otherwise, you may have to pull up a piece of plywood and use liquid nails along with screws.

It is also important to inspect the plywood and make sure that there is no serious damage. If there are any high or low points, they should be corrected at this time because it will also result in high or low points in the cement board.

If there is already a floor over the plywood, it is not necessary to tear it up. You can always attach the cement board over the other floor, such as vinyl flooring. Just be aware that you are adding to the height and the weight of the floor in the process.

Finally, use a wide blade to run over the floor and make sure that there are no nails or screws sticking up above the finished floor level. If there are, make sure that they are set or removed.

How To Install Cement Board Over Plywood

Confused man standing

Now that the preparation is done, it’s time to actually install the cement board over plywood. This is a relatively straightforward process, but it is one that has to be done properly.

Step 1: Mix Thinset – You should use some thin set mortar between the subfloor and the cement board. Most people think that this is for bonding but in reality, is to keep any voids from existing.

You can mix the thinset in a 5-gallon bucket. It is best if you use a mixing paddle to do so but it can be mixed by hand using a small trowel.

Be careful when mixing thinset because it does contain silica. The dry thinset can enter the lungs, so you want to wear a disposable respirator to keep yourself from breathing in that dangerous dust.

Step 2: Seams – Purchase fiberglass mesh tape and use it on all of the seams in the plywood. The mesh tape is sticky on the side that goes down, so it will stay in place. Covering the seams at this time provides you with a greater ability to keep them from shifting in the future.

Step 3: Thinset – Add this point, you will want to use your notched trowel to put down a layer of thin set. Be careful that you don’t put down more than what you are able to cover in a half hour. Otherwise, it can dry.

You may need to continue to mix the thinset in the bucket on occasion to keep it from drying out as well.

Make sure that you have plenty of thinset but not so much that it is going to add too much volume to the underside of the cement board. Remember, you are using the thin set to support the board, not to hold it in place.

Step 4: Place Cement Board – Begin placing the cement board in place in the thinset. As you do so, move the board slightly to embedded in the thinset and then lift it to make sure that the entire board is covered underneath.

The goal at this point is to avoid any type of voids from occurring under the cement board. That could eventually lead to cracking and other issues in the tile that is installed later.

As the thinset dries, it is going to hold the cement board in place. You can still use cement board screws to push through the backer board at approximately 8-inch increments.

The screws should be driven until they are slightly below the surface. This will not cause a problem when you eventually put tile in the area.

The combination of the thinset and the screws is going to make an amazing bond between the cement board and the plywood. Since you already ensured that the plywood was solid, the cement board will also be solid and the tile floor will have zero defects.

If you experience any problems with high points in the cement board, work quickly to remove the thin set and correct the issue in the plywood. You can then start the process again.

ReadyToDiy is the owner of this article. This post was published on 2022-09-09..

It is possible to put cement board over plywood but you have to prepare everything properly in advance. Any flaws that exist in the plywood, such as high or low points will transfer into the cement board. In addition, if you have any squeaks in the plywood floor, you will still hear them once the cement board is down.