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Thinset Under Cement Board for Bathroom: Is It Recommended?

Thinset is a type of mortar you can spread across the floors and walls in the bathroom. It is an adhesive, so you can put it on the subfloor and know the tiles will stay in place when you install them.

Since the thinset is water-resistant, you can use it in the shower and tub or on the bathroom floor without worrying that it will deteriorate and need repairs. It’s ideal to use the thinset under cement board for the bathroom.

Thinset Under Cement Board for Bathroom

Should You Use Thinset? The Quick Answer

Yes, you should use the thinset under cement board for the bathroom because it ensures your floor maintains its quality over the years. You don’t have to worry about tile deterioration or mold growing beneath your bathroom floors. Installing the thinset adds steps to your bathroom makeover, but it pays off.

Should You Use Thinset Under Cement Board?

You should use the thinset under the cement board for the bathroom because it bonds your tile tightly to the floor. The thinset resists water, so you don’t have to worry about mold growing beneath the floor or behind the shower walls. Thinset doesn’t loosen in hot weather, so you don’t have to consider how it might contract in different temperatures.

For all the benefits of thinset, you should use it under cement board. The cement board itself is the right choice as a base for the bathroom floor. You can install the cement board over the plywood subfloor to prevent that wood from getting exposed to heat and moisture.

Skipping cement board in a bathroom installation means you will face some deterioration. The plywood will become less sturdy as it retains moisture from the bathroom above it. As the wood rots, your tiles will get loose, eventually cracking and crumbling until you need to replace them. 

Though you can replace a small section of tile, you’ll find that the wood starts to rot in other areas, too, creating a neverending repair job. It’s better to start on the right foot by laying cement board and applying thinset.

What Thinset Do You Need?

Different types of thinset are available, so you should know what you need before you get to work.

Unmodified Thinset

Unmodified thinset is the simplest type. It uses sand, cement, and water-retaining elements. You can mix the powder with water and have an unmodified thinset ready to apply to your bathroom floor.

Modified Thinset

Modified thinset has additional elements that help it create a stronger bond. Ingredients include sand, cement, water-retaining components, and liquid latex or acrylic polymers that facilitate the bond. Some brands of modified thinset don’t require water before you spread them on the floor.

Modified Thinset

Which Thinset To Choose?

It might sound like a modified thinset is the best choice since it creates such an enduring bond. However, a modified thinset needs air to dry. Unmodified thinset doesn’t need air, so you can spread it on the floor, apply the cement board, put on another layer, then tile the bathroom. Modified thinset needs to get air through the grout joints, so it might not cure for up to 60 days.

How to Use Thinset Under Cement Board in the Bathroom

Most thinset comes as a powder that you have to mix yourself. Know that the thinset starts firming up as soon as you mix it, so you want to work quickly to get the best bond for your tiles. The general rule is that you should only mix as much thinset as you can use in 30 minutes.

You can buy some thinset premixed, but it will cost a bit more than the powder version. If you’re tackling a small space, premixed thinset isn’t too expensive, and you’ll use it all before it hardens. Otherwise, mixing the thinset is best because you can make as much as you need for specific projects and periods. You don’t have to worry that it’ll dry out before use.

Before you get started, walk around the plywood subfloor of your bathroom. Screw in all the boards so they’re secure and don’t squeak when you move. It’s much better to find these problems before you layer more floors on top of your plywood.

Next, ensure the plywood is level and even. Hammer down any boards that stick up and sand the seams so it’s smooth. This step gives you a solid and level foundation for your thinset and cement board.

Apply thinset to the floor in large sections. Scoop out the thinset with a trowel and put it on the floor. Then use a notched trowel to comb the thinset. These ridges give you some breathing room between the plywood floor and the cement board, so air bubbles pop as you push them together.

When you have the cement board covering the bathroom floor, use fiberglass mesh tape on the cement board seams. Smooth out any air you see in the tape to keep the floor even.

Mix up some more thinset and apply it to the cement board. It should even go over the seam tape. Then lay the tile in the thinset in your ideal design.

Apply thinset to the floor in large sections

Let the thinset cure for about 24 to 48 hours before you walk on it. Some thinset is quick-setting and only requires six hours to cure. But it’s better to be safe than sorry, as walking on tile that isn’t set can make the flooring shift or loosen.

Frequently Asked Questions

You’ve learned when you should use thinset under cement board in the bathroom. If you follow the steps above, you’ll have no problem finishing a sturdy bathroom floor. But in case you need more information, check out the answers to these frequently asked questions.

Is grout the same as thinset?

No, grout and thinset are different. Thinset works as an adhesive to connect the tile to the floor substrate. Unless you use translucent glass tiles, you won’t see the thinset in the bathroom. 

Grout, however, is what you put between the tiles. Grout fills the gap to make your bathroom floor smoother and more cohesive (see our guide to filling the gaps in your bathroom ). You’re always able to see grout, and it’s one of the last steps before you finish your bathroom makeover.

What thinset adhesive should I use?

The type of thinset you need depends on what you’re installing. If you’re using a backerboard on the floor, you can use any type of thinset. If you’re working on the walls or with large tile, you’ll need stronger types of tile mortar. You’ll also need to consider what type of tile you’re installing, like if it’s glass tile, natural stone, or exterior-grade tiles.

Are mortar and thinset the same thing?

Thinset is a type of mortar, but not all mortar is thinset. Mortar typically uses sand, concrete, and water to create a binding agent for builders. It’s thicker than thinset and can bond together stronger items, like brick or stone.

Thinset is the mortar that you only use on tiles, while mortar can work as an adhesive for many other building materials. As it’s less thick than other mortar, it’s easy to spread thinset over the bathroom floor and keep it flat and level.

Final Notes

It’s best practice to use a thinset under cement board for the bathroom. You won’t have to worry about tiles loosening or shifting. Thinset is a sturdy adhesive that keeps your floors in top shape for years while also preventing mold from growing due to the trapped bathroom moisture. Make some time to apply thinset when you redo your bathroom.

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