Joint compound and fiberglass mesh tape are the best products to use when sealing the seams where cement board and drywall meet. These seams are usually at the edge of a shower or bathtub area that is exposed to water. While you will want to use thinset mortar and mesh tape to cover the seams between two pieces of cement board, you will use a different tactic when finishing the point where cement board and drywall meet. Cover the seam with mesh tape and then use joint compound over the tape. This will lead to a smooth, paintable finish.
Table of Contents
Why is it Important to Use the Right Materials Where Cement Board and Drywall Meet?
Typical drywall joint sealing techniques, using paper tape and joint compound, won’t hold up on cement board. This is why mesh tape and thinset mortar are typically used where two pieces of cement board meet. However, this method won’t work on a joint between drywall and cement board because thinset mortar leads to a poor drywall finish. You’ll need to strike a balance and use materials that work for both drywall and cement board.
- Paper tape designed for drywall is not ideal for use on cement boards.
- Thinset mortar used to seal seams between two pieces of cement board is not a good material to use on drywall.
- For best results, use a hybrid approach with mesh tape and joint compound to bond drywall and cement board.
If you’re wondering if you can skip this hassle and just tile over drywall in your shower, it’s important to keep in mind that drywall won’t hold up where it is exposed to direct water. Cement board is a great choice for your shower walls.
5 Steps to Tape a Joint Between Cement Board and Drywall
It’s a recipe for disaster if you use drywall in a shower. On the other hand, it’s expensive and impractical to cover all the walls of a bathroom in cement board. For that reason, you will have joints where drywall and cement board meet. There’s nothing to fear. In a few steps, you can seal these joints and finish your bathroom remodel.
Use the Same Thickness Board
When planning your remodel in a room with cement board and drywall, it’s essential that you use drywall with the same thickness as your cement board. Most cement board is 1/2 inch thick. So, it’s essential to use 1/2 inch drywall.
- It’s key that the cement board and drywall meet at a level joint. This makes sealing the joint much easier.
- Most cement board is 1/2 inch thick, so use 1/2 inch drywall to match.
- If your cement board and drywall are of different thicknesses, screw shims to the studs where the thinner material will be installed. Then, install the wall material on top of the shimmed studs.
Even if you purchase two materials that are rated at the same thickness, the actual thickness may be different. Or, you may be replacing drywall with cement board in a room where the drywall is 5/8 inch thick. In this case, make sure the boards meet flush by screwing shims to the studs where the thinner material will be installed. Then, attach the wall board to these thickened studs. You can build up the thickness of the wall this way to create a level joint that is easier to seal.
Cut the Board Precisely
The larger the gap between two pieces of wall material, the harder it is to seal. Measure precisely and take your time cutting pieces around irregular surfaces or curves. You will have a much easier time sealing a joint that is no more than 1/4 inch between two types of board.
- Measure your wall material carefully before marking and cutting, to ensure a close fit.
- Make sure the gaps between your wall boards are no more than 1/4 inch.
- Filling gaps that are larger than 1/4 inch can take several extra coats of joint compound and often results in a poorer finish.
Sealing joints between wall material is the most common source of headaches for new DIYers. To reduce your chance of migraines, take your time measuring and cutting. It’s better to cut your board a little too big and shave off an edge than it is to install wall boards with large gaps between them.
Apply Mesh Tape
Self-adhesive mesh tape is the only choice when sealing a joint where one or more sides are cement board. Paper tape simply won’t hold up on cement board. Although paper tape generally leads to a cleaner finish on drywall, you can still achieve a smooth look if you use mesh tape on a drywall/cement board joint.
- Use this self-adhesive joint tape where cement board and drywall meet.
- Never use paper drywall tape on cement board.
- With proper finishing work, mesh tape can yield a good finish on drywall.
A bonus to using mesh tape is that it sticks to the wall without any joint compound or thinset mortar beneath it. This makes it easier for beginners to seal joints.
Apply Several Coats of Joint Compound
To get a good seal on a seam between drywall and cement board, apply 2–3 coats of joint compound. Wait 24 hours between coats of joint compound to allow it to fully dry. For best results getting a good finish between cement board and drywall, use a setting-type joint compound. This product produces a better finish over mesh tape than all-purpose joint compound.
- Apply 2–3 coats of joint compound to the seam between wall boards.
- Wait at least 24 hours between coats of joint compound.
- Use this setting-type joint compound over mesh tape for best results.
- Do not use thinset mortar to seal a joint between drywall and cement board.
Joint compound is the best choice where drywall meets cement board. This is because you can sand and achieve a smooth finish with joint compound on drywall. If you use thinset mortar at the point where drywall and cement board meet, the thinset mortar will leave a poor finish on the drywall, regardless of how carefully you sand. If you plan on painting your drywall, this can lead to an ugly end result.
Sand the Joint Compound
Once the final coat of joint compound has been allowed to dry for 24 hours, sand it to a smooth finish. To accomplish this, first sand using 120 grit sandpaper, then 150 grit, and then finish off with 220 grit for a seamless joint between drywall and cement backer board.
- Wait 24 hours after applying the final coat of joint compound before sanding.
- Sand with progressively finer grit sandpaper, going from 120, to 150, and finishing with 220.
- If desired, sand between coats of joint compound to make the final sanding easier.
To make your final sanding easier, it’s a good idea to sand between coats of joint compound in step 4. Go over the joint briefly with 150 grit sandpaper to smooth imperfections and make for a smoother final coat.
How Do You Finish a Joint Between Cement Board and Drywall?
The best way to finish a joint where drywall and cement backer board meet is to:
- Make sure the cement board and drywall are the same thickness during installation, to make for a smooth joint that is easily sealed.
- Cut wall materials precisely, to keep seams small.
- Use self-adhesive mesh tape where drywall and cement board meet.
- Spread 2–3 coats of setting-type joint compound over the seam, waiting 24 hours between coats.
- Sand the joint to smooth it, beginning with semi-coarse sandpaper and finishing with fine sandpaper.
This technique will lead to smooth, durable joints that can be finished with tile or paint. Using paper tape or thinset mortar at a joint between cement board and drywall often leads to peeling paper or a poor finish.