Aside from actually doing the repair, choosing the correct products is half the battle. One such issue could be the choice of drywall and cement board. Cement board is like drywall to an extent and offers a far harder surface.
It is more suited to areas where drywall isn’t strong enough or could be exposed to moisture. The second is when it comes to finishing. You probably know you use drywall mud or jointing compound on drywall, yet, “can you use joint compound on cement board?”
You might find it a bad idea if using cement board as the backer for your tile installation. If the cement board is installed in a shower enclosure, bathroom floor, or tub surround, thin-set mortar should be used instead of the drywall joint compound for cement board. (Read Drywall Vs Plaster: How To Tell)
You can, however, mud over cement board and treat it like drywall if it isn’t exposed to moisture.
In short, you’ll use alkali-resistant fiberglass tape and thin-set mortar for joints of walls and floors when using multiple cement boards.
Thin-set is a water-retaining adhesive mortar made of cement, fine sand, and a binding agent.
The drywall compound comprises Gypsum dust, mica, attapulgite, perlite, and starch, which are not as robust when exposed to water.
In our guide, you can learn more about what you need for finishing Durock cement board for the details of corners and board edges.
Can You Apply Drywall Mud Over Cement Board?
A joint compound is essentially gypsum dust mixed to make a frosting-like consistency. It is also called drywall mud or simply “mud” by professionals.
Taping seals joints where you then apply a small amount of the mix. When covered with more joint compound, you can fully hide the seams between the boards.
The joint compound delivers a smooth surface and blends with the rest of the board’s surface. In addition, joint compound, unlike spackle, takes longer to dry, allowing you to work to get the desired finish.
What is Cement Board?
The traditional substrate for ceramic, stone or porcelain tile applied with a thinset mortar adhesive is cement board, which is supplied under trade names such as Hardiebacker, and Durock.
The material is a suitable replacement for drywall and plywood backer materials for wall applications in damp settings, such as tub surrounds and showers. Also, it can be used under floor tile.
Cement board offers a long-lasting foundation, and since it doesn’t contain any organic components, it is resistant to shrinkage, rotting, mold, and decomposition you find with moisture exposure.
If you use the material with ceramic tile, you’ll find the cement board offers the most durable floor or wall surface.
Because it does not increase durability, the drywall joint compound is not suited for sealing the seams between cement boards.
Its sole purpose is to provide a smooth, seamless painting surface. In addition, the drywall compound isn’t waterproof, yet thin-set mortar is.
You use thin-set mortar rather than joint compound if you are installing cement boards where there is moisture, like in your shower or as a tub surround. (Learn How To Fill Gap Between Baseboard And Floor)
What Can I Use For Joints On Cement Board?
Thin-set is a cement-based powder that is mixed with water or latex to form a thick paste.
You’ll use this to set tile and seal the seams between cement boards. The thin-set mortar seals the sheets together and strengthens the seam by pushing it into the gaps between the boards.
Many thin-set formulations contain latex and polymer additives designed to improve bonding strength. As the tiles’ size grows, adequate mortar coverage becomes more crucial.
Fiberglass mesh tape must first be put to all seams and gaps, covered by a thin-set mortar skim layer. The thin-set will fill all the spaces and cracks left when taping.
On the other hand, they create a drywall joint compound that can be used with paper or fiber joint tape to seal drywall sheet joints. It creates a smooth surface on which to paint interior walls.
Can I Use Drywall Tape With Cement Board?
You can use joint compound on cement board in a pinch or at certain times, but never drywall tape. Instead, use thin fiberglass tape designed for cement boards.
How to Install Cement Board
Cement boards work just as well outside as they do inside. To properly install cement board using thin-set mortar instead of joint compound, follow these instructions for use under flooring. Cement board on walls uses the same technique yet doesn’t face the same pressures.
1. Plan the installation.
Begin by sketching a basic layout for the cement boards, considering the size of the boards and the area to be covered.
The joints between the boards must be roughly 8 inches offset from the joints in the subflooring for maximum floor stiffness.
2. Prepare and apply your Thin-Set.
Mix a small amount of thin-set mortar according to the manufacturer’s directions, blending the dry powder with water in a bucket.
Using a notched trowel, apply a sizeable amount of mortar to the subfloor and spread it out evenly.
Cover a small enough area to install the first cement board.
3. Install the First Cement Board
Place the first cement board sheet on top of the mortar bed, rough side facing up. Allow 1/4 inch between the edge of the sheet and the walls.
Fasten the sheet using cement board screws, ensuring the screws are either flush with the sheet or slightly recessed along with the sheet, space the screws eight inches apart.
Using the same process of applying thin-set, placing the sheet, and attaching it in place, continue adding boards in the first row. Using a ruler, space the sheets 1/4 inches apart.
Pro Tip: Use alkali-resistant cement board screws and cement board joint tape. Contact with the boards will cause corrosion or degradation if ordinary screws or drywall joint tape are used.
4. Make your cuts.
Trim the last sheet in the first row using a sharp utility knife and T-square. Score the boards with two or three passes of the knife, then break off the piece in a straight line. Cut the mesh on the rear of the board as needed. (Learn How Long Does It Take For Tile To Set)
5. Tape joints.
Repeat the above until all your boards are in position.
Apply your cement board joint tape across all the gaps between sheets.
Next, cover these using a thin layer of thin-set mortar.
Apply the mortar using a drywall knife. It is essential to ensure the tape is covered and all joints are smooth.
Can You Use Joint Compound On Durock?
When sealing the seams between cement board and drywall, joint compound and fiberglass mesh tape are the finest options.
These seams are frequently found along the exposed border of a shower or bathtub.
Standard drywall joint sealing treatments such as paper tape and joint compound on cement board will not hold up. When two pieces of cement board meet, mesh tape and thinset mortar are typically used.
This procedure, however, will not work on a drywall-to-cement-board joint because thinset mortar results in a poor drywall finish. Instead, you’ll need to balance and use drywall and cement board-compatible materials.
Cement board joint compound is thinset mortar, typically used to seal the seams between two pieces of cement board, is not suitable for use on drywall.
To bond drywall and cement board, use a mixed approach with mesh tape and joint compound for the best results.
Steps For Taping Cement Board and Drywall Joints
Using drywall in a shower is a formula for catastrophe. Covering the walls of a bathroom with cement board is costly and inconvenient.
As a result, there will be joints where the drywall and cement board meet.
1. Use the Same Thickness of Board
When remodeling a room using cement board and drywall, use drywall with the same thickness as your cement board. For example, a cement board is 1/2 inch thick, so use 1/2 inch drywall.
The Cement board and drywall must have a level joint, making joint sealing easier.
Even though two materials are rated at the same thickness, the actual thickness may differ. For example, you may replace 5/8-inch drywall with a cement board.
Make sure the boards meet flat by inserting shims to the studs where you will insert the thinner material. Next, attach the wall board to thicker studs. This increases the wall thickness for an easier-to-seal joint.
2. Cut Your Boards
The wider the space between two pieces of wall material, the harder it is to seal. So, take your time measuring and cutting around curves.
Before marking and cutting wall material, measure for a close fit and ensure the gaps between wall boards are only 1/4 inch.
Filling gaps bigger than 1/4 inch can take several extra applications of joint compound and result in a worse finish.
New DIYers often struggle with sealing wall joints. It is better to cut the board a little too big and shave off the edge than install boards with large gaps you can’t tape and skim coat very well.
3. Apply Mesh Tape
Self-adhesive mesh tape is your only choice for sealing a joint where one or more sides are cement board.
Paper tape can’t hold up on cement board even if paper tape leads to a cleaner finish on drywall.
However, using mesh tape on drywall and cement board joints can still achieve a smooth look.
A bonus to using mesh tape is that it sticks to the wall with no joint compound or thinset mortar beneath it. This makes it easier for beginners to seal joints before dealing with the skim coat.
4. Apply Coats of Joint Compound
Apply 2–3 coats of joint compound to seal a drywall-cement board joint.
Let the first coat of joint compound fully dry for 24 hours between adding more to the taped seam.
Use a setting-type joint compound to finish the cement board and drywall gap. This setting-type product gives a superior finish over mesh tape than all-purpose joint compound.
Where there is a gap between drywall and cement board, use drywall joint compound as this mix can be sanded to a better finish.
If you use thinset mortar where drywall and cement board meet, the thinset mortar will leave a poor finish on the drywall, no matter how thoroughly you sand.
5. Sand the Joint Compound
Sand the final coat of Durock joint compound to a smooth finish after it has dried for 24 hours.
To finish this, sand the drywall and cement backer board with 120 grit sandpaper first, then 150 grit and 220 grit for a smooth joint.
Sand between coats of joint compound if desired to make final sanding easier. (Learn How To Remove Granite Backsplash)
Does Drywall Mud Stick to Hardibacker?
Hardiboard offers even more roughness for mud adhesion than sheetrock and at least as good moisture absorption as sheetrock, so mud should stick nicely. However, you won’t be able to use these sheets in a moisture-filled environment as drywall mud isn’t resistant to moisture.
Besides this, even if using a setting-type joint compound, using this board type in areas where you could use regular drywall doesn’t make financial sense.