Two substrates are suitable for stone veneer application; metal lath and cement board. A metal lath comes in handy if the material beneath is made of gypsum, OSB, or wood because it reinforces the structure.
However, stone veneer can be applied directly if the substrate is a cement board. Metal lath was the traditional method of installing stone veneer. Most people these days prefer to install it over cement board.
Installing stone veneers on cement boards is a relatively straightforward process. However, there is one issue that confuses both expert contractors and DIY enthusiasts – Do you need a scratch coat on cement board?
For years, scratch coats have been used to enhance the bond with the next coat. But is it necessary when you are installing stone veneer over cement board?
Do I Need a Scratch Coat on Cement Board?
A scratch coat is not necessary when you are installing stone veneer on a clean and well-prepared cement board. Stone veneer can easily bond to cement board using mortar as long as there is no dirt or contaminants on the surface.
Therefore, if you create a scratch coat when installing stone veneer to cement board, you will be wasting a lot of effort and time that would have been used in fitting the pieces of stone veneer.
However, a scratch coat is mandatory when installing stone veneer on metal laths. The scratch coat improves the bond between the metal lath and the stone veneer.
Why Do Some Manufacturers Recommend a Scratch Coat on Cement Board?
As mentioned earlier, scratch coats have been used for years to improve the bond with the next coat. Some manufacturers still follow this principle, which is why they may recommend using a scratch coat on cement boards when installing stone veneer.
Even though stone veneers can bond directly to cement board, if the manufacturer recommends a scratch coat, I would advise you to follow their instructions.
Why? Because of the warranty. If you do not follow your manufacturer’s instructions and something goes wrong, they may not honor your claim.
How Do You Apply a Scratch Coat on Cement Board?
If you are used to creating scratch coats on all your projects or your stone veneer manufacturer recommends it, then don’t hesitate to apply it.
To do so, you will need to prepare the scratch coat mixture using cement, sand, and water. Mix it until you achieve a paste consistency.
Apply it on the cement board using a masonry trowel. The ideal thickness of a scratch coat on a cement board should be half or three-quarter inches.
Spread the mixture over the cement board, ensuring the entire area is covered. Once it’s laid on, grab a notched trowel and make vertical or horizontal grooves on the scratch coat. Do this while it’s still wet because it will be easier.
After creating a scratch coat, you should give it about a day for it to dry before you can proceed with the installation of the stone veneers.
Can a Cement Board Suck Moisture Out of a Scratch Coat?
Another challenge you may encounter when installing a scratch coat on cement board is that the latter may suck out the moisture from the mortar.
Therefore, as you wait for the scratch coat to dry, you may notice some parts of the mortar separating from the cement board.
To prevent cement boards from sucking moisture out of the scratch coat, you should always dampen the board with a wet sponge. Therefore, it won’t need to suck moisture from the mortar.
Also, this isn’t a problem that you have to worry about in the first place because a scratch coat isn’t needed when installing stone veneer on cement board.
Will Stone Veneer Bond to Cement Board without a Scratch Coat?
Yes, it will. Many people have installed stone veneer to cement board without a scratch coat, and it has held up well. Therefore, there is absolutely no reason why you should use up a lot of your time and effort to create an unnecessary scratch coat.
Besides saving you time, skipping the scratch coat when installing stone veneer reduces the thickness of the entire structure. If you are working with limited space, you shouldn’t hesitate to skip the part about the scratch coat.
What Else Don’t You Need When Using a Cement Board?
Many expert contractors agree that using a cement board for stone veneer installation makes the work easy. The reason being you only need the cement board, stone veneer, and mortar.
A scratch coat isn’t the only thing that you don’t need to use for this project. Other common materials aren’t necessary, and examples include;
• Metal Lath: If you are fitting stone veneers on cement board, metal lath is the last thing that should be on your mind. Metal lath is often used when stone veneers are about to be installed on a surface that is not rigid or structurally sound.
Examples include drywall, gypsum, or OSB. If you have used cement board, you can testify to how durable this material is. Therefore, it doesn’t need the reinforcement of metal lath.
• Waterproofing Membrane: A waterproofing membrane is also unnecessary when installing stone veneers on cement boards. Unlike drywall, sheetrock, or OSB, cement board isn’t susceptible to water damage.
Cement boards can be exposed to moisture and water 24/7 and still retain their shape. Unless you would like to keep the material behind the cement board moisture-free, there is no need to use a waterproofing membrane.
Can I Use an Adhesive on Cement Board Instead of a Scratch Coat?
Sure, you can. If you are installing stone veneers and want a much stronger bond, you should consider adhesives instead of a scratch coat.
An adhesive made of epoxy or silicone can create a super strong bond with the cement board. The good thing about adhesives is that they are easy to use and will not increase the thickness.
Which Side of the Cement Board Should You Apply a Scratch Coat On?
Cement board isn’t like drywall. It doesn’t have a paper-facing side. Therefore, if you have decided to apply a scratch coat on top of it, it won’t matter which side you apply it on.
Both sides of the cement board can adhere to mortar. Unless you are working on a special type of cement board, you don’t have to be so keen on which side you should work on.
Is Veneer Stone Too Heavy for Cement Board?
Cement board is pretty sturdy. Unlike drywall or sheetrock, it can withstand more weight.
Therefore, you don’t have to worry so much about the weight of the veneer stone overpowering the cement board. As long as you use the correct mortar or adhesive to create a bond, weight shouldn’t be a significant factor.
The most important thing you need to take note of is to ensure the cement board is properly fastened to the wall.
If the cement board is weak, it may fall off due to the weight of the veneer stones. For any project related to cement board, you must ensure the surface is structurally sound.