Mastering Cement Board Outside Corners (Insider Tips & Techniques)

Cement board is well known for its many features such as being durable and water-resistant. However, one of the issues that both professional and DIY builders have raised regarding this construction material, is how difficult it is to finish cement board outside corners.

If you have an upcoming cement board installation project and you’re wondering how you should approach the outside corners, I’ll be sharing with you a detailed guide about everything you need to know.

USG Durock cement board

How Do You Finish Cement Board Outside Corners?

Step 1: Install a metal corner bead

Because we’re talking about the outside corners, I’ll be assuming that you have already installed the cement board on the adjacent walls. So, I’ll be jumping straight into how you can finish the outside corners.

The first step entails installing a metal corner bead. This will act as a transitional piece from one cement board to the other.

The reason I recommend the installation of a metal corner bead is that it creates a straight and structurally sound outside corner.

So, cut the metal bead to the desired height of the outside corner wall and secure it with fasteners or screws.

Step 2: Cover the joints between the cement board and metal corner bead with fiberglass mesh tape

Some people prefer to skip this step but it’s very important. Considering that the metal bead and cement board are made out of different materials, a fiberglass mesh tape contributes to reinforcing the structural integrity of the outside corner.

Fiberglass mesh tape also prevents the development of cracks.

Step 3: Cover the cement board outside the corner with a thinset

To get an even and smooth finish on your outside corner, prepare some thinset mortar and apply it using a flat trowel.

Make sure that you only apply thin layers of thinset that cover the metal corner bead and mesh tape. If you apply thicker layers, you may end up with a bulging outside corner.

Step 4: Feather the edges of the outside corner and sand the thinset

Unless you’re a pro, it’s usually a bit difficult to get a smoother finish in the above step. So, you’ll need to feather out the edges.

And to get a smoother finish, sand down the thinset once it dries. The latter step also allows you to correct any errors such as bumps and rough spots.

And that’s it! You’ll have successfully finished your cement board outside the corner. From here, you can waterproof the wall, install tile, or finish it however you see fit.

What Other Transitional Pieces Can You Use on the Cement Board Outside Corners?

Metal corner beads are the go-to transitional pieces for cement board outside corners. But they are not the only option in the market. Here are some alternatives you can consider;

• Vinyl corner beads – These are just like metal beads only that they’re made out of plastic. Even though they are not as durable as metal corner beads, they are rust-free and easy to install. You can also get them in various colors.

• Schluter RONDEC – Another popular transitional piece that people like using on cement boards outside corners is Schluter RONDEC. After installation, RONDEC forms a symmetrically rounded corner which is quite aesthetically appealing, if you ask me.

• Bullnose trims – These are mostly used to transition between tiles. So, if you’re going to tile over the cement board, I would highly recommend this option.

Bullnose trims don’t just look good but they also do a great job of protecting the outside corners from damage.

• Wood trim – If you want to create a sophisticated cement board outside corner finish, you can opt to use a wooden trim.

Wood is well known for its rustic design and it will make those corners stand out. Please note that a wood trim won’t thrive in high-moisture areas.

Whichever transitional piece you decide to use on a cement board outside the corner, make sure that it is installed correctly.

As you prioritize aesthetics, don’t overlook the importance of longevity when finishing a cement board outside the corner.

Do Cement Board Outside Corners Need to Be Waterproofed?

Of course, the cement board outside corners must be waterproofed. Whether you use a metal, vinyl, or wooden transitional piece, waterproofing is crucial in preventing moisture penetration and water damage.

If you don’t waterproof the outside corners, you can run into serious issues such as mold growth.

The cement board itself is water-resistant but not waterproof. That means it can withstand moisture exposure without getting damage but it will also let water pass through because it is porous. So, you need to waterproof the adjacent cement boards as well as the corners.

Today, there are many waterproofing products you can use in such a project. You can use a waterproof membrane like Kerdi or a liquid membrane like RedGard.

In case this cement board outside corner is on the exterior part of your home, you should consider installing flashing to prevent direct exposure to rain or snow. Waterproofing the cement board outside corners goes a long way in ensuring the longevity of your structure.

What are the Challenges Associated with Finishing a Cement Board Outside Corner?

As I said earlier, finishing a cement board outside the corner isn’t easy. This is an opinion that’s shared by both professional contractors and experienced DIY enthusiasts.

If it’s your first time tackling such a task, it’s a good idea to familiarize yourself with some of these common obstacles you may run into and how you can overcome them;

• Achieving a smooth finish – When two cement boards are meeting at an outside corner, getting a smooth finish isn’t a walk in the park.

But the good news is that I have already shared with you a detailed guide on how to accomplish this. So, I won’t dwell so much on this.

• Cutting the cement board – To achieve the perfect angle on the outside corners of the cement board, you need to work with strict measurements and precise cuts. This needs a lot of skill.

• Cracks – This is a very common issue when it comes to cement board outside corners. If you don’t use a transitional piece or fiberglass mesh tape, the corner will lack proper structural integrity and will eventually start to crack.

• Installing tile – If you think that finishing a cement board outside the corner is challenging, wait until it’s time to lay tile. Quite often, people discover that the tiles are not even after installation.

• Misalignment along the joints – If one cement board happens to be misaligned, creating a seamless joint will be quite challenging. And you may have to remove the cement board and re-install it.

• Durability – Outside corners are more susceptible to damage. Therefore, unless you use additional materials to reinforce the structural strength, those outside corners may not last very long.

USG Durock cement board

How is Finishing a Cement Board Outside Corner Different from an Inside Corner?

Even though the materials used to finish both an inside and outside corner are the same, the approach is very different. Outside cement board corners have an obtuse angle which makes finishing them more difficult.

On the other hand, a cement board inside the corner has more of an acute angle which makes it easier to create a smoother finish.

Maintenance Tips for Cement Board Outside Corners

After going through all the work that’s involved in finishing a cement board outside the corner, it’s crucial to take necessary measures to ensure that it lasts as long as possible.

Here are some maintenance tips that you should keep in mind;

Always ensure that the corner is sealed and waterproofed. Moisture can weaken the strength of the mortar and transitional piece. It can also promote mold growth.

Repair cracks as soon as you notice them. The thing about cracks is that they spread with time. But if you repair them, you can prevent the cracks from going much further along the corners.

Frequently inspect the corners. Through regular inspection, you can identify signs of wear and prevent them from worsening.

It’s also advisable to ensure that your cement board outside corner is clean and free of debris at all times. Don’t also forget to recoat the walls every few years to maintain the aesthetic appeal.

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