CMU is an abbreviation for concrete masonry units. It’s commonly known as cinder block, concrete block, and other names amongst contractors.
CMU refers to a standard-sized rectangular block that is used for construction. It’s usually made out of Portland cement, water, and fillers.
If you are interested in building your residential or commercial property with CMU, you may be interested in finishing it with stucco.
In today’s article, we will be taking you through the process of applying stucco over CMU. We will also share the benefits you will enjoy as well as the risks associated with this project.
Can You Apply Stucco Over CMU?
Of course, you can apply stucco over CMU. All you need to do is ensure that there is sufficient bond between the stucco and CMU and it will last for years.
Concrete masonry units have lots of unique properties but they are not aesthetically pleasing. That’s why most people prefer to finish them with a product such as Stucco.
How to Apply Stucco Over CMU?
There are two main methods of applying stucco over CMU;
• Direct Application
In this method, all you have to do is apply stucco directly to a CMU. A bond will be formed when the CMU sucks water from the fresh stucco. The use of a bonding agent can also enhance the adhesion between these two materials.
A direct application of stucco over CMU is the easiest and quickest method. However, you have to be 100% sure about the bond created. Otherwise, the stucco will not last.
• Lathed Application
If you doubt stucco’s ability to adhere to CMU, you can always introduce a metal lath. The lath will in this scenario act as a surface to hold the stucco. Lathed applications offer a stronger bond between stucco and CMU.
Like the above method, this one also has some drawbacks. Metal laths are susceptible to corrosion and when that happens eventually, the stucco can start breaking into smaller pieces.
You can choose any of the methods described above, depending on your preferences. However, there is a professional way of applying stucco over CMU that comes with minimal risks. This is a method that I highly recommend because it’s less tiresome and time-consuming.
Here is the best method of how to apply stucco over CMU;
Step 1: Clean the CMU
In any stucco application, the surface you intend to apply it over must be thoroughly cleaned. The presence of debris and other particles on CMU can affect adhesion to stucco. Therefore, grab a pressure washer and start blasting the cinder blocks.
Step 2: Rough Up the CMU Surface
Some people go directly to the next step, but I prefer scuffing up the surface. Whenever you are applying a finish over a CMU wall, it’s crucial to rough up the surface.
This gives the stucco something to bite onto. On a rough concrete masonry unit, you are guaranteed better adhesion to stucco. You can use sandpaper to lightly scuff up the cinder block.
Step 3: Apply a Bonding Agent
I find the process of applying stucco over CMU much easier when I use a bonding agent. There are many options for you to choose from.
Examples include the Quikrete concrete bonding adhesive or the DRYLOK Latex bonding agent. These products guarantee a strong bond between the stucco and the CMU that will last for years.
Please note that each bonding agent may have a different application process. Follow the instructions provided by the manufacturer.
Step 4: Apply the Stucco
Once all the above steps have been completed, you can dive straight into the application of the stucco over the CMU. For those who are not familiar with the stucco application, you usually start with the scratch coat.
It should have a rough texture so that it can adhere to the next coat. The scratch coat should be left to dry for about a day.
Then you can apply the brown coat which is a bit thicker. The brown coat should be leveled and smoothened with a trowel. Finally comes the finish coat.
Step 5: Let It Dry and Provide the Regular Maintenance
Give your stucco enough time to dry and your CMU wall should look better with the new finish. Even though stucco is durable, it requires regular touchups such as misting it with water to prevent cracking.
What are the Benefits of Finishing CMU with Stucco?
There are lots of other materials you can use to finish cinder blocks. But why would you use stucco? Here are some of the benefits property owners can enjoy from covering CMU with stucco;
• Improvement in exterior aesthetics – Stucco comes in multiple colors, textures, and designs. Unlike the dull look of CMU, stucco can spice up the exterior of your home. With stucco, you can create a unique architectural design over the CMU walls.
• It can reduce the transfer of heat – Did you know that stucco has some insulation properties? When you cover stucco over CMU, it minimizes the transfer of heat from the inside and the outside. This will keep your energy bills relatively low.
• It protects CMU from wear and tear – CMU is durable. However, with time it will feel the effects of weather, impact, and other factors. With stucco on the surface, CMU will be protected from these elements and should serve you for much longer.
• Stucco is an affordable exterior finish – Compared to other finishes that can be used over CMU, stucco is relatively affordable and low maintenance. Stucco doesn’t need to be cleaned every week. Also, if applied correctly, it can last for years without repairs.
What Risks Are Involved When Applying Stucco Over CMU?
Before embarking on any project, it’s a good idea to evaluate the risks involved. Like all other projects, when covering CMU with stucco, there are a couple of things that may go wrong. Below is a list of the risks associated and how you can avoid them;
• Formation of cracks – One of the biggest challenges of using stucco over CMU is the development of cracks. If the CMU wall experiences any sort of movement, cracks can easily form on the stucco.
That’s why it’s crucial to inspect and fix any structural issues with the CMU before applying stucco. Poor adhesion can also lead to cracking. Make sure you use a bonding agent and you thoroughly scuff up the CMU.
• Moisture penetration – The other issue you may find yourself dealing with is the penetration of water. Stucco is a porous finish. This means that it easily allows water to pass through.
As moisture seeps through stucco, it can get trapped between CMU walls leading to mold and damaging the cinder blocks. To prevent this problem, the CMU wall has to be thoroughly flashed.
• Insufficient bonding strength – If the stucco isn’t strongly bonded to the CMU, it can easily delaminate or fall off. If you follow the method I shared with you earlier, this isn’t a problem you have to worry about.
Insufficient bonding strength is often caused by poor workmanship. As long as you use a bonding agent and scuff up the CMU surface, the stucco will adhere perfectly.
If you are worried about running into these risks, you should hire a professional stucco contractor. Applying stucco over CMU is a job that can be handled by any DIY enthusiast.
However, if you doubt your ability, it’s better to seek professional help than end up with a poorly installed stucco finish.
What Stucco Alternatives Can You Use Over CMU?
Stucco isn’t the only finish you can apply over CMU. Other finishes look better and are more functional. Take paint for example. It’s easier to apply and it’s quite affordable.
Paint also comes in various colors and designs. You can also use veneers, cladding panels, vinyl siding, and plaster over CMU. Before settling for stucco, explore these alternatives and figure out which one is better.