How to fix an uneven floor using a self-levelling compound

It's not uncommon to be preparing to install some new flooring, only to find yourself faced with an uneven floor. Dips and bumps in a floor can be caused by many factors, but whatever the reason for their existence, one thing is certain: you won’t be able to install the flooring correctly until the uneven floor has been fixed.

The best and simplest way to do this is by applying a self-levelling compound. Whether you have an uneven bathroom floor, uneven floors in an old house or your garage floor is cracked and uneven, we have the solutions to help.

Why use a self-levelling compound for uneven subfloors?

A self-levelling compound is a polymer-modified cement solution that has high flow characteristics. It's applied prior to the flooring being installed and when applied correctly, creates a smooth surface for you to work on.

There are many types of self-levelling compounds available, all with different characteristics. It is crucial that you choose the right one for your application, and that you follow its particular set of instructions to ensure it is applied correctly.

Things to consider when using a self-levelling compound to fix uneven floors

Always remember that a 'smooth' floor isn't necessarily level, and a 'level' floor isn't necessarily smooth. The British Standards relevant to the flooring industry specify a “surface regularity” - this is a measurement of a floor's smoothness, not how level it is.

If slopes in the floor are particularly pronounced, you may need an alternative solution as most self-levelling compounds are designed to work at depths of 3mm-10mm, although some deep fill self-levelling compounds can be applied to depths of up to 50mm-60mm.

Other factors to consider include where the floor is located and what conditions it will experience over its lifetime, what floor covering is being installed, and how long you have to apply the self-levelling compound.

Whatever your needs, the key to choosing the right self-levelling compound is to assess the particular application and do your research.

Step-by-step: how to use a self-levelling compound when fixing uneven floors

Once you’ve selected the right self-levelling compound for you, it's important to understand how best to apply it. Whether you need to fix an uneven bathroom floor, uneven garage floor, or uneven kitchen floor, there are some simple steps you need to undertake.

Firstly, if the subfloor contains residual moisture with a reading of 75% RH or above, a liquid DPM should be used.

By choosing to use a floor primer, you will also enhance the bond between the self-levelling compound and the subfloor, and create a barrier to stop air escaping, which means the self-levelling compound can cure correctly.

To apply self-levelling compound to a floor, you will need the following equipment:

An electric drill
A mixing paddle
A mixing bucket
A trowel
A spiked roller
PPE

Prepare the area by vacuuming to ensure there is no dust, as this can cause adhesion issues
If skirting boards can’t be removed, masking tape should be applied to protect the surface
Add the appropriate amount of water to a suitably sized mixing bucket, and pour the powder into the water
Mix using an electric drill and whisk for two minutes, ensuring that the compound is uniform in colour and consistency
Pour onto the prepared surface in a continuous motion, then spread using a steel trowel to the desired thickness. Work the self-levelling compound into the corners and ensure that there is good coverage
A spiked roller can be used once the self-levelling compound has been applied. This removes trapped air bubbles to improve the overall finish. (If using a spiked roller, ensure that you do not drag the roller across the surface, but gently bring it back and forth). Note - we don't recommend the use of a spiked roller on fibre-reinforced compounds, as this will pull fibres to the surface.
Once applied, the product must be left to dry for the recommended amount of time before it can be walked on and before a floor covering can be applied

FIND OUT MORE ABOUT HOW TO APPLY A SELF-LEVELLING COMPOUND IN THIS VIDEO

Everything you need to know when fixing an uneven subfloor with self-levelling compound

Now that we’ve explained how to apply a self-levelling compound onto an uneven floor, discover some of the frequently asked questions around the topic.

How do I know if my floor is uneven?

Besides a visual inspection, there are many ways to identify an uneven floor, including the use of a beam level, phone apps, or even placing a golf ball or a marble down in the centre of the floor and seeing if it rolls. However, it's important to remember that the primary function of a self-levelling compound is to smooth imperfections in a floor so that a floor covering can be applied - it's not designed to dramatically change the levels of a floor.

Does self-levelling compound need a spiked roller?

Using a spiked roller is highly recommended. This will remove trapped air bubbles to improve the overall finish and create a smoother, sounder surface on which to lay the flooring. However, we don't recommend the use of a spiked roller on fibre-reinforced compounds, as this will pull fibres to the surface.

I have some self-levelling compound that is out of date, can I still use Iit?

No. Always make sure that the self-levelling compound you use is within date, otherwise there is a risk it could fail. Furthermore, should the product fail to perform, it will not be guaranteed by the manufacturer, meaning you will be left to fix the problem.

How can I prevent cracked self-levelling compound?

The most likely causes of cracked self-levelling compound are poor bonding to the substrate below, and movement in the subfloor. Poor bonding to the substrate can often be rectified by the use of a floor primer, such as our Cempolay Universal Primer.

Bostik Cempolay Universal Primer

A water-based, solvent-free, acrylic dispersion floor primer and bonding agent that’s suitable for use prior to the application of any Bostik self-levelling compound.

When applied undiluted, it acts as a bonding agent on non-porous surfaces such as power floated concrete, ceramic tiles, quarry tiles, rigid metal, terrazzo, asphalt and surface membranes. In diluted form, Bostik Cempolay Universal Primer can be used as a primer on porous surfaces and will dry within one hour depending on the conditions.

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Where this is a risk of movement in the subfloor, and you're concerned about a self-levelling compound cracking, the best solution is to use a fibre-reinforced self-levelling compound, such as our Cempolatex Fibreflex. This has been specially designed for use on floors where vibration and minimal thermal movement could be problematic.

Bostik Cempolatex Fibreflex

A one part, rapid setting, self levelling compound developed for use on subfloors where vibration and minimal thermal movement can be problematic.

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