How to achieve a perfectly even subfloor

Every floor finishing company knows that the quality of the end result depends heavily on the quality of the subfloor. A flat base is essential, especially when installing flexible floor coverings. That is why it is important to pay close attention to the pre-treatment of the subfloor. By levelling it correctly, you can create a perfectly smooth subfloor.

Levelling: first assess the screed

The first step – before you can start levelling – is to assess the substrate.

Is the quality of the screed good?
The tensile strength should be at least 0.5 Nm per mm2. If necessary, apply a special screed stabiliser.
Does the screed need to be cleaned?
For example, due to impurities such as oil, grease or plaster residue.
Are there any shrinkage cracks in the screed?
If so, it is important to grind them out, apply cramps and fill the cracks with a two-component epoxy, such as RENO P520 EASY.
Check the residual moisture content of the floor.
If the floor is still too damp, or if there is rising damp, a two-component epoxy moisture barrier (HYTEC E336 XTREM) can be applied to a cement-bound screed. This is not permitted on an anhydrite floor, as it could cause it to 'rot'.
Is the screed 'too porous'?
If so, use a special adhesive primer for porous substrates (GRIP A500 MULTI) to eliminate the absorbent effect of the screed. For closed substrates, a primer such as GRIP A936 XPRESS is suitable.

Determine the right type of levelling compound

The best levelling compound to use depends on these four elements.

How intensively is the space used?

For busy areas with heavy foot traffic or high castor wheel loads (office chairs or hospital beds), it is best to choose a levelling compound with a higher pressure resistance. Bostik SL C980 XPS has a pressure strength classification of C40.

The type of screed is also important.

Is it cement-bound or anhydrite? In the case of flexible (wooden) floors, use a fibre-reinforced levelling compound with a higher flexural strength, such as Bostik SL C740 FIBER MAXI.

Another determining factor is the desired layer thickness.

Certain levelling compounds can be applied in a single operation to a thickness of 4 to 5 cm. Levelling compound SL C940 XL can bridge a height difference of 50 mm.

Finally, the available drying time also plays a role.

Fast-drying levelling compounds can be further processed just 2 to 4 hours after application. One example is SL C990 SPRINTER.

Levelling compound in combination with underfloor heating

Any type of levelling compound can be used in combination with underfloor heating, but it is important that the hot water pipes are sufficiently covered by the screed, otherwise the floor surface could become too hot.

Applying the right levelling compound in the right way results in a durable and smoothly finished floor with a longer lifespan.

How to apply the levelling compound

After mixing the levelling compound with the correct amount of clean water, leave the mixture to rest for a while. Then mix again briefly and you will have a homogeneous mixture.

The levelling compound can be spread to the desired thickness with a flat trowel or flat, non-toothed squeegee. After 5 to 10 minutes, the levelling compound can be worked with a spiked roller.

Ensure that the levelling compound can dry naturally, without draughts or direct sunlight. A second coat can be applied without primer as soon as the first coat is walkable but not yet completely dry.

Finishing with floor covering

After levelling, you are ready to apply your floor covering to a perfectly smooth surface. Floor covering – whether tiles, wood or flexible floor covering – benefits from a perfectly smooth subfloor. This ensures a smooth and durable result that will remain beautiful for a long time.