Filling, levelling, smoothing


Expert preparation of a substrate also includes the surface levelling. Depending on the surface condition, it may be necessary in some cases to produce layers of several centimeters thickness. In other cases, a fine smoothing layer of 1 to 2 mm is sufficient. Application of a levelling layer is also required if the substrate is non-absorbent. The minimum layer thickness for producing an absorbent substrate is 2 to 3 mm.


A basic prerequisite for the use of levelling compounds is a stable substrate that has already been mechanically treated and primed.

The main tasks of levelling compounds


Non-slump levelling compounds with a stiff consistency (Thomsit RS 88) have been developed for filling holes or for producing slopes. Their consistency can be adjusted by the addition of water. They harden within a short time (approx. 30 - 60 min).


Levelling compounds often need to level out heavily cracked, uneven surfaces or floors with a significant slope. For large-area application, Thomsit offers self-levelling high-build compounds like Thomsit DS 40 that feature pourable consistency.


The most common application, however, is the smoothing of slightly uneven substrates. For this purpose, a self-smoothing compound is applied in layers of 1 to 3 mm thickness. It ensures that later no unsightly marks, caused by irregularities in the substrate, show through the floor covering.

Classification of levelling compounds according to their binding agent

Cementitious levelling compounds

These standard levelling compounds offer a clear advantage. Due to their cement content, they rapidly bind the gauging water even when applied in thick layers. Today, there are even fast-setting levelling compounds which contain accelerated cements. These ensure that the readiness for flooring installation is achieved very fast (after approx. 1 - 3 hours). Depending on their formulation, these compounds can cover special areas of application. For example, they are fiber-reinforced for use on wooden subfloors or come with a high polymer content to withstand heavy loads (pallet truck or forklift traffic).

Thomsit’s range of cementitious levelling compounds includes e.g. Thomsit DX, XXL Power, FA 97, SL 85 and XXL Xpress.

Gypsum-based levelling compounds

Gypsum-based levelling compounds are primarily intended for use on gypsum substrates (calcium sulfate screeds, anhydrite screeds). Thanks to their very low shrinkage, they are suitable for almost all existing subfloors and dry screeds. Gypsum compounds also come with fiber additives for use on wooden subfloors. However, when applied in thick layers (> 3 mm), they tend to dry more slowly than other levelling compounds (please refer to the respective Technical Data Sheet).

Thomsit’s range of gypsum-based levelling compounds includes e.g. Thomsit AS 1 and AS 2.

Dispersion levelling compounds

Dispersion-based levelling compounds are ready-to-use polymer/filler mixtures that are preferably used as a migration barrier on old floor coverings (e.g. as a protective layer against plasticizer migration from old PVC flooring). They can only be applied in very thin layers up to 1 mm thickness in a single operation. They are generally intended to level out the texture of the old floor covering underneath. Dispersion fillers produce non-absorbent substrates.

Thomsit’s range of dispersion fillers includes for instance Thomsit FF 69 Flex Finish.

Reaction resin levelling compounds

2-component polyurethane levelling compounds are special products which, due to their elastic consistency, are preferably used on resilient subfloors such as damping underlays or sports floors. In addition, these water- and tension-free compounds can be used for coating metal floors and certain types of asphalt surfaces. Layers produced with reaction resin compounds are not absorbent.

Thomsit’s range of reaction resin products includes for instance Thomsit S 810.



Attention: Not all substrates are suitable for thick coats of levelling compound. In the case of mastic asphalts, for example, a layer thickness of 5 mm should not be exceeded; otherwise crack formation in the screed can be expected. In such cases, or if particularly thick levelling layers are to be applied, always seek technical advice first.

Tips for the application of levelling compounds


No matter which compound you choose: its application is subject to certain conditions. An important factor of influence is the room climate. About half the gauging water used for preparing a levelling compound needs to evaporate; the rest is chemically bound by the binder, e.g. cement. For this reason, two measures must be taken on site. Firstly, ensure that excess water can escape from the rooms via windows, doors and air circulation. Secondly, provide appropriate ambient conditions like air humidity (ideally 50-60 % RH) and room temperature (ideally 20 °C).


Significant deviations from these conditions will negatively affect the strength of the levelling compounds, resulting in unstable areas or in an accumulation of polymers on the surface (color changes, streaks).


The materials should also be stored under optimal climatic conditions. In summer, make sure that the installation products, in particular the levelling compounds, are not exposed to the blazing sun or stored in a closed vehicle.


Excessive heat will significantly impair the flow properties of the levelling compound so that a smooth, level surface can no longer be produced. The same effect occurs when the levelling compound is too cold. The optimum storage temperature is 15 to 25 °C.

Would you have known?

A levelling compound layer of 2 mm thickness contains approx. 0.8 liters of water per m². If the room size is 20 m² and if about half the water needs to evaporate, this means that as many as 8 liters have to be removed! This clearly illustrates the need for good air circulation and ventilation when working with levelling compounds.

Low-dust levelling compounds

As past studies conducted by the BG Bau (Employers' Liability Insurance Association of the Construction Industry) have shown, relatively high dust concentrations can occur temporarily when mixing dry mortars, tile adhesives or fillers indoors. Once stirred up, the finest particles will remain in the room air for hours and affect the respiratory tract.

In any case, occupational safety and health legislation clearly postulates the so-called STOP principle. In this acronym, "S" stands for substitution, "T" for technical measures, "O" for organizational precautions and "P" for personal protective equipment. The order of priority has been clearly defined by the legislator: Can a harmful product be replaced by a less harmful one, i.e. be substituted? This is undoubtedly the case in the category of levelling compounds.

Once again, Thomsit is setting new standards in terms of occupational safety and sustainability. Take for example our low-dust levelling compound Thomsit XXL Power: it reduces the amount of dust released by up to 90 percent! This applies to the entire mixing process – from pouring out the content of the bag to mixing or pumping the compound. In this way, we protect the user’s health and at the same time provide him with a convincing argument for the contract negotiations.


When carrying out refurbishment projects, dust and dirt count among the main concerns of building clients – in particular when doing renovation work in private homes.


Further information can be found under the keyword "Staubarm" (low dust) on the website (BG Bau = Employers' Liability Insurance Association of the Construction Industry).

Mixing the levelling compound


Mineral levelling compounds are mixed with clean, cold water at the predefined mixing ratio, using a suitable stirring device for approx. 2 to 3 minutes. When deviating too much from the amount of gauging water specified by the manufacturer, this will inevitably lead to complaints (poor flowability, significant loss in strength, negative effects on the surface quality).


When applying layers of more than 10 mm thickness, it may be useful to admix up to 30 % fire-dried quartz sand (grain size 0-2 mm) to levelling compounds such as Thomsit XXL Power or Thomsit SL 85.


The admixture of dry sand does not require any additional gauging water. Excessive amounts of water always result in reduced strength and longer drying times. Self-levelling compounds react to water overdose by the heavier components settling at the bottom and the lighter ones floating to the surface. This results in the formation of an unstable surface edge zone and a weakening of the structural strength.

Machine application


Levelling compounds can be applied ergonomically and quickly using mortar pumps such as duo-mix 2000 by m-tec mathis technik gmbh ( Due to the efficiency of mortar pumps, it makes sense to use them from a surface area of about 500 m² and a layer thickness of 3 mm. For smaller areas (200-500 m²), pump application only makes sense if layers well over 10-15 mm thickness need to be installed. Further information on the pump application of levelling compounds can be obtained from our technical advisors.


Applying the levelling compound


After mixing, the required amount of levelling compound is applied on the substrate and spread with a suitable tool. This can be a smoothing trowel or pointed finishing trowel. Even more preferable are squeegees or notched screed rakes whose notch size already determines the layer thickness. With the help of such squeegees attached to telescopic handles, the compound can be spread ergonomically in a standing position. Basically, all levelling compounds require a certain layer thickness before the self-levelling effect is achieved. To produce sufficiently smooth floors, it is therefore necessary to apply layers of at least 2 mm, preferably 3 mm.

Particularly in large rooms, the time needed for mixing and applying the next batch should be as short as possible in order to avoid unsightly, visible transitions between the areas.

Trowel application

Squeegee application

Spiked roller

No matter if the product was mixed manually or by pump, applied by squeegee or trowel: it is always advisable to finish fresh surfaces with a spiked roller in order to produce particularly smooth, even surfaces. In addition, any transitions between the individual batches can be adjusted so that later no marks show through the floor covering.

If it is necessary to apply a further levelling coat, wait until the first layer has dried. A perfect bond between the layers is ensured by applying an intermediate priming coat with Thomsit R 766 Multi-Purpose Primer.

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