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FAQ - subfloor preparation

  • What is a screed?

    A load-bearing / working level made of one or more layers of screed cement.

  • What is the purpose and function of a screed?

    Screeds are used to level out existing subfloors and bring them to the required level. Furthermore, it has to fulfil the function of heat and sound insulation guidelines and to hold a floor covering.

  • What are the most common screed types on the market?

    • cement screed

    • anhydrite screed

    • poured asphalt screeds

  • What are the characteristics of the different screed types?

    Cement screed: The cement screed is usually recognisable by its cement-grey colour. Since the cement screed shrinks in the drying phase, the screed layer is required to install a false joint in door openings, columns and in any area over 6m x 6m field area. The dummy joints are usually closed again as soon as the screed is ready for laying!

    Anhydrite screed: Anhydrite screeds are recognisable by their light, creamy beige colour. The anhydrite screed can also be installed quickly and without joints in larger surfaces. The surface of the screed must be sanded and vacuumed before further treatment unless otherwise specified by the manufacturer.

    Poured asphalt: Poured asphalt can be recognised by its tar-black colour and its typical asphalt smell. It can be treated immediately after cooling. As a substrate for floor coverings, the surface must be rubbed with quartz sand in order to ensure a later bond between the screed and the floor. 

  • What requirements are placed on the substrate before a floor covering is laid?

    The subfloor must be solid, clean, permanently dry, stable, free from cracks, even and resistant to tension and compression!

  • What testing methods are available for surface strength?

    Grid scoring test, wire brush test and hammer impact test

  • Which test method is used to determine the level of moisture balance?

    An electronic measuring device can be used for orienting testing of the remaining moisture (e.g. CM measurement method, KRL method).

  • What is the permitted moisture balance of a newly installed screed?

    • Cement screed <2.0 CM-%

    • Cement heating screed <1.8 CM-%

    • Anhydrite screed <0.5CM-%

    • Anhydrite heating screed <0.3CM-%

  • Do I have to level out a subfloor if I install an elastic floor covering for gluing?

    If an elastic floor covering for gluing is installed, the substrate must be smoothed, vacuumed, primed and levelled. Levelling is necessary to achieve an even and absorbent substrate. The levelling material can also be used for leveling and filling holes.

  • How do I create a professional and ready-to-install subfloor for laying an elastic floor covering for gluing on a newly installed screed?

    In general, a cleanliness sanding is always recommended in order to remove possible dirt such as paint residues, plaster, etc. The surface must then be cleaned of dust and abrasive layers with a powerful industrial vacuum cleaner. Now apply a primer (mixing ratio according to manufacturer's instructions) to the substrate, ensuring that it is applied evenly and without puddles. With a new well sanded and dust-free mastic asphalt, a primer can be dispensed with. After the primer has completely dried, the filler work can now begin. First, level out all larger unevennesses and incorrect heights to adjacent rooms with a stable filling compound. If you are not working in the wet method, please apply primer again to the leveled surfaces. The substrate can then be treated with a smoothing and levelling compound with a minimum thickness of 2 mm. In order to achieve a high level surface it is recommended to use the squeegee technique.

  • How do I treat an existing substrate?

    In the case of old subfloors, thorough substrate testing and following preparation for a permanently damage-free floor covering installation is very important. After you have removed the old floor coverings including their base layer, existing adhesive layers must be removed by sanding/milling. Afterwards, the dust and grinding films must be removed from the substrate with a powerful industrial vacuum cleaner. The next step is to apply a self-adhesive edge strip to maintain the expansion of the subfloor and to ensure that the levelling compound does not create any sound bridges (This is not required in all countries. Please check the country-specific regulations.). The substrate can then be primed (mixing proportion according to manufacturer's instructions), ensuring that it is applied evenly and without puddles. After the primer has completely dried, it is usually possible to start filling. First, level out all larger unevenness’s and incorrect heights to the adjoining rooms with a stable filling compound. If you are not working in the wet method, please apply primer again to the leveled surfaces. The substrate can then be treated with a smoothing and levelling compound with a minimum thickness of 2 mm. In order to achieve a high-level surface, it is recommended to use the scraper technique. The filling process therefore virtually eliminates any interaction !

  • What causes cracks in the subsurface and how should I treat them?

    The cracks (even small micro cracks) can be caused by various reasons, e.g. uneven thickness of the screed, drying too quickly, incorrect composition of the screed mortar during installation and
    incorrect installation of the underlay. By appropriate work the cracks can be closed by dowelling/epoxy stitching with a 2-component reaction resin. With an angle grinder, slots are made in the substrate approx. every 15-20cm, with a depth of 3/4 of the screed! Then these are vacuumed out and a screed clip is inserted. In the next step, the joints including the screed clip are closed with the 2-component reaction resin and sprinkled with quartz sand. The quartz sand that has not been bonded must then be vacuumed off. Now the filling work can begin!

  • Which subfloors are suitable for laying vinyl floors & design floor coverings?

    The requirements for professional installation are a permanently firm, dry, level, leveled substrate. Functional floors such as terrazzo, stone or wooden subfloors can only be regarded as suitable subfloors after appropriate pre-treatment.
    If using dispersion adhesives of sufficient thickness of at least 2 mm, dense non-absorbent substrates such as poured asphalt must be levelled. When using the various primers, levelling compounds and fillers, the installation guidelines of the manufacturers are absolute and must be followed.

  • What is a movement joint?

    Movement joints are all types of joints that allow movement between building parts, surface parts or individual structural parts and reduce harmful tensions. Therefore, movement joints must not be closed and must be brought though to the finished floor surface by means of suitable profiles or by closing with elastic sealants with backfilling.

  • What are fake joints?

    Because cement and magnesium screeds shrink in their setting phase during the drying phase due to the chemical processes and do not tear and break off uncontrolled at the weakest point, so-called predetermined breaking points are installed! Fake joints are made by cutting into the fresh screed to one third of its thickness with a trowel.These dummy or predetermined breaking joints must be closed force-locked before laying.