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How to prepare floors for tiles and slabs

The floor must fulfil the following conditions: it must be sufficiently load-bearing, dry, even and free of dust and grease.

Crucial to know

acceptable unevenness of floor surface is only 2 mm per 1 m

Tools needed

moisture measuring set

fast setting mortar

a primer

self levelling compound

Step 1

Moisture in the Subfloor

Measuring moisture in the subfloor

Measuring the moisture is in the substrate is a must, because it influences the following decisions:

  • Which type of primer is needed?
  • What leveling compound should be applied?
  • Which type of tile or slab can be installed?

There are two methods for on-site moisture measurement:

  1. CM method (calcium carbide method). The moisture content of screeds is measured with the help of a CM moisture meter. 
  2. CRH method (corresponding relative humidity). Moisture is measured with a humidity meter and an electronic data logger.

Step 2

Flat and even Floor Substrate

Grinding, repairs and filling up voids

First of all you need flat and even surface. Grinding is necessary when the floor is uneven, when a sintered layer or coating must be removed, or for producing a more absorbent (open-pored) floor. 

Then you need to repair cracks and holes if there are any in the subfloor and fill up voids around shower and bathtub drainages. You can use one of fine filling compounds CD 25 or CD 26, or fast setting mortar CX 5 or CX 20, possibly rapid hardening mortar CN 83 – depending on a kind of damages.

Step 3

Areas where a slope is needed

Creating slope

If you are going to tile a balcony, a terrace or a shower in a bathroom, you probably have to make a slope. It is the right moment to do it. For creating slope use rapid-hardening screed CN 83 (layer thickness 5 to 30 mm).

In the case of linear shower drainage, the slope is on one or two sides, depending on the location of the shower channel, and should be at least 2.5% towards the drain. Point drainage requires a slope from four sides – to each edge of the grid (so-called envelope drain), which should be at least 2%. Please also pay attention to the recommendations of the drainage manufacturer regarding the type of slope.

Architects often call for a 1% slope on outdoor balconies and terraces, but this minimal slope does not provide sufficient drainage. Generally, positive drainage requires a slope of at least 2%.

Step 4

Check substrate absorbency to apply the right Primer

Check the floor absorbency and apply the right primer

Priming is applied for surface strengthening and/or for regulation of absorption level. It is used also as a binding mean to join individual floor and wall coverings. To apply the right primer, you need to know how absorbent is your floor. Depending on a kind of substrate and special purposes, choose a proper primer:

  • For absorbent screeds and concrete floors use dispersion water-based primer R 777 or the universal one – CT 17.
  • For absorbent, impervious, dusting and unstable surfaces use PU primer R 740.
  • Against rising moisture and for surfaces exposed to mechanical and chemical stresses use epoxy primer R 755.
  • For universal purposes use a multi-purpose primer R 766 or a universal primer CT 19.
  • For difficult substrates (PVC, tile-on-tile, chipboard etc.) use special primer CT 19 or CN 94.

Step 5

Apply self levelling Compound

Levelling uneven substrate

Now you can check the floor evenness. If there are depressions or level differences exceeding 2 mm per 1 m, you have to apply self levelling compound. Use CN 68, CN 69 Super Nivel, or CN 72 Nivel Expert depending on your needs. More thin layer self levelling compounds by Ceresit you can find here.

Tips & Tricks

For professionals

Fixing cracks & assembling construction elements.

For quick repair of cracks and other damages as well as for fixing construction elements and pipes, both indoor and outdoor, we recommend fast setting mortars CX 5 and CX 20.

CX 5 Express: with setting time 5 min.
CX 20 Comfort: with setting time 20 min.

products used

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    Fine-grained one component filler to smooth out concrete and reinforced concrete, to fill cracks/voids and repair damaged substrates. The range of application is from 5 to 30 mm. 
  • ${/content/heliux/adhesive/ceresit/com/en/products/mortars-and-auxiliaries/central-pdp#/ceresit-cd-26/SAP_0201VLC013L8/searchResult.productFields.MainImageAltText}
    Coarse-grained one component filler to smooth out concrete and reinforced concrete and to fill cracks/voids and repair damaged substrate. 
  • ${/content/heliux/adhesive/ceresit/com/en/products/flooring/central-pdp#/ceresit-cn-68/SAP_0201TAC013Y9/searchResult.productFields.MainImageAltText}
    Self-smoothing levelling compound for levelling calcium sulphate, fast drying screeds and cement surfaces, in layers of 2 to 20 mm. 
  • ${/content/heliux/adhesive/ceresit/com/en/products/flooring/central-pdp#/ceresit-cn-69/SAP_0201TAC013Z0/searchResult.productFields.MainImageAltText}
    Self-levelling compound for medium traffic loads in 2 to 10 mm layer thickness for indoor application 
  • ${/content/heliux/adhesive/ceresit/com/en/products/flooring/central-pdp#/ceresit-cn-83/SAP_0201TBC014A0/searchResult.productFields.MainImageAltText}
    Rapid-hardening cement mortar for repairs of cementitious substrates with layer thicknesses of 5 to 30 mm. 
  • ${/content/heliux/adhesive/ceresit/com/en/products/flooring/central-pdp#/ceresit-cn-94/SAP_0201TNC014A4/searchResult.productFields.MainImageAltText}
    Special primer for reliable bonding of floor levelling compounds, ceramic and natural stones on critical surfaces. 
  • ${/content/heliux/adhesive/ceresit/com/en/products/interoir-finishing/central-pdp#/ceresit-ct-17/SAP_0201JAC014F1/searchResult.productFields.MainImageAltText}
    Primer for surface strengthening of all absorbent substrates for indoor and outdoor application, prior to fixing ceramic tiles, pouring floors or fixing thermal insulation boards. 
  • ${/content/heliux/adhesive/ceresit/com/en/products/tiling/central-pdp#/ceresit-ct-19/SAP_0201SOC014F8/searchResult.productFields.MainImageAltText}
    Fast-acting special primer for a secure bond between ceramic coverings, natural stones, wall & floor screeding compounds and on difficult substrates. 
  • ${/content/heliux/adhesive/ceresit/com/en/products/mortars-and-auxiliaries/central-pdp#/ceresit-cx-20/SAP_0201VBC014O3/searchResult.productFields.MainImageAltText}
    Assembly and repair mortar. Mortar for precise assembly of elements and filling up cavities, characterized by a comfortable setting time. 
  • ${/content/heliux/adhesive/ceresit/com/en/products/mortars-and-auxiliaries/central-pdp#/ceresit-cx-5/SAP_0201VBC014O4/searchResult.productFields.MainImageAltText}
    Universal fast-setting mortar for quick, easy and strong fixing and assembling with perfect consistency for horizontal and vertical substrates (indoor & outdoor applications). 
  • ${/content/heliux/adhesive/ceresit/com/en/products/flooring/central-pdp#/ceresit-r-740/SAP_0201TNC015L9/searchResult.productFields.MainImageAltText}
    CERESIT R 740
    One-component PUR fast barrier primer for absorbent and impervious substrates, suitable for highly absorbent substrates. 
  • ${/content/heliux/adhesive/ceresit/com/en/products/flooring/central-pdp#/ceresit-r-755/SAP_0201TNC015M0/searchResult.productFields.MainImageAltText}
    CERESIT R 755
    Reliable moisture barrier effect: up to 6 CM% on cement screeds, up to 7 CM% on concrete floors. Also suitable for producing an epoxy resin screed. 

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