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Natural Stone & Porcelain Tile Help & Advice

How To Prepare Your Floor For Tiling

You may have natural limestone tiles, or architectural porcelain tiles such as our Cement Chiaro range, but no matter which tiles you have, you need to prepare the floor before fixing them. This article will tell you how to do just that.

Before you even think about fixing the tiles you need to assess whether the floor is suitable to tile onto and what remedial action is required to ensure your project is a success. Failure to assess and prepare your substrate correctly may result in the floor tiles cracking, coming loose, or the tile adhesive de-bonding from the substrate.

The surface you're faced with can present a number of situations: an existing tiled floor which is uneven due to 'lippage', or an untiled floor with a new screed, or floorboards where tiles are required as the new flooring.

Tiling Onto Existing Tiles

'Lippage' is the industry term for floor tiles which, either due to bad workmanship or naturally occurring differences in the size of the tiles, have some edges and corners higher or lower than others, creating an uneven surface. There are three options to resolve this:

There is no technical reason why, assuming the existing tiles and substrate are sound and solid, that new tiles can't be fixed on top of existing tiles. One situation where you may wish to tile over other tiles is in an older property where the base is flagstones or natural limestone tiles where lifting the tiles may damage the substrate below and add additional time and costs to the project.

Ensure the tiles are firmly bonded to the original substrate, that any loose tiles are made good, the surface is free from dust, dirt and contaminants and that any sealer treatments have been removed. If you're happy the tiles are even, flat, and smooth, then simply apply a coat of bonding agent such as Kerakolll Primer A which will help the floor tile adhesive form a strong bond with the new floor tile.

If the existing floor tiles are sound but are uneven, have wide grout joints, or have tumbled edges, such as Dijon tumbled limestone tiles, an option is to apply a self-levelling compound to give a flat and smooth surface to tile onto.

Types Of Floor Screeds

Calcium sulphate (anhydrite) screeds tend to be lighter in colour and smoother in texture than traditional sand and cement screed. Anhydrite screeds have become quite common as they offer benefits over sand/ cement screeds: they are relatively easy to lay, low cost, fast-drying, pumpable, self-levelling and offer minimal shrinkage.

Sand and cement screeds, often known as a 'traditional' screed, tend to be darker in colour and rougher in texture than anhydrite screeds. The traditional sand and cement floor screed is usually a cementitious material made from a 1:3 or 1:4.5 ratio of cement to sharp sand.

As well as differences in the shade and texture, anhydrite screeds tend to not require joints as the screed is pumped and can cover larger area without the need for stress joints. Be sure to check the depth of the screed, normally accessible at a doorway. If the depth is greater than 70mm it is likely to be a sand and cement screed. It's essential that you confirm the floor screed type with the customer before starting to tile, even if you have already performed these manual tests.

Why The Floor Screed Type Matters

If floor tiles are fixed with a cement-based tile adhesive and applied directly onto a anhydrite screed, cement in the tile adhesive will react with the gypsum in the screed resulting in a mineral called ettringite being formed where the two meet. This change is sufficient to cause a complete de-bonding of the cementitious adhesive from the anhydrite screed. As such, a specialist gypsum based floor tile adhesive suitable for anhydrite screeds must be used. Alternatively, a suitable bonding agent such Kerakoll Primer A can be used to form a barrier between the substrate and the adhesive.

The screed is generally defined as a self-smoothing 'levelling' screed, used solely to give a flat even surface to fix floor tiles onto. A 'levelling' screed doesn't contribute to the structural performance of the floor and tends to be thinner in depth than a thicker 'wearing' screed which adds strength and stability to the floor.

Tiling Over Underfloor Heating

It’s essential that you know whether any underfloor heating been commissioned and brought up to full working temperature. You need to be confident this has happened as if not the screed may fail, resulting in tiles coming loose. Key considerations before fixing floor tiles to screeds:

  • Is the screed dry? If the screed is new, the general rule is allowing a drying time of 24 hours for every 1mm thickness. For new and old screeds alike it's prudent to perform a moisture test.
  • Is the screed damaged? Damaged screed should be replaced. Cracked screed should be filled, or a suitable anti-fracture matting such as Kerakoll Idrobuild Tex should be fitted.
  • Is the screed surface free from contaminants? Dust and dirt should be removed from old screeds. New screeds may have laitance, a surface layer of fine particles displaced as water evaporates.
  • Is the screed flat, level and smooth? An even and consistent surface will make fixing floor tiles easier and quicker.
  • What primer should you use? The use of a bonding agent such as Kerakoll Primer A is key to ensuring adhesive can perform to its optimum. We don't recommend using PVR as a primer.

When fixing floor tiles onto a screed with wet under-floor heating flexible adhesive and grout should be used as they will absorb the minute changes in movement which occurs as the underfloor heating heats and cools.

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