Whether you're fixing Dijon limestone tiles or architectural porcelain tiles such Cementi Chiaro and the floor is uneven, you need to assess if the floor is suitable to tile onto and what action is required to ensure your tiling 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 any of three scenarios: an existing tiled floor which is uneven due to 'lippage', an un-tiled floor with a new screed or floorboards where tiles are required as the new floor.
Tiling Onto Existing Tiles
'Lippage' is the industry term for floor tiles which, either due to bad workmanship, or naturally occurring differences in the calibration of tiles, have some edges and corners higher or lower than others, creating an uneven surface on which to tile. You have three options to resolve the situation: remove the tiles and lay a new substrate, apply a self-levelling compound, or angle grind away the uneven areas until the surface is more even.
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 tiles is in an older property where the base is flagstones or natural limestone tiles. In this situation lifting the tiles may damage the substrate below and add additional time and costs to project.
Ensure the tiles are firmly bonded to the original substrate, 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 Kerakoll 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.
Choosing Your Screed
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 screed, 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 it will be pumped and can cover larger area with the need for stress joints. In addition, check out 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. Nonetheless, having conducted these manual assessments, it's essential you confirm the screed type with the customer before starting to tile.
Why do you need to determine the type of screed? If floor tiles are fixed with a cement-based adhesive and applied directly onto an anhydrite screed, cement in the tile adhesive reacts with the gypsum in the screed resulting in a mineral called ettringite being formed where the two meet. The interface change is sufficient to cause a complete de-bond of the cementitious adhesive away from the anhydrite screed. Therefore, a specialist gypsum-based floor tile adhesive suitable for anhydrite screeds must be used. Alternatively, a suitable bonding agent such Kerakol 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.
Another question you need to ask the customer or the builder is whether the under-floor heating has 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.
Before fixing floor tiles to screeds, there are some things that you need to take into account:
- Is the screed dry? If the screed is new the general rule is allow a drying time of 1 day 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 Kerakol 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 Kerakol 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.