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Tiling Onto Screeds With Wet Underfloor Heating

30 November

Whether you're fixing natural stone limestone tiles, or architectural concrete-effect porcelain tiles such as our Cementone Grey and wet under floor heating has been installed, you need to assess whether your substrate is a calcium sulphate (anhydrite) screed, or a traditional sand and cement screed, and prepare it suitably 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.

Telling The Different Screeds Apart

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' screeds, 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 is pumped and can cover larger areas without the need for stress joints. Be sure to check the depth of the screed. This is 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 still essential that you confirm the screed type with the customer before starting to tile.

Why Determine The Type Of Screed?

Why do you need to determine the type of screed before tiling over wet underfloor heating? Simply put, if floor tiles are fixed with a cement-based adhesive and applied directly onto a anhydrite screed, cement in the tile adhesive will react with the gypsum in the screed, forming a mineral called ettringite where the two meet. The interface change is enough to cause a complete debond of the cementitious adhesive from the 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.

Other Considerations Before Tiling Over Underfloor Heating

You need to find out whether the underfloor heating has been commissioned and brought up to full working temperature before you start work. It’s vital that you can 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:

  1. Is the screed dry: If the screed is new, the general rule is to allow a drying time of one day for every millimetre of thickness. For new and old screeds alike, it's prudent to perform a moisture test
  2. 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
  3. Is the screed surface clean and 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
  4. Is the screed flat, level and smooth: An even and consistent surface will make fixing floor tiles easier and quicker
  5. What primer you should 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.

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