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Tiling over electric under floor heating

07 August

Whether you're fixing natural stone Cappuchino marble tiles, or classic polished porcelain tiles such as Marmo Venatino and electric under floor heating has been installed, you need to assess your substrate and consider a number of other factors before you begin fixing floor tiles.

Check Your Substrates

The substrate could be calcium sulphate (anhydrite) or a traditional sand and cement screed, a concrete floor, a tilebacker/ insulation board, or plywood. 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.

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 is 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 a anhydrite screed, cement in the tile adhesive reacts with the gypsum in the screed resulting in a mineral called ettringite being formed at the where the two meet. The interface change is sufficient to cause a complete debond 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.

Plywood should be a minimum of 6mm thick and screwed at 100mm intervals to prevent 'bounce', and sealed with a SBR bonding agent such as Kerakoll Primer A.

Tile backer boards are the perfect option to fix underfloor heating to. Manufactured using high density extruded polystyrene which provides excellent insulating properties making electric under floor heating efficient and effective. Tilebacker/insulation boards should be fitted using fixing washers if laying onto wooden substrate, or tile adhesive if fixing to screeds or concrete.

Finishing Up

Another question you need to ask the customer or the builder is whether the underfloor heating been commissioned and tested by a qualified electrician. You need to be confident that this has happened as, if not, the screed may fail resulting in tiles coming loose. It's prudent to test the system again during and after installation to ensure no damage has occurred.

When fixing floor tiles over electric 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. Spreading of adhesive should be done with a plastic rather than metal trowel, to avoid nicking the cable. Adhesive should be spread evenly to avoid air pockets which will cause hot spots and result in failure of the cable.

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