Thanks for your question Pete! This is a common problem for many tradesmen, especially when tiling bathrooms or wet rooms, but luckily it is quite easy and simple to solve – with the right products of course!
For going over an electric underfloor heating mat system we would recommend using our LX-360 Fibre Leveller. This is a great product for this application because it can be used from 3mm up to 60mm, and because it is flexible and fibre reinforced it can cope with movement of the background and thermal changes that occur with underfloor heating systems.
Before laying the electric underfloor heating mat, it is crucial to ensure that the background you are going onto is dry (less than 75% relative humidity), sound, clean and free from dust, grease of other contaminants. Depending on what background surface you may need to prime the surface with Dunlop SBR Bonding Agent before laying the mat.
For example plywood overlays should be primed with two coats of SBR diluted with a ratio of 1:2 with water. Cement:sand screeds and concrete surfaces should be primed with a diluted primer such 1:4 with water, while Calcium Sulphate and Anhydrite screeds must be primed with a suitable primer such as in two coats. Dilute the first coat (1 part bonding agent to 4 parts water). Allow to dry before applying the second coat (diluted 1 part bonding agent to 2 parts water).
Remember that calcium sulphate and anhydrite screeds need to be fully dry before tiling (that is less than 75% relative humidity) and laitance is removed with a scarifier.
Once the surface is fully prepared only then can you lay the electric underfloor heating mat as per the manufacturer’s instructions.
You can then go about levelling the floor as needed with Dunlop LX-360 Fibre Leveller. Mix as per the pack instructions and pour to the required depth. Make sure you take care not to damage the cables, and because of the fibres present in the leveller, there’s also no need to use a spike roller. Just float out the mixture and it will find its own level.
I hope that helps!