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This floor should have been screeded to encase the pipes and provide a flat floor prior to tiling using a flexible self-levelling compound (or other) suitable for underfloor heating.

The use of a tile clip-system for installing the tiles is highly unsuitable as it has created hollow voids underneath the bowed tiles where the adhesive will have little or no contact with the floor.

The result is a hollow void of no support to the tiles due to little adhesive contact, nor heat transfer from the pipes to the tiles.