Underfloor heating under tiles: what to fit, and what goes wrong

Electric mats or a wet system

Electric mats or loose cable are the usual choice in a bathroom, kitchen or single room retrofit. They add only 3-5mm of height, go down in a morning, and run off a fused spur with a floor probe thermostat.

Wet (hydronic) underfloor heating pays back better across a whole ground floor or an extension, but it needs a screed or a routed panel system and much more height. Retrofitting it into one small room rarely makes sense.

Either way, tiles are the right floor finish: porcelain conducts heat well and is unaffected by the thermal cycling that makes timber and vinyl move.

Insulation boards are not optional

Without an insulation board under the mat, roughly half your heat goes downwards into the slab. A 6-10mm XPS tile backer board under the heating cuts warm-up from around an hour to ten or fifteen minutes and lowers running costs for the life of the floor.

The board also gives the mat a flat, stable bed. On a timber floor it does a second job: it stiffens the deck and reduces the deflection that cracks tiles.

The build-up we use

On a concrete floor: primer, insulation board bonded in flexible adhesive, heating mat taped down, floor probe in a conduit between two cable runs, then a self-levelling compound suitable for underfloor heating over the cable — or the tiles bedded directly in a solid bed of S1 flexible adhesive.

On a timber floor: 15-18mm WBP ply or backer board first, then insulation board, then the mat, then a decoupling membrane before tiling. Heated timber floors move twice — structurally and thermally — and the decoupling layer is what stops that movement reaching the tile.

Never trap a cable in a void. Any air gap around the cable is a hot spot, and hot spots are what burn out mats.

Adhesive, grout and movement

Use an S1 or S2 flexible cement-based adhesive rated for underfloor heating, and a flexible grout. Ready-mixed tub adhesive should never go over heating — it cures by drying and will not set properly under the tile.

Perimeter movement joints at every wall, threshold and upstand, filled with flexible silicone rather than grout. On floors over about 40m², or where the floor runs through a doorway into another heated zone, intermediate movement joints go in as well.

Testing and commissioning — the step that gets skipped

The mat's resistance is tested three times: on delivery, after it is laid, and after tiling. A record of those readings is what an installer warranty depends on, and it is the only way to prove a cable was intact before the floor went down.

After tiling, leave the heating completely off for at least seven days while the adhesive and grout cure. Then bring it up gradually — around 5°C a day over a week — until it reaches its working temperature.

Firing a new heated floor straight up to 27°C is the single most common cause of hollow tiles and cracked grout lines we get called out to. It cannot be undone afterwards.

Running temperature and thermostats

Cap the floor surface temperature at about 27°C for occupied areas — higher is uncomfortable underfoot and hard on the adhesive. A programmable thermostat with a floor probe is worth the extra over an air-sensing one, because it protects the floor as well as the room.

Underfloor heating is slow to respond by design. Set it to hold a low background temperature rather than switching it on and off like a radiator.

Fitting it with your tiles

We install electric underfloor heating as part of bathroom, kitchen and wet room tiling across London, Essex and Cambridge, and we tile over wet systems installed by your heating engineer. Send the room dimensions and a photo of the existing floor and we will price the heating and the tiling together.