Start With the Subfloor, Not the Mat

Underfloor heating is wonderfully simple in principle: a thin element sits beneath the tile, warms up, and turns a cold kitchen or bathroom floor into the most pleasant spot in the house. What catches people out is that everything above and below that element has to be right, or the system will be slow, patchy, or expensive to run.

Before you buy anything, check what you are fixing to. A solid concrete or screed base needs to be sound, dry and free of dust and loose material. Timber floors are workable, but only if the boards are rigid — any flex will eventually crack tiles and can damage the heating element. If in doubt, overboard timber with cement-faced tile backer boards screwed down at 200–300mm centres, and remember that a suspended timber floor needs ventilation kept clear underneath.

It also pays to think about heat loss. Electric mats are usually sized at around 150 W/m², which copes well with a well-insulated room. A draughty Victorian hallway or a conservatory with single glazing will need more, or it will simply never reach a comfortable temperature.

Insulation Boards: The Step Worth Paying For

Insulation boards sit between the subfloor and the heating element, and they are the single best value upgrade in the whole job. Without them, a large share of the heat goes downwards into the slab rather than up into the room.

• Choose the right thickness. 6mm cement-faced boards are the usual choice where floor height is tight; 10mm or 20mm perform better if you can afford the extra build-up.
• Check the ceiling height. Add up boards, adhesive, element, levelling compound and tile — it is easy to lose 25–30mm, which matters at a doorway and under a door swing.
• Stagger the joints. Lay boards in a brick-bond pattern, butt them tight, and tape the joints with the manufacturer's scrim tape so no levelling compound leaks through.
• Prime as instructed. Most board faces need a primer before adhesive or levelling compound, and skipping it is a common cause of debonding.

Fit a flexible foam perimeter strip around the edge of the room before tiling. It gives the floor somewhere to move and is cheap insurance against tenting tiles later.

Heating Mats Versus Loose Cable

Self-adhesive mats are the practical choice for regular rectangular rooms: they roll out quickly, the cable spacing is fixed, and you can cut the mesh (never the cable) to turn corners. Loose cable suits awkward shapes, small bathrooms and rooms with lots of obstacles, but it takes far longer to lay and you must keep the spacing even. Water-based systems are brilliant for whole-house projects, but for a single tiled room electric is usually simpler, cheaper and faster to install.

Two rules matter more than anything else:

• Never run heating under fixed furniture. Leave a clear margin of at least 50–100mm from kitchen units, baths, shower trays, WCs and fitted wardrobes. Trapped heat has nowhere to go and can damage both floor and furniture.
• Cover the walkable area, not every last centimetre. Aim for roughly 80–90% coverage of the open floor, keeping runs clear of the toilet base and door thresholds where the cable could be pinched.

Photograph the layout before you cover anything. Years from now, when someone wants to drill a hole for a towel rail, those photos will save the day.

Thermostat and Floor Probe Placement

The thermostat needs a permanent live feed, so plan the wiring before the walls are finished. Fit it on an internal wall, away from direct sunlight, radiators, ovens and draughts, at around 1.2–1.5m from the floor.

The floor probe is the part people get wrong. It should sit in a conduit — a short length of plastic tube — set into the floor between two runs of cable, roughly a third of the way into the room from a wall. It must never touch the heating cable itself, or the thermostat will read the element temperature rather than the floor, and it must be replaceable without lifting tiles. Many tilers fit a second probe alongside as a spare. Programmable and smart thermostats are worth the extra cost, since scheduled heating and floor temperature limiting both cut running costs noticeably.

Tiling Over the Warm Floor

Once mats are down and tested, cover them with a self-levelling compound to a minimum of 3mm over the highest point of the cable. Let it cure fully — usually at least 24 hours, and longer in cool, humid conditions — before tiling.

Use a flexible, cement-based adhesive, ideally S1 or S2 rated, mixed precisely and applied with a suitable notched trowel. Press tiles down with a slight twist and check for full bed coverage; hollow spots above a heating mat are a recipe for cracked tiles. Leave an expansion joint at the perimeter and at doorways, and use flexible grout. For larger areas, add movement joints at the intervals your adhesive manufacturer recommends, typically every 20–25m² internally.

Testing, Commissioning and the First Heat-Up

Test the mat's resistance and insulation before laying, after laying, and again after tiling. Record the readings — if a cable is damaged later, a change in those numbers tells you immediately.

Then be patient. Wait at least seven days after tiling before switching the heating on, and start low: many installers begin at around 20°C and raise the floor temperature by no more than 5°C per day. This gentle warm-up lets the adhesive and grout cure without thermal shock. After that, run the system on a schedule rather than constantly, and set the floor limit to suit your tile — porcelain and ceramic handle warmth beautifully, but there is no need to run them hot.