How to install electric underfloor heating

Inspired by a detailed UK self-build guide. This article draws on the "HOW TO INSTALL ELECTRIC UNDERFLOOR HEATING" video from the Build with A&E channel, a popular UK self-build series. Their close-up footage of the mat routing and thermostat wiring makes the tricky bits much clearer than most written guides.

1. Choose the right system for your room

Electric underfloor heating comes as either a loose cable stapled to a mat, or a pre-spaced cable bonded to a fibreglass mesh roll. The mesh mat type is easier for DIY and suits most bathroom and kitchen tiling projects. Size the mat to cover the floor area you actually walk on, not the full room — heating under baths, vanity units, and fitted furniture wastes power.

Check the wattage of the mat against your circuit. Most bathroom mats up to 4 m² run on a standard fused spur at 230 V. Larger installations may need a dedicated circuit and must be installed by a qualified electrician under Part P.

2. Plan the mat layout before you start

Sketch the room to scale on paper and mark the fixed furniture positions. Plan the mat to fill the open floor, leaving at least 50 mm away from bath panels, shower trays, and walls to prevent heat build-up near combustibles.

The cold lead — the unheated cable that runs from the mat to the thermostat backbox — needs a route up the wall. Mark where it will chase into the plaster or run behind a small conduit before you commit to a layout.

3. Prepare the subfloor

The subfloor needs to be solid, flat, and dry. Dips of more than 3 mm over a metre run should be filled with floor levelling compound and allowed to cure fully. An uneven floor beneath the mat causes localised hot spots which can shorten the element’s life.

If the subfloor is old concrete with rising damp, apply a damp proof membrane first. Electric heating and moisture do not mix.

4. Fix the thermostat back box and run the cold lead

The thermostat typically mounts on the wall at about 1.2 m, on the same wall as the nearest socket. Fix the back box to the wall, then chase a channel for the conduit that carries both the cold lead from the mat and the floor sensor probe.

The sensor probe sits in its own small conduit loop out on the floor area covered by the mat. Position it between two cable runs, roughly in the middle of the heated area. You need to be able to retrieve the sensor without lifting the floor if it ever fails.

5. Roll out and fix the heating mat

Unroll the mat starting near the cold lead exit point. Follow your planned layout, turning the mat by cutting the mesh only — never cut the cable itself. The cable should snake back and forth across the floor with even spacing between runs.

Fix the mat with the adhesive patches provided or with cable staples into the subfloor. Keep the runs parallel and avoid crossing cables over each other. Check the resistance with a multimeter before and after laying — it should match the figure on the mat label. Any significant change means a cable has been damaged.

6. Tile over the mat

Apply tile adhesive directly onto the mat using a notched trowel. Press the adhesive firmly into the spaces between the cable runs so there are no voids beneath the tiles. Voids act as insulation against the cable and cause hot spots.

Use a flexible tile adhesive rated for underfloor heating. Standard adhesive can crack under thermal cycling. Allow the adhesive to cure fully before switching the heating on — usually 24–48 hours, longer for thick beds.

7. Connect the thermostat

Wire the cold lead and sensor into the thermostat following the manufacturer’s diagram. Most electric UFH thermostats take a standard switched live feed from a fused spur. The fused spur should be accessible and in a suitable position outside the bathroom zone if the room is a wet area.

Connecting the thermostat to mains power must be done by a competent person. In a bathroom, zone rules apply and the work needs to be notified under Part P of the Building Regulations unless you are a qualified electrician.

8. Test the system and set the schedule

Switch on and check the floor sensor is reading a temperature. Set the floor limit to no more than 27°C to protect the adhesive curing properly in the first week. After that, the typical comfortable floor temperature is 22–24°C.

Programme the thermostat to come on 30 minutes before you get up and switch off when the floor is not in use. A tiled floor holds heat well, so short bursts are more efficient than running it all day.

When to call a handyman

Call Richard if the circuit needs upgrading, if the bathroom zone rules are unclear, or if you would like the tiling done at the same time to save disruption. The mat laying itself is manageable as a DIY task for a careful person. The electrical connection at the consumer unit side is not.