Installing Underfloor Heating Under Tiles

To install underfloor heating under tiles, lay insulation boards on a clean, sound subfloor, test the heating, then roll out the mat or lay the loose wire, leaving a 100mm unheated border. Fit the floor sensor, test again, and tile over it. A qualified electrician makes the final connection through an RCD.

Choosing the right system for a tiled floor

Heating mats

In larger rooms, our 150W or 200W heating mats are the best solution. The mats are rolled out, cut and turned to fit the area. For the heating to work as a primary or secondary heat source, we recommend heating 80–90% of the available floor.

The mats are 500mm wide, so how much of the floor you can cover depends on how many 500mm runs fit in the room. Never order a mat for 100% of the floor, because it won’t fit. Allow at least a 100mm unheated border around the room when working out what you need.

Loose wire

In awkward or smaller rooms, like cloakrooms and WCs, loose wire is the choice of most installers. It isn’t fixed to a mesh, so it’s very versatile and can be laid wall to wall. As with mats, lay it directly onto insulation boards for the best efficiency and response.

If you’re not sure which suits your room, give our technical team a call.

What you need

The heating mat or loose wire, a thermostat with its floor sensor, insulation boards, flexible tile adhesive, and a multimeter to test the heating at each stage. Lay the heating directly onto XPS or cement-coated tile backer insulation boards.

200W/M² Electric Underfloor Heating Mat

From: £22.99 Inc. VAT
£19.16 Ex. VAT

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Tile Backer Underfloor Insulation Boards

From: £44.95 Inc. VAT
£37.46 Ex. VAT

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M5 Digital Underfloor Heating Thermostat

£25.99 Inc. VAT
£21.66 Ex. VAT

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Digital Multimeter

£11.98 Inc. VAT
£9.98 Ex. VAT

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How to install underfloor heating under tiles, step by step

1. Test the heating before you start. Take a resistance reading of the mat or cable with a multimeter and write it on the warranty page at the back of your instructions. It shows the heating was working when you received it. If the mat is the wrong size, or the readings don’t match the test results on the box label, stop and return it for the correct size. Once a mat is unrolled it becomes the installer’s responsibility and can’t be returned for a different size.
2. Prepare the subfloor. It must be clean, sound and suitable for the floor covering. We highly recommend insulation over a concrete subfloor. Either board is suitable under a tiled floor. We recommend 10mm, but if height is an issue 6mm will be enough. Tile backer boards can be screwed down, and both types can be stuck down with a flexible adhesive.
3. Measure the floor. Measure accurately, allowing a 100mm or more unheated border around the edge, and check the mat is the right size for the area. The mat size and wattage are on the box and the mat label.
4. Lay the mat. Start close to the thermostat position if you can. If that isn’t practical, the cold tail can be extended with a suitable electrical wire. Unroll the mat slowly, peeling off the adhesive backing tape as you go, and press it firmly onto the floor. Make sure the heating element isn’t twisted or stressed anywhere.
5. Turn the mat at the walls. To change direction or bring the mat back on itself, cut through the mesh backing. Never cut the heating cable.
6. Work around awkward areas. If the area is an irregular shape or there’s an obstacle to avoid, the cable can be taken off the mesh and laid loose, held in place with a suitable fixing tape. Keep the same spacing as on the mesh so the heat is even. Never lay cables closer than 50mm apart. If they get too close, touch or overlap, the cable may overheat.
7. Test again. Once the heating is down, repeat the resistance and continuity test and note it on the warranty page. The readings should match the ones from step 1.
8. Fit the floor sensor. Fit the floor probe supplied with the thermostat between two heating wires, no more than 30mm from one of them. Keep it away from anything else that could affect it, like hot water pipes. Then check the probe’s resistance to confirm it works, and note the reading on the warranty card.
9. Connect multiple mats. If you’re fitting more than one mat, they can be connected in parallel into a suitable junction box, with one spur run to the thermostat position, ready to be wired in.
10. The electrics. The system must run through a residual current device (RCD), fitted to the electrical regulations in force at the time. All electrical connections must be made by a qualified electrician. It’s the installer’s or electrician’s responsibility to make sure the system is fitted correctly and that any other materials used are suitable for the heating.
11. Tile, then test once more. Lay the tiles in flexible tile adhesive, then test the heating again after the floor is finished.

Laying loose wire: working out the spacing

Test the cable for continuity and resistance before you unroll it, and note the reading on the warranty page. Then work out the spacing between cable runs:

Spacing (mm) = 1000 ÷ cable length (m) × heated area (m²)

The answer should come out between 50mm and 80mm. It’s a guide to the spacing you need for the best coverage. Never cut or shorten the heating cable. To heat a bigger area, increase the spacing. To heat a smaller area, reduce it. Only the black cold connection ends can be shortened or lengthened.

Testing and your warranty

You can test the heating as often as you like, but to complete the warranty it must be tested three times: before it’s laid, after it’s laid, and after the floor covering is down. Record every reading accurately on the warranty page.

Installing under tiles: FAQs

Want to know what it all costs? See our electric underfloor heating cost guide, or call us on +44 (0)1803 316060 and we’ll help you choose.