Laying electric underfloor heating (heating mat)

The cable warms up as current flows through it and passes the heat into the adhesive and the tiles. The whole layer is thin, usually only a few millimetres, so the floor responds quickly. You feel the warmth underfoot much sooner than with a thick screed full of pipes.

Taking the chill off or heating the room

In a bathroom the mat usually just stops the tiles feeling cold. A radiator or towel rail does the main heating. In a well-insulated house a mat can be the main heat source. In that case the output per square metre and the total consumption should be worked out from the room's heat loss, not guessed. Mats come in different outputs, commonly about 100 to 200 W/m². Pick the higher output if the floor really has to heat the room and the lower one if it is just for comfort.

Single-core and twin-core cable

With twin-core cable only one cold tail leaves the mat. The cold tail is the non-heating lead that runs to the thermostat. Laying is simpler because the mat does not have to return to its starting point. A single-core mat has a cold tail at each end, so the layout must bring both ends back to the thermostat. In a small bathroom with several corners that is an extra headache, which is why twin-core mats are now more common.

When a mat is not the right choice

Do you want to heat a large floor area across the whole house and already run a heat pump or boiler? Then wet underfloor heating is usually cheaper to run. Mats belong mainly under ceramic tiles and stone. Under laminate, wood or vinyl, heating films are more usual. The other option is to embed the mat in levelling compound and lay the floor covering on top of that. Either way, keep to the maximum temperature the covering's manufacturer allows.

Laying the mat itself is quick. The quality of the result depends largely on what was done beforehand.

The subfloor

The subfloor must be firm, dry, flat and free of dust. Old tiles can sometimes stay if they are well bonded and do not sound hollow. They need sanding and a coat of bonding primer. Level out larger dips and humps with compound before the mat goes down. Otherwise adhesive collects in the hollows, and on the high spots the cable ends up under only a thin skin.

Insulation under the mat

If there is an unheated space below, such as a cellar, or an uninsulated ground-bearing slab, much of the heat goes downwards. The floor then warms slowly and running costs go up. In that case it pays to lay thin insulation boards made for use under tiles, for example XPS boards with a cement facing. Between two heated flats this is usually not essential, but insulation still shortens the warm-up time.

What the electrician and plumber must do first

• A back box for the thermostat is set in the wall, normally out of reach of splashing water, with a supply cable run to it from the consumer unit.
• A chase with a conduit for the floor sensor runs from the bottom of the box down into the floor, along with a chase for the cold tail.
• Water pipes and wastes in the floor are finished, pressure-tested and covered, so nobody has to drill or cut over them later.
• The positions of the shower drain, floor gully and any bath are fixed, because the mat does not go under them.

Planning the area

Choose the mat size by the free floor area, not the room size. From the bathroom area, subtract the bath, shower tray, WC, any fixed vanity unit and a strip along the walls. The cable does not belong under furniture without legs, kitchen units or fitted wardrobes. Heat would build up underneath, the cable would overheat and the furniture would suffer. Most manufacturers recommend keeping a few centimetres away from walls and fixed objects. The exact distance is in the instructions.

If you live in a block of flats, ask your electrician whether the main supply and the riser can take another appliance. For larger areas, also speak to the building management or owners' association. A normal indoor floor renovation rarely needs permission. If the floor is part of a bigger alteration, check with the local building authority what the local building regulations require.

Testing before laying

Before the mat is unrolled, the installer measures the resistance of the heating conductor and the insulation resistance. The readings are compared with the label and written into the warranty card. This first test proves the mat arrived in good order. Without it, a warranty claim struggles to show when the cable was damaged.

Priming and unrolling the mat

The subfloor gets a coat of primer. It evens out absorbency and helps the self-adhesive mesh stick. Once it is dry, the mat is unrolled from the point where the cold tail comes out. At the wall the mesh is cut, the mat is turned and runs back the other way. Only the mesh is ever cut, never the heating cable. To get round a corner or a drain, the cable can be carefully freed from the mesh and laid loose, held with tape.

Rules for running the cable

• The cable must never cross or touch itself.
• Spacing between the loops stays even, otherwise you get warmer and cooler strips.
• The cable is never bent sharply and never runs across movement joints.
• The joint between the heating cable and the cold tail is normally laid in the floor and embedded, not in the wall.
• The cold tail must be long enough to reach the thermostat box without extending it.

The floor sensor

The temperature sensor sits in a flexible conduit with a sealed end. The conduit runs from the wall into the floor. It ends roughly halfway between two cable loops, usually a few tens of centimetres from the wall. The sensor must not touch the cable, or it measures the cable rather than the floor. Thanks to the conduit, a faulty sensor can be pulled out and replaced later without lifting tiles.

Second test, photos and a sketch

Once the mat is laid out, both resistances are measured again. Then take several photos from different angles and make a sketch showing where the cable, the joint and the sensor are. A few years later you may need to drill for a door stop or fix a cabinet. That sketch will save a lot of trouble.

Straight into tile adhesive

Here the adhesive goes directly over the mat and the tiles are bedded into it. This gives the lowest build-up, but there is a catch. The notched trowel easily snags the cable and damages it. It is also hard to make sure the cable is fully surrounded with no air pockets under the tiles. So this method suits smaller areas and an experienced tiler. It needs a flexible adhesive suitable for underfloor heating, usually class C2 S1 to EN 12004.

A layer of compound first, then tiles

In practice a two-step method is more common. The mat is first covered with self-levelling compound, which coats the cable evenly and without air. The layer is usually a few millimetres above the cable, according to the manufacturer. Once it has cured, you have a flat, solid base. The tiler can fix tiles without any risk to the cable. Heat spreads more evenly and the tiles hold better.

Where the waterproofing goes

In a bathroom, and especially in a walk-in shower with a linear drain, the floor needs waterproofing (tanking). The usual order is: lay the mat, cover it with compound, then apply the liquid membrane with reinforcing tape in the corners on top of that layer. Some system manufacturers allow a different order. The deciding factor is the instructions for the specific system. Adhesive, compound and waterproofing must also be compatible with each other. Before drilling anywhere through the waterproofing and near the cable, check the sketch first.

A heating mat is an electrical appliance wired permanently into the mains. Only a qualified electrician may connect it to the circuit and fit the protection. That includes an RCD, which is essential in a bathroom, and placing the thermostat correctly for the bathroom zones. In many countries bathroom circuits are notifiable work, so check local rules. After connecting, the electrician tests the circuit and issues an electrical installation certificate or similar test record. Keep it with the mat's warranty card.

Which thermostat to choose

The thermostat should have a floor sensor, even if it also measures air temperature. Under tiles you set a comfortable floor temperature. With wood or vinyl, keep to the maximum surface temperature their manufacturer gives, which is often much lower. A programmable thermostat warms the floor in the morning and evening when you are at home and keeps it cooler the rest of the time. On a time-of-use tariff, set the programme around the off-peak hours. Your electricity supplier can tell you the details of your tariff.

Plenty of the groundwork is something you can do yourself, as the DIY guide below shows. The connection and the testing stay with the professional.

Two bathrooms with the same floor area can take very different amounts of work.

• Simple shape. A rectangular free area without projections is covered quickly: the mat is just unrolled and turned.
• Lots of corners and obstacles. Drains, soil stacks, recesses and angled walls mean a lot of mesh cutting and laying cable by hand.
• Condition of the subfloor. Flat, sound concrete only needs priming. Old tiles must be checked and treated, and an uneven floor must be levelled.
• Insulation boards. If insulation goes under the mat, that adds another layer and time for bonding.
• No preparation in the wall. Without a thermostat box and sensor chase, someone has to cut into a finished wall.
• Floor build-up height. At thresholds and adjoining floors, check in advance that the new layers fit and the door still opens.
• Small area. Even a small toilet needs all the tests and settings, so the work per square metre is relatively greater.

• Cable damaged during tiling. A trowel notch or a sharp tile edge nicks the insulation. The heating may work until moisture trips the RCD. That is why the cable is tested after embedding and again after tiling.
• Sensor without conduit or touching the cable. The thermostat switches off too early and the floor stays cold, or a faulty sensor cannot be replaced.
• Air under the cable. Where the cable is not fully embedded it overheats. This can shorten its life and crack the adhesive.
• Mat under furniture. Under a cabinet without legs, heat builds up, the furniture warps and the cable suffers.
• Switching on too early. Heating before the adhesive or compound has cured makes it dry unevenly. Tiles can come loose or the grout can crack.
• No insulation over a cold space. The floor takes a long time to warm up and running costs are higher than the owner expected.
• Joint in the wall or a short cold tail. Extending the cable outside the floor often causes faults later, and most manufacturers advise against it.
• No sketch. Years later someone drills blind and goes through the cable. A specialist then has to repair it, and part of the floor often has to come up.

In a typical bathroom, priming, laying the mat, fitting the sensor and testing usually take a few hours. What takes longer is everything that has to dry. Primer usually needs a few hours, according to the manufacturer. Levelling compound can often be walked on within about a day. It needs longer before tiling, and longer still before it can be heated.

The first switch-on follows the instructions for the adhesive and compound. For thin layers of adhesive and compound, manufacturers usually state about a week. Thicker cement-based layers can need several weeks. Warm the floor up gently the first time. Set the thermostat low and raise the temperature step by step over several days. That way any remaining moisture evaporates evenly and the tiles are not put under stress.

All in all, expect several days to a few weeks between laying the mat and having a warm floor. A short installation does not mean the bathroom is fully usable the next day.

An electric mat is not a start-to-finish DIY job, because the connection and the tests must be done by a qualified electrician. You can still save time and avoid problems by preparing the room well and looking after the mat while the trades work.

1. Clear the room completely and remove anything fixed to the floor that will not go back in the same place.
2. Remove old floor tiles if they are loose or sound hollow, and sweep and vacuum the subfloor thoroughly.
3. Measure the room and draw a plan of the free floor area, leaving out the bath, shower tray, WC, fixed units and a strip along the walls.
4. Mark where the thermostat, cold tail and sensor conduit will go and agree these positions with the electrician.
5. Agree the order of work with the plumber, electrician and tiler, so that no pipes or chases are left for after the mat is down.
6. Check door clearances and thresholds against the expected height of the new floor build-up.
7. Keep the mat's warranty card ready and ask for every resistance reading to be written into it.
8. Photograph the laid mat from several angles before it is covered, and keep the photos with the sketch.
9. Keep people in work boots off the laid mat and never stand buckets, tools or a ladder on it.
10. Leave the heating off until the curing time on the adhesive and compound has passed.

After handover, most of the mat is hidden under tiles. So insist on getting all the records and check that everything works before the tradespeople leave.

• Warranty card with resistance readings recorded before laying, after laying and after embedding or tiling
• Readings within the tolerance stated on the manufacturer's label
• Photos and a sketch showing the cable, the joint and the sensor
• Sensor in a conduit with a sealed end and not touching the cable
• Electrician's installation certificate or test record and a working RCD
• Thermostat showing the floor temperature and set for the type of floor covering
• Clear instructions on when and how to bring the heating up gradually, based on the adhesive and compound used
• After the first warm-up, an evenly warm floor without noticeably cold or hot spots
• Thermostat manual and contact details of the installer in case of a fault