Underfloor heating and tiling

Typical jobs

• Laying an electric heating mat in a bathroom, ready for the electrician
• Insulation boards under a heating mat on a concrete floor
• Embedding a heating mat in flexible adhesive before tiling
• Tiling over a wet underfloor heating system laid in a new floor
• Testing the mat's resistance before and after tiling

Electric underfloor heating under tiles is usually a thin mat of heating cable on a mesh, laid on the floor just before tiling. On a concrete floor, insulation boards go down first so the heat goes up into the room rather than down into the slab. The mat is rolled out to a plan that keeps it clear of the toilet, the bath and any fixed units, and the cable is never cut or crossed. It is then embedded in flexible tile adhesive or a thin levelling compound, and the tiles are laid on top in flexible adhesive.

This is the part a tiler does. The tiler leaves the mat's cold tail and the floor sensor lead at the wall; the electrician fits the thermostat and makes every connection. The mat's resistance is measured with a meter before it is covered, after it is covered and after tiling, so a damaged cable is found while it can still be reached. Keep the readings: the mat maker's guarantee usually depends on them. Connecting the system to the mains is electrical work. In England it must meet Part P of the Building Regulations, and the work is notifiable if it needs a new circuit or a new consumer unit, or falls in the zone round a bath or shower, within 0.6 m of it and up to 2.25 m high: an electrician registered with a competent person scheme certifies it, or it is notified to building control. Wales has its own version of Part P with a wider list of notifiable work, and Scotland and Northern Ireland set their own building standards. To find an electrician, see www.yourelectrician.trade.

Wet underfloor heating, warm water through pipes, is usually laid in a new floor by the heating trade, though low-profile overlay systems exist for existing floors, and the tiler tiles once that floor is dry and has been commissioned and allowed to cool as the floor maker says. In both cases the floor should not be heated to full temperature soon after tiling; adhesive and grout makers give a waiting time and a warm-up schedule.

On a heated floor the tiler and the electrician each do a part, so settle who does what before the mat goes down.

• Will you lay the mat and the insulation boards?
• Will you test the mat's resistance before and after tiling, and give me the readings?
• Which adhesive goes over the heating, and how long before it can be switched on?
• Who is arranging the electrician, and will they see the mat plan first?

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