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How do you tile onto underfloor heating?

Underfloor heating enjoys continuing popularity, both for new build and refurbishment projects. It has no radiators or heaters on show, is relatively economical to run, requires minimal maintenance and offers even heat distribution. It can be installed beneath virtually any floor type including wood, laminate and, naturally, tiling. Fixing tiles with underfloor heating is not difficult, but care and preparation is needed.

The traditional method of heating a floor is via hot water pipes. These can be laid into a screed, set into flooring-grade chipboard or laid in the cavity between the joists of a suspended floor. A number of more modern solutions are available, including electrical underfloor heating cables, with the most popular being the cable system, laid onto a thermal insulating board. Also available are heating mats, which need a bonding adhesive for the substrate and also for priming the surface of the mat. Whichever underfloor heating method is selected, the first step must always be to check with the manufacturer that the system is suitable for use with the proposed substrate, within the relevant environment and is stable enough to receive a rigid tile finish.

Hot water pipes
Floors with underfloor heating generally operate below 30º C, with the heating elements typically incorporated before tile fixing begins in a suitable reinforced floating screed, normally a traditional sand/cement screed. This should have a minimum thickness of 75 mm. The screed will have to be allowed a drying period of at least three weeks, if containing Portland cement to BS EN 197-2 2000 Cem1. This can be reduced by the use of task-specific products: if BAL Quickset Cement is used, the drying requirement is reduced to one week.

After drying, the screed should be heated gradually. The temperature should be raised, increased by no more than 5º C each day, to a temperature of 25º C and maintained at that level for three days before being allowed to cool naturally to room temperature. The heating system must have been turned off or, in cold weather, turned down to below 15º C. Tiling can then begin. Once tiling is completed, the floor should then be left for at least 14 days before being brought to its operating temperature, raised at a maximum rate of 5º C per day.

Cable systems and heating mats
When undertile heating cables are laid over traditional wooden flooring, the cables are normally bedded directly into a layer of flexible adhesive. When tiling onto a solid floor, it may be appropriate to bed the cables into a suitable smoothing compound, such as BAL Ultrabase. Heating mats are normally constructed of flexible heating elements encapsulated in very thin polyester. Such mats have to be bonded to the substrate with an appropriate contact adhesive. The mat surfaces will then need priming, normally using the same contact adhesive, before tiles are laid using the recommended adhesive.

Movement joints
Whichever type of underfloor heating is installed, movement control joints will be essential. These should be incorporated at all screed perimeters and around upstands or anything which penetrates the screed. Such joints are normally created while the screed is being laid by installing pre-formed expanded polystyrene strips against the wall or upstand.  These are typically 10 mm thick and it is essential that the movement control joints penetrate the full depth of the screed.

Movement control joints in floors should then be incorporated (as per British Standard BS 5385: Part 3: 2007: Clauses 6.8 and 7.1.6) at all perimeters and upstands, to coincide with those in the base screed. Intermediate movement control joints should also be incorporated within the tiling, into bays of size not greater than 40 m² with an edge length no greater than 8 m (in accordance with clause Such joints should penetrate through the thickness of the tile adhesive bed and the reinforced base screed.

The most appropriate adhesive will depend upon the circumstances of the project. For concrete substrates, cement:sand screeds or for floors overlaid with plywood of 15 mm or more, there are a number of possible adhesives. The inherent nature of the installation means that the tiling will be subject to regular variations in temperature, so it requires either a flexible adhesive or a cementitious adhesive with an admixture — either BAL Single Part Flexible, or BAL Gold Star with BAL Admix AD1, for example. Where there is a need for fast fixing, a rapid-setting adhesive will be appropriate, not forgetting the need for flexibility; either BAL Rapidset Flexible or BAL Rapidset with BAL Admix AD1 would be suitable. There are a number of highly polymer-modified BAL grouts available.

The choice of both adhesive and grout is restricted where tiling is onto a single layer of timber. In this case, BAL Fastflex is recommended before grouting with BAL Wide Joint Grout with BAL Admix GT1 diluted at 1:1 with water, providing the floor is capable of supporting a rigid tile finish.

After completion of the tiling, allow 14 days for drying — for BAL adhesives — before bringing the floor up initially to its normal operating temperature. Temperature increase should be gradual, rising at 5º C per day. 

There are a great many proprietary underfloor heating systems available. Some may recommend alternative drying times, screed thicknesses, or particular adhesives or grouts. Such recommendations should be followed when these systems are installed and, when in doubt, consult the system manufacturer. For any questions concerning the use of adhesives or grouts with underfloor heating, the BAL Technical Advisory Service is always ready to give free, impartial advice during normal working hours: 0845 600 1222.


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