Robert Merry, Technical Committee member of the Stone Federation of Great Britain and owner of The Stone Consultants, discusses the difficulties faced when paring underfloor heating systems with natural stone and how to overcome such issues.

The two most popular forms of underfloor heating currently on offer are electric heating mats and water fed pipes. Installing either of these with natural stone can be a challenge.

First of all, let’s look at electrical underfloor heating (EUFH) mats. On timber floors, traditionally with joists and overlaid boards, there are several stages to consider. The boards need to be sufficiently stable to prevent deflection in the surface, which can result in cracking or de-bonding of the tiles. Joists will need stiffening with additional noggins at 300mm centres, 2no. layers of WBP ply, cross laid, prepared properly and screwed at 300mm centres with stainless steel screws before you even start to consider the bedding ‘sandwich’ on top.

On top of the prepared joists and installed boards, it is essential to install a slip membrane. If you are dealing with a wet area, then you will need to consider if the selected slip membrane is a waterproofing layer and how that is sealed at the perimeters, paying particular attention to compatibility with the waterproofing products on the walls and drain outlets. I would venture these joints are the most likely to develop leaks and therefore should be warranted.

Some manufacturers of slip membranes stipulate that there is to be no direct contact between the electrical heating mat and their product. However, there are exceptions. One manufacturer produces a slip membrane with the heating element already incorporated into the membrane, but, if you’re not using this system, a separating layer is required. The Stone Federation recommend the EUFH is placed under the slip membrane. Check with the manufacturer and also ask if there is any reduction in performance if you use this method.

Securing the heating mat in position needs planning. Exposed and unsecured mats can be damaged, particularly when installing heavy stone slabs as the finishing layer. A coat of latex might be appropriate if you are installing the heating mat on top of the slip membrane.

And finally, having made your ‘sandwich and topping’, you will want to turn it all on. However, you must be careful not to damage the finished product. Turn the heating mat on at 5°C a day until it reaches its optimum heating level. Any faster and the floor may suffer the effects of thermal shock.

Hot water pipes buried in screeds are the second most common form of underfloor heating. The tiling systems are very similar. Waterproofing issues remain the same and preparation of the floor to SR1 is essential. Be careful of the screed type used. Fast-drying calcium sulphate screeds need further preparation to remove laitance and you should also check the moisture content before tiling.

One fundamental difference between the two systems is how the heating system is turned on and when. Screeds can shrink as they dry, and the activation of the heating system can accelerate the process. This can lead to cracks in the screed, which need repairing before tiling.

In all cases, the system needs to be heated at 5°C a day, until the optimum temperature is reached and then cooled to room temperature over two or three days, prior to laying any natural stone. It should be maintained at 15°C while installing the stone and then left to cool for 10 days to allow the bedding and jointing materials to cure/dry.

If none of this is possible due to the building programme, then an uncoupling membrane can be used. However, this then changes the nature of the floor from a bonded to an unbonded floor and implications for loading and use should be considered. You should still only turn the heating system on at 5°C a day.

One last thing to consider is that underfloor heating pipes, laid in troughs between timber joists wont usually work with a stone floor. This is because you can’t stiffen the floor with noggins or screw the boards at 300mm centres.

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The stone installer needs to have selected a compatible adhesive and grout and be installing on a surface prepared to SR 1 to achieve the correct bed thickness. The bed has to be full under the stone with no voids and no ‘dot and dabbing’.

How do you check the heating mat is working all the way through this process? Damaged circuits are difficult to repair if buried under tiles, membranes and latex. I have seen alarms connected to circuits which are activated if broken. Recognising the early warning signs of damage will help to prevent expensive rectification work at a later date.

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It’s a complicated process with many conflicting recommendations. The Stone Consultants offers advice and guidance on stone-related issues to the construction industry. If you need any assistance with specifications for underfloor heating systems with stone, don’t hesitate to contact me on 07771997621 or by emailing info@ x

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