In this regular column, Consultant Barry Hunt answers reader enquiries. If you have a question, email it to: nss@qmj.co.uk marked for Barry\'s attention

Q: What is the best form of underfloor heating to use with stone?

This is a difficult question to answer as there are so many different systems appearing on the market, each claiming to be better, greener, more efficient and easier to install than the others.

The problem with all these systems is that much of the advice provided with them is very general and what may work with some types of floor coverings does not necessarily work when using stone.

Even when advice is provided for stone tiling, this may not take into account the wide variety of stones available and the range of properties that can encompass.

The first choice to make when considering heating is whether to opt for a water or electric system.

Electric systems appear to have many advantages over water systems, especially if something goes wrong.

Water systems need to be embedded within a relatively thick layer of screed to ensure sufficient coverage over the pipes, so if there is a leak for any reason it can be difficult to identify without lifting the whole floor.

Deeper embedment also means that heat flow is less responsive and there is greater opportunity for energy loss before the heat reaches the stone surface.

Another problem with water systems is hot spots, especially where pipes may be concentrated close to a manifold or passing through doorways, leading to excessive differential expansion and ultimately cracking.

With electrical systems possibly the biggest problem is the lack of stiffness that may be inadvertently produced, especially when laying on to timber substrates using thin-bed adhesives.

It is essential that a decoupling membrane is employed to provide relief for the expected differential movement between the flexible timber and inflexible stone and bedding.

Current advice is that the decoupling membrane can be positioned either below or above the heating system. However, it generally makes more sense to place it below to provide a greater final thickness to the rigid part of the construction.

Whether you are using electric or water, it is essential to check the system works properly before laying the stone finish.

The temperature must be brought up slowly to final operating temperature by no more than 5∞C per day. But this should not be carried out until after the bedding materials have been sufficiently cured. If the screed is very thick, the heating system may be used to speed drying, although the relative humidity must be checked to confirm sufficient drying has actually occurred before tiling commences. Stone Federation guidance provides further advice on running heating systems.

Finally, once the heating system is in and working, do not forget to install your stone properly.

Possibly the two most important aspects are making sure that full bedding is achieved to provide even transmission of heat and that adequate allowance is made for movement.

Reference : Natural Stone Flooring. Code of Practice for the design and installation of internal flooring. Published by Stone Federation Great Britain, June 2005.

Barry Hunt is a Chartered Geologist and Chartered Surveyor who has spent 20 years investigating issues relating to natural stone and other construction materials. He now runs IBIS, an independent geomaterials consultancy undertaking commissions worldwide to provide consultancy, inspection and testing advice. Tel: 020 8518 8646

The advice offered in answer to readers\' questions is intended to provide helpful insights but should not be regarded as complete or definitive. Professional advice should always be sought with all stone-related issues as each problem will inevitably have its own unique set of circumstances."