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: I am installing stone around a bath. Are there any problems I should be particularly aware of?

A number of problems are presented when using stone to surround baths. There has to be a robust construction that still allows access to any plumbing and other services that may be present. At the same time the construction must allow for inevitable movement that will occur when the bath is filled.

A fairly average sized bath filled with water typically weighs at least a quarter of a tonne, while half a tonne or more can be achieved with larger baths. And larger baths will also require more stone to cover them, increasing the loading.

Another major issue is the potential for deflection when the underlying supporting floor structure is of timber, which is common in domestic properties. If the adjacent floor covering is also stone, it may be susceptible to cracking as the floor structure deflects.

Deflection will be more pronounced if the bath is positioned centrally rather than against, or close to, a wall. Free-standing baths will also probably require the greatest quantity of stone to cover them.

The installation of noggings at a maximum of 300mm centres between the joists should stiffen most timber floors sufficiently where a bath is to be installed. Screwing down two layers of ply in accordance with Stone Federation’s recommendations* also helps to increase stiffness.

However, every floor is different and it is a good idea to consult a structural engineer to check the loadings and related deflections for a given floor construction.

Reducing the spacing of noggings and increasing the thickness of the ply layers helps to increase stiffness and reduce the potential for damaging deflection.

Decoupling layers used with timber substrates will help reduce the potential for cracking from substrate movements. However, these must not be solely relied upon as they are designed to cope with horizontal rather than vertical movements.

The frames and backings for bath surrounds are typically of timber but can comprise metal. Very rarely solid concrete may be combined with a concrete floor. Ideally, timber should be protected from damp by using a tanking system suitable for adhering stone to. All metal items used should be non-corroding.

All joints between the completed bath surround and any walls and floors should not be hard grouted. They can either be left open or filled with an appropriate flexible joint filler, depending on the design aesthetic.

Bath edges and any overhangs are best filled rather than left open. The joints can be as little as 3mm wide and the outer surface struck flush to the adjacent stone for the neatest and least obtrusive finish.

Wedges of flexible jointing materials are unnecessary and should be avoided as they will fail.

Finally, no matter how good the selected stone is, prolonged exposure to moisture can result in darkening from damp patches that lead to staining. The variety of chemicals that may be used can also have an impact on stone performance. Stone surrounds should be thoroughly sealed and it is important that such treatments are regularly maintained.

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