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 struggling with my levels on a job. What are the thinnest natural stone flooring tiles I can safely use?

There is no minimum limit for the thickness, or ‘thinness’, of natural stone tiles. The requirements for modular tiles given in BS EN 12057 are for tiles of 12mm thickness and less, but the standard says they cannot have any other dimension greater than 610mm. Having thin tiles greater than 610mm long and wide is impractical as the bigger they get the more susceptible they are to breakage and warping in use and braking when being laid.

If your requirements demand very thin tiles you must remember that resistance to breakage works on an inverse square law so that a tile only 6mm thick has a quarter the breaking resistance of a 12mm thick tile of the same size.

Reducing the tile dimensions will help to restore some of the resistance to failure, though smaller tiles are often not considered as attractive.

The fundamental strength of the material to be used is another consideration. It may not be possible to cut porous limestones and sandstones thinly unless the lengths and widths are very small. More dense limestone may be more amenable to thinner cutting but some features, such as veining and stylolites that often create lines of weakness, may exert a greater influence than they normally do with thicker tiles. Verdes and brecciated stones would probably also be best avoided. Most igneous stones, such as granite and basalt, should have sufficient strength, as should most marbles, quartzites and slates.

Another factor to be wary of is the translucency of a stone when cut thinly. You can see through some of the larger crystalline materials when they are cut to thicknesses less than their typical crystal grain size. The colour of the bedding mortar may actually show through the tiles and provide a duller than expected appearance, especially if the adhesive is darker in colour than the stone. Therefore it is generally better to select more finely crystalline and/or opaque stones.

Riven and other textured surfaces also help to reduce translucency.

It is important to ensure a fully supporting bed is achieved with very thin stone tiles.

The thinnest natural stone tiles you could probably use with reasonable dimensions would be made from slate. Possibly you could go as thin as 3mm, but the slate would have to be of high quality with consistent cleavage. Any unsupported corners, for example, would require probably only a concentrated loading of 40kg to cause damage. In practice, 4mm is probably the minimum thickness that really should be attempted. The only problem with slate is the lack of colour choice – usually various shades of dark grey with a few greens and purples also available.

The surface on which thin tiles are laid needs to be of the highest quality and even, as the tolerances you will be working with will be minimal. It is imperative that an SR1 finish is achieved if the laying surface is a screed, which means a tolerance of no more than 3mm change below a 2m straightedge.

Reference

BS EN 12057:2004. Natural stone products – Modular tiles – Requirements. Published by the British Standards Institution, London.

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
Email: info@ibis4u.co.uk
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.