Natural stone: no need to live in fear

Natural stone: no need to live in fear

January 2019

By Colin Stanyard, TTA Technical Committee

As an architect or designer, have you ever considered specifying or selecting natural stone but were unsure of the material’s suitability? As an installer have you ever been approached to fix natural stone but were uncertain about how to go about it?  

To prevent sleepless nights and reduce the number of recurrent nightmares, The Tile Association has recently published a document entitled “Guide to the Installation and Fixing of Natural Stone Tiles and Slabs to Internal Walls and Floors”.

Published in cooperation with The Stone Federation of Great Britain, and The British Adhesives and Sealants Association (BASA), the comprehensive paper has been written with the aim of providing best practice advice for all parties involved in the process of designing, building or refurbishing natural stone projects and includes guidance for installing natural stone on internal walls and floors and includes wet areas and fixing at height in both residential and commercial situations.

The document is conveniently divided into sections and can, therefore, readily be used for reference on any given topic without the necessity to read through the full document, although this is to be recommended to get the best from this publication.

A brief description of the main stone types and the obligatory section on the very important and relevant topic of CE marking is followed by the section “Design Considerations”.  It shouldn’t come as a surprise to any installer that the sub-floor construction should be rigid and capable of supporting the added load of the tiling system as well as the anticipated static/dynamic loading in service.  But did you know that the maximum deflection of the sub-floor is less than that acceptable for ceramic floor tiles?  Generally the construction of the sub-floor should be designed to limit deflection to 1/720 of the span, although in practice these values may be too high for most natural stone.

This section also looks at the use of stone in public spaces in relation to slip performance and conformance with part M of the Building Regulations, as well as giving consideration to Health and Safety regarding safe handling related to weight and restricted access.  A tick list is provided to ensure all points are covered when selecting the stone for a particular area with the section rounded off with a brief overview of installation requirements.

Once the type and size of stone has been selected, the next step would be to move on to the installation.  The next sections deal with floor and wall substrates in turn.

The floor substrate section highlights the preparation and drying times of cement:sand screeds and calcium sulphate screeds as well as use of tilebacker boards.  Important information on the use of both water-fed and electrical cable heating systems is provided.  Were you aware that heated screeds require a minimum 21 day drying period before the commissioning process can begin or that extended drying times and/or commissioning periods will be required where marble, limestone or travertine tiles are to be installed?  Or that uncoupling membranes are required where electrical cable heating systems are installed as detailed in BS 5385-5?  Further information on the use of uncoupling membranes can be found in section 6.2 of the document.

You are very likely to be aware that acoustic flooring systems are becoming increasingly important to ensure designed systems comply with Approved Document E of the Building Regulations.  However, how often have you thought beyond acoustic performance alone?  Did you know, when it is necessary to fix tiles directly to acoustic matting it is important to consider that the mat will not allow vertical movement under the increased weight, and that the type of acoustic flooring system should be sufficiently robust to withstand normal static, dynamic and impact loads.  Not all acoustic systems are suitable for use with natural stone tiles/slabs, e.g. floating floor systems.  The manufacturer should be consulted to assess the anticipated levels of deflection or compression of the underlying materials.

Density and thickness
When contemplating the installation of natural stone tiles on walls, it is important to remember that the density and thickness of stone tiles are often greater than ceramic tiles used.  Therefore it must be ensured that the substrate plus any surface applied coatings have adequate strength to support the weight of the stone tiles, for example it is unlikely that gypsum plaster will have sufficient cohesive strength to support their weight.  Any wet applied wall substrates must additionally be fully dry and cured if cracked or stained finishes are to be avoided.  A range of different wall substrate types are covered in the document including cement-based render and tilebacker boards. It also highlights that plywood should not be used for direct tiling as indicated in the recent publication of BS 5385-1.

The topics of resin-backed stone and resin-impregnated stone are discussed to ensure the reader is aware of their differences and of the special requirements necessary in their handling.  Failure to give proper consideration to these materials will likely lead to failure in service.

Once you are satisfied that the stone is the correct choice for the environment to which it will be exposed and that the substrate is of the correct type and correctly prepared, ensure that the most appropriate type of adhesive and grout are selected.  For example certain types of stone may require a white rapid-setting cement-based adhesive or a reaction resin type.  In wall locations there may also be a requirement for additional mechanical fixings.

As with any tiled installation, movement joints will be required in both wall and floor locations and their positioning in combination with BS 5385-1 (walls) and BS5385-5 (floors) is discussed in the document.  It is also important where wet applied sealants are used, that they are composed of a material compatible with the type of stone installed.  For example did you know that acetoxy curing sealants are not suitable for use with calcareous stone, such as limestone?

In contrast to a ceramic tiled installation, stone finishes may require a whole host of different considerations such as pre-cleaning, sealing and/or the application of impregnators.  For example, were you aware that inadequate pre-cleaning may leave residues of adhesive, grout or quarry dust trapped within the small capillaries of the stone?  The consequence of sealing or impregnating an incompletely cleaned stone is a poor surface appearance e.g. staining or shadowing beneath the sealer.  Stones must also be completely dry.

Many stones are likely to require the application of a sealant prior to grouting e.g. porous material will require the application of an impregnating product.  Once the grouting process is complete and the stone surface is clean and dry, a final seal may be applied as necessary.  Following a few basic rules will ensure a successful installation.

As with any tiled finish, it is not a self-cleaning system.  Therefore, it is essential that the correct cleaning regime is followed and maintained at the frequency recommended by the stone supplier and the manufacturer of the cleaning materials.  The use of materials inappropriate for the type of stone is likely to have a detrimental effect on the appearance.

The TTA document finishes with a very useful troubleshooting section focusing on commonly occurring problems encountered following the installation of natural stone.  Problems, such as discolouration and/or darkening of stone following the application of an impregnating sealer are discussed.  This occurrence is usually due to the product being applied before the stone and/or its bedding has fully dried thus trapping moisture.

Rectification of this problem would generally involve chemical removal of the impregnating sealer and the allowance of the stone to dry before a re-application.  Other issues such as whitening, streaking and oily finishes are also introduced in the troubleshooting section.

In summary, if selected, handled, installed, sealed and maintained correctly, natural stone materials will last for many years.
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