Specify Natural Stone Wall and Floor Tiling with Confidence - ARDEX UK Ltd


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The aim of this CPD seminar is to enable listeners to make informed choices on the selection of materials when specifying natural stone tiling, by considering the impact that natural stone tiles, backgrounds and finished environments will have on the adhesive and grout specification. It will also demonstrate how to avoid common installation failures, through optimum product specification and guidance on best practice. Having listened to this CPD the audience should feel confident about selecting appropriate solutions in most situations.


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Ardex UK Ltd History

Before commencing this CPD we just like to set the scene regarding Ardex still a family owned business. Ardex group employs approximately 1600 employees across 50 sites and has global sales in excess of £450,000,000. Ardex has been at the forefront of the UK market for flooring and tiling products for 50 years, servicing the market from its manufacturing base in Haverhill’s Suffolk.


Being ISO 9001 accredited you can rely on the fact that all Ardex products are manufactured to a recognised quality standard. We also take every appropriate step we can to minimise our impact on the environment. Our new products have been formulated to achieve greater coverage, so that less product is required during installation and serious consideration is given to the sources of raw materials we use.


Ardex provides customers with quality products and a Technical Support network you can rely on. Our guiding principle is to have excellence in all that we do, no matter how big or small the job. Ardex are the specialist manufacturers of products specifically designed for natural stone tiling. You can always rely on our ongoing support with your specifications. We work closely with the stone Federation Basa and the tile Association to promote professionalism, technical standards and best practice within the tiling industry. Both federations provide a source of useful information relating to natural stone tiling.


Technical standards and best practice are detailed in BS5385 and code of practice BS8000 section 11.2 1990. In addition to internal stone wall tiling in part one of BS5385 part 5 focuses on the design and installation of natural stone and agglomerate stone flooring. Part 3 provides practical guidance on the installation of screeds to receive stone floor tiling, and part 4 provides advice on tiling in special conditions, for example wet locations.


National building specification clauses provide best practice, guidance and support for your tiling specifications. M40 covers stone, concrete, quarry, ceramic tiling and mosaic. M41 covers terrazzo tiling, and in situ terrazzo.


Although natural stone has always been a popular choice, recently it has had a resurgence within tiling. Nowadays modern ceramic tile production may mean it's difficult to tell a natural stone product from a replica. However, it's important to identify natural stone and specifically the type of natural stone to be able to create a robust specification. For example, can you tell whether this is natural stone or replica tile?


There is an EN standard which defines the naming requirements for natural stone which is mandatory for all CE marked products, however, some stone is imported from outside the EU where the standard is not required. The terms referred to in the standard don't tend to be layman terms but will supply the traditional name of the stone. Usually the tile manufacturers will provide you with further guidance if you're in doubt.


Another important consideration when specifying tiles is slip resistance. There is little guidance in the current British standard relating to slip resistance, and there is no standard worldwide test. Different testing methods include the tortoise, pendulum and ramp; however, the tile Association has published a useful technical guide called the slip resistance of hard floor coverings.


Natural Stone Tiles

Let's take a look at the different types of natural stone. Stone is a natural quarried product and will vary from tile to tile even when mined from the same quarry. It can have a tendency for warping staining or discoloration, and this will also vary within a typical supply. With this in mind we will take you through the key watchpoints by providing you with a basic understanding of the types of natural stone tiles available, the considerations when specifying the adhesive, grout and installation requirements.


Let's take a look at the origin of natural stone. Sedimentary rocks for example limestone are formed through a consolidation of sediments, seashells and other minerals, and as a result each stone will show its own colours and markings. There is often surface pitting on these types of stone.


Sandstone is a hard stone and can be particularly porous, and like many stone tiles it will require specialist treatment to reduce the porosity of its surface. Metamorphic rocks modified by heat and pressure such as marble, often have mineral veining. Some marble tiles can be weak and may have a mesh backing or resin on the reverse of the tile to strengthen them. Such backings need to be checked to ensure that any backing mesh and its water resistant glue, does not occupy more than 25% of the area. Some green marbles for example have been known to be adversely affected by moisture, since they contain clays. Seek technical advice when specifying these, as they will probably require special fixing.


Igneous rocks. These are rocks formed from molten rock magma for example granite, contain approximately 60% feldspar (colour), 30% quartz (hardness) and 10% mica. It's the mica which leads us to take additional considerations with natural stone, due to its tendency to discolour.


Slate may be riven or unriven. Riven slate tends to vary in thickness and as with all uncalibrated stone, the variety and thickness requires consideration for the installation and levelling products required. Some sources of slate may contain oil which can lead to the initial adhesion being lost as the oil migrates to the tile adhesive interface. It's always sensible to trial a small area first.


Honed stones are machined to create a smooth even surface, detailed guidance on travertine is available from the tile Association. Agglomerated stone, which used to be referred to as conglomerate stone, often looks like polished granite or terrazzo. The tiles are made by mixing marble or granite chips with polyester or epoxy resin. A note of caution is required here some products contain resins and mineral fillers that may expand on wetting. This can lead to warping resulting in lipping at the tile joints. It is also worth mentioning here that the British standard recommends low alkalinity rapid drying systems with agglomerated stone.


Remember that stone is a natural product so every natural stone tile will have a unique appearance due to the natural variation of tiles. It's important to ensure sufficient is specified in one batch and that the tiles are mixed, so that shades are blended effectively.


Specifying Tile Adhesives for Natural Stone

Let's take a look at the key considerations when specifying tiling adhesives for natural stone. The backgrounds for tiling stone tiles and the situations where we place tiling, can influence the choice of tile adhesive or grout and the specification.


It's worth reviewing basic adhesion principles here. There are three main types of ceramic tiling adhesives dispersion, cement based and reaction resin. The first 2 are the most common. The reason it's worth taking a moment to talk about them, is because they develop adhesion in different ways. Dispersion otherwise called ready mixed or paste adhesives is for wall use and found most commonly in the DIY or domestic tiling market. They need to dry to develop adhesion.


Cement based adhesives are commonly used in the contract market for both wall and floor tiling. These bagged adhesives, when mixed with water, form a mortar which sets and hardens. Reaction resin adhesives commonly referred to as epoxy or polyurethane, are generally two component systems which react together when mixed to gain adhesion. These are used in specialist areas. This very basic principle is important as considerations such as the body of the tile type, its level of water absorption, the tile size or weight, the width of the grout joint, coupled with the porosity of the background or the end tiling situation will affect your choice when specifying, especially for dispersion or paste products.


As we continue through this CPD, we will build onto this initial framework. At the end of the day although not a visible part, it's the correct choice of adhesive and correct installation that literally holds the overall project in place.


Although we've addressed basic adhesion properties earlier, it's important to see that by adding additional polymers, and formulating the products to suit given situations, we can develop a range of enhanced properties for adhesives and grouts for any installation, for example we may start with a standard adhesive and modify the formulation to improve adhesion. Basic polymer modified adhesives were developed in the late 1960s. We may start with a standard setting adhesive and develop to make it rapid setting, as in 1972.


We may need less water staining on a tile or need to reduce the risk of warping, so we use rapid drying technology. We may start with a standard adhesive but realise that we need more flexibility due to thermal expansion when tiling on to heated screeds. Ultimately, we may have even added all these elements, but we want to deliver fail safe technology and strength, so we add additional elements and develop unique fibre reinforced adhesive performance as in 2008. At this point it's useful to familiarise ourselves with the classifications within the European standards.


Adhesives are firstly characterised by type for example CD or R denoting cementitious dispersion or reaction resin. Then by their additional properties FTE.

  • F - Fast setting
  • T - Reduced slip
  • E - Extended open time.


Whether it be an adhesive or grout classification a level one product has normal characteristics and a level two product has additional characteristics. A C1 adhesive will have a minimum tensile strength of 0.5 newtons per millimetre squared; whereas, a C2 will reach a minimum tensile strength of 1 Newton per mm squared. In addition to this there are also other properties such as enhanced slip, faster setting or longer open time.


It's important to point out that S2 adhesives due to the fact that they are highly deformable are not recommended for heavily trafficked areas as stated in BS5385. We can provide further technical guidance when required.


C2 TE S1 adhesive equal cementitious adhesive with additional properties, reduced slip and extended open time, which is deformable. Quality products will reach and be classified to the EN standards however the EN classifications could only provide a guideline level of performance. Some products will meet and exceed the standards but are also formulated to provide specialist solutions for natural and agglomerated stone.


Some natural stone and agglomerate stone tiles such as those based on marble and limestone, are moisture sensitive and prone to water staining and warping. This is important to consider when specifying the grout or adhesive. This is important to consider when specifying the grout or adhesive, and the unique rapid dry technology range of adhesives, grouts and screeds, was developed to avoid this issue and cope with the demands of natural stone fixing. These are clever products easy to use and guaranteed to perform.


Water staining can be an issue with some stone tiles. Staining can sometimes be an issue with natural stone, and there could be many reasons which lead to it. It may be incorrect surface treatment or an inadequate cleaning regime or moisture from the fixing products or coming from the base. Unfortunately, we often see the effects of staining of stone tiles which is irreversible.


Here we can see an example of blob fixing, where normal cement adhesive has been used. The water from the adhesive has penetrated into this porous tile. We would never condone blob fixing as we explain later in this CPD. However, the problem of water staining into the tile can be avoided by the use of our rapid dry technology adhesives and grouts. Designed for use with natural stone. The rapid drive formula system binds the mix water with the adhesive or grout virtually eliminating the risk of water staining and guaranteeing the best possible finish to all natural stone applications. This rapid dry formula should not be confused with adhesives which are solely rapid setting or rapid hardening.


It is also important to remember that some stones such as marble may be translucent and that a grey based adhesive will have a tendency to show through the stone for translucent tiles, or if in any doubt specify a white product from our range.


Common Tiling Backgrounds

It is also important to understand the backgrounds we need to bond our stone tiles too and how they will impact on your specification choice of adhesive. Here we will review the most common backgrounds. Always specify that backgrounds are free from debris, clean, sound, dimensionally stable, important when considering expansion and shrinkage, which can be caused by the effects of changes in moisture content. For example, avoid wood based materials such as sheets and boards that expand or contract with changes in atmospheric humidity, as explained in BS5385.


Dimensionally stable is also important when considering movement under loads. A lack of rigidity of the supporting background will incur unacceptable deflections under loads and impacts. Stone tiling is basically a rigid finish and therefore requires rigid support. Background should also be level, dry, appropriate for the given conditions. When specifying onto boards and membranes always refer to the manufacturer’s instructions.


Always check for surface regularity as specified in BS5385. Often stones may be varying in thickness for example uncalibrated stones. The adhesive bed must be adequately prepared as part of the installation to compensate for these variations, so that the required surface regularity of the finished tiling may be achieved.


A range of smoothing compounds are available if thinner layers need to be made up. Any holes can be filled with a rapid repair mortar. As you need a flat base to tile on smoothing compounds are an ideal cost effective way to achieve this.


Some adhesives may be used for pre-smoothing although this is convenient it may not be the most cost effective method. Commonly there are two types two part latex or powder water based. Technical Support is available to provide detailed guidance. Many compounds can be pumped at a rate of 300 to 400 metres squared per hour, ideal for large projects.


Rapid drying and rapid hardening systems are available to help with your installation times. Traditional screeds require a minimum of three weeks prior to tiling. Background shrinkage and recommended drying times can cause practical timing problems on site. With this in mind a number of rapid drying and rapid hardening screeds and renders have been formulated. Bonded screed rapid hardening screeds will allow installation after three hours. Concrete base must have finished shrinking six weeks minimum.


Unbounded screed rapid drying screeds will allow installation after 24 hours, even if the concrete base is still shrinking.


Floating screed rapid drying screeds will allow installation after 24 hours, even if the concrete base is still shrinking.


Note that some high performance screeds are only appropriate in dry locations. Rapid hardening renders will also allow tiling after just 2 hours.


Not all adhesives are suitable for very thick bed applications and uncalibrated natural stone can require this. That's why rapid drying and hardening natural stone adhesives were formulated for bed thicknesses of up to 30mm and for translucent tiles. These products can be granted just three hours after fixing. Details on this and all the unique rapid dry products are available on the Ardex CPD Academy. www.adexcpdacademy.com.


It's always important to assess the key considerations relating to the background, such as: 

  1. What's the porosity of the background?
  2. Is the background prone to movement for example dimensional stability, expansion and shrinkage, loading?
  3. Is the background able to support the weight of tiling?
  4. What’s the surface condition of the background?


BS 8000 workmanship on building sites, which is currently under review contains useful information on drying times of backgrounds, surface regularity of backgrounds and the install tiling, as well as differences in height either side of tile joints.


Full guidance on backgrounds and drying times is covered in our CPD seminar, Specifying Ceramic Tiling with Confidence. Available on the Ardex CPD Academy www.ardexcpdacademy.com.


Some natural stone tiles are heavy and it's important to check that the substrate to be tiled is dimensionally stable, and able to support the weight of tiling. There are some specific recommended weights for wall tiling as detailed here. The ideal substrate for tiling is render, concrete or concrete block work, for other backgrounds guidance is given to weight limits as follows.


The maximum weight of tiling which can be supported by a dry well adhered gypsum plaster background, is 20 kilos per metre squared. That is equivalent to ceramic tiles with a maximum thickness of 8mm plus tile adhesive, or natural stone tiles with a maximum thickness of 7mm plus tile adhesive. The weight of tiling directly where plasterboard background should not exceed 32 kilos per metre squared. That is equivalent to ceramic tile and adhesive with a maximum thickness of 12.5mm and natural stone and adhesive with a maximum thickness of 10mm, hence in general it's always best not to skim plaster that will be tiled.


Specifying Tile Grouts For Natural Stone

Specifying tile grout is the final most visible part of the fixing process and it's important to get it right. Beautiful stone tiles will look fantastic when grouted well and with the right products. As with all tile adhesive selection, it's equally important to consider the parameters of the tile and the finished environment. Although dispersion already mixed crowd tend to be popular within the DIY sector, professionals will use a cementitious or reaction resin system due to a number of issues related to ready mix grouts. A highly polymer modified grout may be required. Nowadays it's more likely that the flexible polymers are built in or added as a bottle admix. However, in the case of natural stone it's also important to consider the water absorption levels of the natural stone. Remember a natural stone tile may be water sensitive, so a rapid dry technology grout should be selected from the range. A range of grouts has been specifically developed for natural stone.


The two main types of grout are cementitious or reaction resin. Both types of grout are specified, however the most commonly used are cement based. Reaction resin grouts are usually epoxide based and used in specialist areas, such as swimming pool surrounds and specialist industrial locations. However, we continue to invest our resources in research and development to innovate and invent new technologies. We have a number of exciting innovations in the pipeline which we will be able to discuss in more detail in the near future.


Unfortunately, we often see the effects of staining of stone tiles. Here we can see an example of the water staining where normal cement grout has been used. The water from the grout has penetrated into the edges of this porous stone tile, causing a picture framing effect. Again our rapid dry formula system will bind the mix water with the grout. Virtually eliminating the risk of water staining and guaranteeing the best possible finish to all natural stone applications. You will likely need a rapid drying marble and natural stone grout available in a shade which is formulated to complement most natural stones, and virtually eliminates the risk of water staining. One such grout on the range is also able to be walked on after just 90 minutes.


Also note that the use of inappropriate movement joint sealants can lead to edge staining. Specially formulated sealants are available for use with moisture sensitive stones. Always use a rapid dry grout to avoid the risk of staining. Stone tiles should be carefully checked for grout retention on the surface, especially if a pigmented or contrasting coloured grout is used. It's advisable to trial a small area to begin with. Some natural stones will require sealing prior to grouting.


In some cases, applying the grout to the whole surface of the stone tiles can result in a more uniform appearance, butt jointing of tiles is not recommended. Consider movement implications and where necessary the adhesives ability to lose water through the tile joints to form an adequate bond.


BS5385 recommends the following minimum grout joints for tiles, wall 2mm, floor 3mm. However, it's important for us to recognise the grout joints these days are much thinner than the British standard recommendation. If this is the requirement for specification, then it's essential that backgrounds and bases are dimensionally stable with no thermal movement.


Finer aggregate grouts are suitable for normal joint widths 1 to 5mm and the coarser great aggregates from 3 to 10mm. Of course, for tiles prone to scratching, the grout should not contain hard aggregates.


There are grouts formulated specially for natural stone. To avoid possible picture framing where the edges of the tiles are stained by water from the grout penetrating the surface, specify a rapid drying product from the range which is specially designed for this purpose.


Through investment in R&D we have designed grout formulations, which retain their colour integrity and, through regulated production processes, raw materials and quality control systems we are able to provide excellent colour consistency across the range.


These grounds are rapid drying, available in stone shades. Durable, walkable after only 90 minutes. Does not contain hard aggregates risk of scratching softer stone tiles is minimised. Remember when a useful tool when specifying is a grout colour selector which clearly demonstrates the finished colour.


Installation Best Practice

Now we've covered some of the key considerations prior to commencing tiling. Let's take a look at installation best practice. Firstly, will cover movement joints. Movement joints should be situated over existing structural movement joints. Where tiling abuts other materials, where tiling is continuous across junctions of other background materials, in large tiled areas, at internal vertical corners, where stresses are likely to be concentrated.


All movement joints should be incorporated in line with BS5385. However, it's important at the design stage to consider the impact of the movement joints on the appearance of the finished ceramic tiling. With water sensitive stones a specialist sealant may be required.


Let's now review the adhesive application, the size of notched trowel chosen depends on the surface being tiled, the profile on the back of the tile and a degree of coverage required. For tiling walls in dry internal conditions, the adhesive contact should be a minimum of 50% of the back of the tile, ideally 75%. However, when tiling to floors all external locations and wet areas such as showers and swimming pools, solid bed fixing should be achieved with 100% coverage to the back of the tile. It's advisable to lift a tile now and again to check solid bed fixing is achieved. Ensure that the adhesive has been applied in ribs and the back of the tile buttered with a thin layer of adhesive, before it is firmly pressed in place with a twisting and sliding action. Within the open time of the adhesive prior to a skin forming. This method of fixing allows full contact of the adhesive with the back of the tile and substrate.


One method of achieving solid bed fixing is to butter the back of the tile. For floor tiling however with pourable adhesives this is not necessary.


There is a trend towards very large format natural stone floor tiles with varied thicknesses. In order to achieve a solid thick bed under these tiles pourable adhesive has been introduced within the rapid dry range. To help succeed in solid bed fixing without the need to butter the back of the tile, particularly when rapid drying these can effectively speed the installation time.


Here we see an example of how solid bed fixing can be achieved by using modern pourable floor tile adhesives. A lack of solid bed fixing can leave the installation prone to impact damage in the future or cracking tiles.


We would never condone blob fixing of tiles. It often leads to cracking and delimination, as the tiles are subject to a lack of support and differential stresses.


Onsite Support

Having researched and produced the specification and contractors guidance, once the project is underway site visits are essential.

  • Monitor best practice to ensure that your specification is followed.
  • Future trip hazards, cracked tiles etc are avoided.
  • Movement joints are incorporated.
  • The background is specified.
  • The tiles are as specified.
  • The adhesive installation is tiled with solid bed fixing using the advice trowels and specified products.
  • The grout joints are as specified and adequately filled with the specified products and well cleaned off.
  • The contractor is aware of COSHH and health and safety on site.
  • A cleaning and maintenance regime is observed, especially when specifying natural stone.


Tile aftercare is important for natural stones. Many stones will require sealing or waxing following installation. This process can benefit slip resistance, colour enhancement and restore a polished effect. Impregnating sealers will reduce water permeability.


In Summary

It's important to bear in mind that the majority of quality natural stones do not give rise to problems associated with fixing natural stones. However, it's often difficult to identify the type of stone, so it's advisable to air on the side of caution and specify a rapid dry product or seek further technical advice.


In summary, we've mentioned some key points throughout the seminar:

  • For installation, solid bed fixing essential, check the appropriate thickness of the adhesive bed.
  • Regarding the adhesive, specify rapid dry technology and specify white adhesives for translucent stones.
  • Regarding the grouting, specify rapid dry technology. Use the grout colours and swatch samples available.
  • Regarding sealing, most natural stones will require sealing, use specific silicone sealants designed for natural stone.
  • Natural stone tiles can create a perfect finish for most environments, of course when they are specified and installed with guaranteed high performance fixing systems.
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