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Using stone successfully with swimming pools

Stone makes a fine companion to swimming pool areas but it needs some consideration.

Stone Federation Great Britain has published a guide to the use of natural stone in swimming pool areas. Here, the author, Barry Hunt, highlights some of the ways of successfully using stone in swimming pool areas.

A significant demand has developed for natural stone to provide the decorative finishes to swimming pools and their surroundings. However, this environment presents some of the highest threats in both domestic and commercial settings. Slipping, tripping and stumbling are the principal threats in the critical pool apron area where a minor accident can lead to major consequences if a person pitches into the pool. Therefore, it is no wonder that the design, installation and maintenance responsibilities are fraught with difficult decisions.

When it comes to the use of natural stone, such decisions are more difficult given the apparent lack of available strong guidance. Hence the publication of a new Guide by Stone Federation Great Britain, Natural Stone in Swimming Pools – its selection, design, use and maintenance.

The water used in swimming pools is subject to human interaction that requires it to be filtered and treated with chemicals – processes that can place a toll on natural stone finishes.

Yet most pools are built without any knowledge of how filtration systems and the chemicals deployed with them will interact with the stone and the ancillary materials used to fix it.

Those providing the water systems will say the stone is nothing to do with them, while those supplying the stone will say the chemicals are nothing to do with them.

So, what is a designer to do?

This is where Stone Federation Great Britain has stepped in to break the deadlock and produced a guide on the use of natural stone in and around swimming pool locations. The advice provided is based on wide ranging and typically successful experience in the use of natural stone for swimming pool installations. Advice has also been sought from specialists in water treatment systems.

British Standards relating to the design, supply and installation of natural stone have been invoked where applicable, incorporating requirements of current good practice.

The new Guide has also taken advice from existing Stone Federation guides, international sources of information and current health and safety legislation.

The new Guide discusses stone selection, design, installation, protection and maintenance, all of which is far from straightforward given the many different types of stone available.

The emphasis of the advice is on how to avoid what appear to be the most common errors – aesthetic disappointment or outright failure of the stone installation.

Types of natural stone

The Guide has sought to lessen the potential impact of the generalisation of stone types when providing advice on stone performance. For example, ‘granite’ is a term that the trade applies to almost every igneous rock type and thus is potentially misleading because there are different types of igneous rock. A stone such as a dolerite will have a completely different set of minerals of different size and with different reaction potentials to a true granite such as DeLank from Cornwall. 

In the Guide, igneous, sedimentary and metamorphic stones have each been split into five categories. The pros and cons of the various stone types are discussed in relation to the mineralogy, chemistry, crystal grain size, interlock and other features.

Potentially, the most important aspect is the presence of voids, higher porosity, or some other feature that increases water absorption and especially the potential for water to flow through a stone.

Types of pools and usage

There are many important considerations which depend on the type and use of a pool, from natural pools to specialist teaching pools. In the Guide, the principal types of pools are introduced and discussed. In some instances it is found that the use of stone may be inappropriate, which is an important consideration in its own right.

The design of a pool requires minimal dimensions where diving occurs, space allowance per swimmer, accommodation for the pool to drain but with a slope that does not cause people using the pool to lose  balance, and many ancillary issues such as plant access, changing areas, wind breaks, storage and sunshine that have to be considered.

There is also a wide range of potential influences that may have an impact on the use of natural stone, such as impact from sub-aqua equipment, disabled facilities and items such as moveable floors and wave machines.

Unfortunately, it is difficult to provide solid advice on such issues as every situation appears to be unique. So they remain as a set a considerations.

Water treatments and balancing

The treatment of water is governed by two main bodies: Swimming Pools Allied Trades Association (SPATA) and Pool Water Treatment Advisory Group (PWTAG).  Between them they have produced a number of standards and codes of practice, helping to detail how pools of different sizes and uses should be treated.

The SF Guide draws on this advice to explain how water is balanced so it is comfortable to swim in, is non-corrosive and non-scaling. Water correctly balanced is not just healthy for pool users, it should also benefit the various materials used in its construction by preventing corrosion and decay.

With use, pool water wants to become more acidic, which creates a continuing battle to keep water slightly alkaline while preventing the build-up of dissolved salts. 

The water balance is affected by temperature, pH, total alkalinity, calcium hardness and the total dissolved solids – and there are limits for all these parameters.

For example, the ideal situation is water with a pH between 7.3 and 7.5 with an absolute ideal of 7.4 (slightly alkaline). A value of 7.2 is considered to be the lowest pH that should be exhibited and 7.8 the highest.

The Guide provides many other limits and discusses the chemicals used and the Langelier Saturation Index employed to assess the quality of water.

Principal pool systems

There are four principal swimming pool systems currently used that have different potential effects on natural stone: ionic purification, active oxygen, ozone and hydrogen peroxide.

These systems are briefly explained and their general performance in relation to natural stone is assessed in the Guide. 

Experience suggests that the most effective systems where natural stone has been used are the active oxygen process and hydrogen peroxide.

Most problems with swimming pool systems manifest within the first six months or when there has been a change of user.

Problems with stonework occur primarily through either direct or indirect contact with water, regardless of the pool system being used. 

Externally, frost is the greatest issue and it is how water is managed that is the key to good performance.

The concentration of salts where drying occurs can lead to disruption and staining is another key consideration. However, staining may sometimes occur as a direct consequence of the chemicals being used and metals, particularly copper, coming out of solution.

Local environmental differences

There are five environmental zones in and surrounding a swimming pool considered in the Guide. They are:

  • The Osmosis Zone, the area that is constantly immersed unless the pool is drained. This can experience an apparently spontaneous growth of crystals.
  • The Capillary Rise Zone, immediately above the water line where water along with dissolved salts may be transported via natural capillaries between the constituent grains and / or crystals, and is where staining and discolouration may be particularly strong. The Capillary Rise Zone may also be the zone most affected by frost and salt attack, warranting the selection of more robust materials at this point.
  • Moving away from the pool itself there is the Splash Zone, where splashed water from the pool should be taken away from the pool edge by positive falls so that it does not wash back into the pool and bring with it dirt and other materials.  This region of random cyclic wetting and drying may be subject to increased salts precipitation, while the quenching of hot stone surfaces may increase surface weathering. Importantly, changes from dry to wet may increase the potential for slipping and stumbling, so careful consideration of the changes in performance between wet and dry surfaces is required. The Guide suggests some limits.
  • The Environment Zone is that part of the stone construction surrounding a swimming pool. This will behave in the way that is expected of either internal or external flooring and paving design. 
  • Beyond this we have the Feeder Zone, the area that is outside the natural stone construction that may be influencing stone performance.

Hydrostatic pressure

Imbalances of hydrostatic pressure are believed to be the principal cause of finishes suffering detachment, usually by popping. 

When a pool is filled with water it exerts a downwards pressure on the floor and lateral pressure on the walls. Where the substrate materials become saturated, this moisture becomes pressurised. When the pool water is removed suddenly there is no confining pressure to the pressurised substrate materials and if this internal pressure cannot be released it pushes against the surface finish.

Hydrostatic pressure build up is unavoidable and therefore any solutions to the problems created need to seek to minimise the potential effects.

The factors involved in hydrostatic pressure build up and how to relieve it are discussed in the Guide, however, this can lead to some designs that may create new problems.

Stone performance considerations

There can be a belief that the swimming pool water will ‘catch’ or ‘save’ you if you fall in while running around a swimming pool. For good reason, therefore, there is a well-known set of rules prohibiting such activities. Unfortunately, as much as it is hoped that rules are adhered to, it is predictable that they will not be. Swimming pools should, therefore, be designed, wherever possible, to minimise the potential risks.

There are no performance requirements for the use of natural stone in and around swimming pools. However, there are some performance requirements for external natural stone pavements in BS 7533 that could be applied to pool surrounds, whether internal or external.

The advice is relatively simple and has been used as a basis for discussion in trying to identify those properties and levels of performance that might be sought from the use of natural stone.

Whatever the environment of use, slip resistance is of prime importance, followed by abrasion resistance and resistance to salt crystallisation and chemical attack, especially that leading to discolouration and staining issues.

The effects of frost also need to be taken into account for external environments. Maximum and minimum values are offered for a range of tests based on current standards and health and safety requirements. These differ for different areas around the pool.

The application of treatments, usually applied to stone surfaces, is a major subject for discussion in the Guide because of the numerous products on the market which have minimal research backing. The scarcity of appropriate tests to back up the many claims that are made is disconcerting.

Wherever a treatment is applied, there remains the risk of it being circumvented in some way, resulting in the trapping of moisture, concentration of salts and other effects that may result in enhanced deterioration and, in the worst cases, premature failure.

The Guide also considers properties such as albedo (how bright the stone is when reflecting sunlight). This is important in external locations because a highly reflective surface that reduces vision due to glare must be considered a potential danger. Then there is thermal absorption, which could heat stones to temperatures that make them impossible to walk on in bare feet. The Guide offers potential solutions to these problems.

Ancillary materials used in and around swimming pools

This section of the Guide discusses the materials used directly in combination with the natural stone in swimming pool environments, notably adhesives, grouts, sealants, treatments and fixings.

It is not intended to include the many items that are found in and around pools (lighting, ladders, covers and so on). This section of the Guide has the potential for considerable expansion in the future as new products are developed to deal with the many issues presented by swimming pool environments.

Assessing stone for use in and around swimming pools

All natural stone for use in and around swimming pools should have been tested to British and European Standards for tiles, slabs, pavers, setts, kerbs and / or cladding. 

As already stated, there is no performance requirement document for natural stone used in and around swimming pools, so there is no guidance on how the various test results should be applied to swimming pool environments in order to be sure a given stone will perform adequately. In reality, each stone must be individually scrutinised and assessed within the design being presented.

This section of the Guide discusses the tests that might be most useful when trying to assess natural stone performance – for example the thermal shock test – which could assess the effects of quenching of hot stone when splashed.

It is hoped the suggested tests and limits applied can be refined as more information becomes available regarding performance, good or bad.

Installation – principal considerations

At the end of the Guide there is a series of summary recommendations for a range of situations within the different pool zones and employing different designs.

These recommendations are based on the various discussions presented in the Guide in the hope they will help minimise the potential for any problems.

In conclusion

Many of the issues relating to natural stone are just as applicable to other materials, which are not necessarily any less potentially problematic.

The very real advantages of using natural stone in a pool area should not be overlooked. Unlike many artificial materials, stone can be cut, shaped and textured.  Curved surfaces can be created rather than resorting to mosaic work, which is more susceptible to potential moisture flow to the backing materials. Natural stone is often easier to replace with a matching material than ceramic and other materials that can go out of production when they are replaced by next season’s new lines. And, of course, natural stone has a great variety of choice and an unmatched aesthetic.

The Guide covers a lot of material but it is only scratching the surface of best practice for swimming pool construction incorporating natural stone finishes.

It has not attempted to provide typical drawn details because there are just too many potential variations available.

This is very much the beginning of something that will, no doubt, evolve with future editions. Better understanding should allow the correct recognition of issues arising and the best ways both to deal with them and (ideally) how to avoid them in the first place.

The Guide is a welcome addition to the documents that Stone Federation Great Britain has developed for natural stone use.