Choosing natural stone for swimming pools – what you need to know

Architects and garden designers are fielding more pool project enquiries than they were even just a few years ago, and demand is rising. Google Trends data shows searches for home pools have risen 184% year on year, and my team’s recent experience at UK pool and spa exhibitions suggests more homeowners and developers are investing in wellness-led outdoor spaces and integrated pool designs.

But, while the aesthetic ambitions are rising, the technical demands of specifying natural stone for pool environments are frequently underestimated – sometimes with costly consequences.

The Stone Federation of Great Britain’s (SFGB) guide to natural stone in swimming-pool environments set out a clear framework in 2024. Its central message remains highly relevant: a swimming pool is not just a challenging environment for stone, it is one of the most chemically, thermally and physically aggressive environments that natural stone will ever encounter. The reality on live projects today is that many designs still ignore these fundamentals.

The four zones

The SFGB guide identifies four distinct zones on any pool project, each presenting its own risks.

The underwater zone is relatively stable in use, but when a pool is drained, hydrostatic pressure from groundwater can cause debonding, a failure mode that still surprises specifiers and clients.

The capillary rise zone, just above the waterline, is the most aggressive. Dissolved salts are drawn upward by evaporation and can migrate up to two metres, causing staining and long-term surface damage.

The splash zone experiences cyclic wetting and drying, with salt precipitation accelerating surface degradation.

The peripheral zone, meanwhile – the poolside surround – must withstand frost, chemical splash, drainage loading and consistent foot traffic.

This zone framework is something we walk through with every specifier we work with. The mistakes tend to happen when the stone is chosen for how it looks in isolation, without thinking about where in the pool environment it is going. A stone that is perfect for the surround might be entirely wrong for a water-line feature, and vice versa.

Stone suitability – not all options are equal

The SFGB guidance also rates stone types for suitability across different pool settings. Marble, granite, quartzite and slate are rated A, meaning suitable for all situations.

Limestone, one of the most popular choices on high-end pool projects, varies significantly. Some limestones are rated A, others as low as D, depending on their porosity and mineral composition. On the other hand, sandstone, frequently requested by clients drawn to its warmth and texture, is rated E, meaning it is likely to be difficult in pool environments.

This variation is rarely visible from a sample board alone – and it is the kind of knowledge that separates a specialist supplier from a generalist one.

Slip resistance, chemical compatibility and low-energy systems

For poolside surfaces, the SFGB guide recommends a minimum pendulum test value (PTV) of 40, a safety threshold that has become even more important as designers push for sleeker finishes and darker stones on contemporary pools. The finish of the stone, whether honed, flamed, tumbled or bush hammered, directly determines whether that threshold can be achieved.

On chemical compatibility, the guide is equally clear. Pool chemicals can react with both stone and fixing systems, and epoxy resin grout is specifically recommended for pool environments due to its acid resistance and impermeability. White cementitious grout, a common default, is explicitly not recommended.

For external UK pools, stone must also pass freeze–thaw resistance testing to BS EN 12371 over a minimum of 56 cycles.

Many pool projects now include energy-efficient heating systems, such as air-source heat pumps, and the refurbishment of existing pools where structures are being retained. These choices can change the temperature profile, water chemistry and maintenance regime of the pool, and integrating stone specification with these newer elements is part of what makes the overall package so technically demanding.

These are not edge case concerns; they are now standard conditions on virtually every UK outdoor pool project. The grout alone is something that as specialists we have to correct on a surprising number of incoming enquiries. It is one of the first questions we ask, because getting it wrong is expensive to put right.

In the face of such complexity, a single point of specialist expertise across specification, fabrication and compliance can be very useful to designers.

Why designers are turning to specialists

For practices under pressure to deliver visually ambitious, wellness focussed outdoor spaces, the message is simple: the design story may start with the pool, but the technical story begins with the stone – and getting specification of that is right is no longer optional.