Why do pool tiles and grout stain and deteriorate?

At a glance

The common causes, and what each one typically looks like on a pool:

• Low pH or aggressive water: soft, sandy grout sitting below the tile face.
• High pH or hard water: white or grey scale, heaviest at the waterline.
• Metals in the water: orange, brown, blue-green or purple-black stains, often after the pool has been shocked.
• Sulphate attack: eroding grout and a white cloud in the water when joints are brushed.
• High chlorine or bromine: faded or patchy grout colour and weakened joints.
• Salt: white deposits on the surround and grout loss near the waterline.
• Unsuitable acids or abrasive cleaning: dull, rough or etched tiles.
• Sunlight and UV: faded grout, faster scale build-up and more algae.

pH and water balance

pH shows how acidic or alkaline the water is. Published guidance for tiled pools commonly gives an ideal range of about 7.2 to 7.8, but the exact target is set by the pool's water treatment specialist.

pH works together with alkalinity, calcium hardness and temperature. Together these decide whether the water is balanced, which is often checked using the Langelier saturation index (LSI).

• pH too low (aggressive water): the water dissolves calcium, so it slowly draws it out of cement grout and render. Joints become soft, sandy and recessed. It can also corrode heaters and metal fittings, releasing metals that later stain the tiles.
• pH too high (scale-forming water): calcium comes out of the water and forms scale on tiles and grout. Dissolved metals also stain surfaces faster.

Scale

Scale forms when water holds more calcium than it can keep dissolved. Evaporation concentrates the minerals, so scale is usually worst at the waterline and on the surround, where water is splashed and left to dry.

There are two main types. Calcium carbonate is white and flaky, fizzes when acid touches it and is relatively easy to remove. Calcium silicate is grey-white, does not fizz and is much harder to remove. Knowing which one is present helps choose a safe removal method.

Metal staining

Iron, copper and manganese can be dissolved in pool water without being visible. When chlorine reacts with them, they come out of the water and settle on the tiles and grout. This often shows up after the pool has been shocked, and it happens faster when pH is high.

• Iron: orange, brown or red.
• Copper: blue, green or turquoise.
• Manganese: purple or black.
• Common sources: fill water, corroding heaters and fittings, copper-based algaecides and copper ionisers.

Sulphate attack

Sulphates build up in pool water when sulphate-based chemicals are used, such as sodium bisulphate (dry acid) to lower pH or aluminium sulphate as a coagulant. PWTAG has documented a growing number of pool problems linked to them.

Over time, sulphate weakens cement-based grout from within. The first sign is usually grout eroding, with a white cloud in the water when joints are brushed. BS 5385-4 sets a maximum sulphate level for tiled pools, and epoxy grout is often used where sulphate is a risk.

Chlorine, bromine, salt and magnesium

• Chlorine: very high levels can harm cement-based materials and bleach coloured grout, so joints above and below the waterline can end up different shades.
• Bromine: often used in spas, and very aggressive to grouts and adhesives.
• Salt: in salt-chlorinated pools, salt is left in the pores as water dries. As it repeatedly dissolves and crystallises, it can cause white deposits, grout loss near the waterline, flaking natural stone and corroded fittings.
• Magnesium: in mineral pools, magnesium can replace calcium in cement grout and leave it powdery, so epoxy grout and adhesive are recommended.

Cleaning acids and abrasion

The wrong cleaning product or method can cause damage that cannot be undone.

Acid cleaners are widely used on pools and are generally suitable for porcelain and ceramic tiles when correctly diluted, tested on a small area and thoroughly rinsed. However, some tile manufacturers warn that certain acids, including hydrofluoric, hydrochloric and phosphoric acid, can damage particular glazes and glass tiles. Acid should never be used on natural stone. Once a glaze has been etched, it cannot be restored.

Abrasion does similar damage. Pumice stones, wire brushes and heavy scrubbing can leave tiles dull and rough. Every method should be tested first and matched to both the deposit and the tile.

UV and sunlight

UV affects the colour and strength of grouts and other polymer-based products, and can fade pigments over time. This applies to outdoor pools and also to indoor pools with large glazed areas or rooflights, because standard glass and many clear roof materials let part of the UV spectrum through.

Sunlight also speeds up evaporation, so scale and residues build up faster, and it encourages algae. Where a pool gets a lot of direct sun, check the manufacturer's data for UV and colour stability when choosing grout and sealant.

Why some staining becomes permanent

A sound glaze is dense and smooth, so most deposits sit on top and can be removed with the right method. Once a tile's face has been etched, worn or attacked, it becomes rougher and more open. Scale, metals and residues then build up in the damaged surface and become ingrained, especially in sunny areas.

At that stage even specialist cleaning may not fully remove the staining, and replacing the affected tiles may be the only way to restore the finish. Balanced water, careful cleaning and dealing with deposits early are the best prevention.

How we help

Water treatment is the responsibility of the pool operator and their water treatment specialist, working to PWTAG guidance. Our role is the tiles and grout.

Every pool is different, so we start with an assessment. We identify the likely cause, test cleaning methods on a small area, and carry out chemical cleaning, repairs and regrouting. Where water chemistry looks to be a factor, we explain what we have found so it can be discussed with your water treatment specialist.

Common questions