Nick Bratt, National Technical Sales Manager for tile adhesive manufacturer Palace Chemicals, outlines the importance of the correct use of materials and good practice, as defined in the relevant British Standards, when it comes to tanking a bathroom or wetroom.
With the continuing growth in popularity of wetrooms, we are seeing a corresponding increase in incidents where the installation has not been satisfactory, and often this is caused by water or moisture getting into the substrate behind the tiles. These issues are very difficult to solve post-installation but can be easily prevented at the design stage by following the guidance contained within the British Standard relating to tiling.
BS 5385-1 2018 states that for all new or refurbished properties, a proprietary tanking system should be used. BS 5385-3, concerning floor tiling, was updated in 2024. One of the key amendments is for direct tiling on to plywood and other wood-based sheets or boards, an intermediate layer, such as an uncoupling membrane, reinforced tanking system or tile backer board should be used, providing the plywood is rigid and stable enough to carry the expected in service load.
The new standard also states that timber is not recommended as a suitable substrate for floor tiling in wet, frequently damp, or high humidity areas. For wetroom floors, they should be laid to falls with gradients of between 180° and 135° to ensure that the water flows towards the a drain, taking note that a gradient any greater than 135° is unsuitable and possibly dangerous.
This requirement impacts on the design of wetrooms in particular since they are subject to repeated and persistent wetting, and a suitable proprietary tanking system should be used prior to tiling. This should be laid continuously around upstands and points where services pass through the floor, to create a seamless seal, so that there is no risk of water leaking through to the substrate.
If using other intermediate substrates, such as moisture resistant boards, the manufacturer’s technical data and/or the manufacturer itself should always be consulted, as these would usually state that a tanking system should be used. The recent updated release from NHBC has sparked some controversy due to the wording around flow rates of a shower and moisture resistant boards, as these contravene the British Standards guidance for tanking.
For walls which surround a bath where a shower is not fitted then the substrate should be suitable for the application and moisture resistant.
It is important to recognise that water resistant adhesives and grouts are designed to allow water to pass through them without them breaking down, hence ‘water resistant’, but are not a substitute for a waterproof tanking system, as clearly stated in BS 5385 Part 4.
If in doubt, tank it out! That way, any and every project that has wet, frequently damp, or high humidity areas will always have the added insurance policy that there will not be any leaks!