Home and dry


Andrew Scanlon of Everbuild answers some of the basement waterproofing queries asked by self-builders to help them avoid getting out of their depth in their projects

Basement conversions have long been a popular way to increase the usable space in a home, used for everything from living rooms and home offices, to kitchens and even bedrooms. Although different projects will require different approaches, one important factor to consider is how you are going to protect your new subterranean space from possible water damage and the associated problems of mould and dampness. Here are some answers to some of the key questions.


It’s important to remember that by their very nature, basements are built below the ground and are at risk of water ingress and becoming damp. The most common source of water in basements is seepage from the groundwater held in the soil around the foundations of your home, although other factors such as pipes and a high water table can put some projects at a greater risk than others. That’s why it’s important to seek expert advice before you start work to get a better idea of what you are dealing with. For the majority of projects though, choosing a high-performance waterproofing treatment can provide all the protection you need.


Waterproofing covers lots of different methods such as cavity drainage systems, but the term ‘tanking’ is most commonly used when referring to basement or cellar conversions. The tanking process involves applying a special waterproofing ‘slurry’ coating to prevent exterior water from penetrating the walls and floors. It’s important that the whole room is treated, not just the area where there is existing evidence of water damage as this will not stop the problem – water will enter the fabric of a building any way – and anywhere – it can, unless every possible route is sealed off. Tanking is an effective way of preventing water ingress into a property as, as the term suggests, it can create an impenetrable, watertight barrier.


Tanking slurry prevents water droplets from seeping through the walls and floors into the basement by forming insoluble crystals which water can’t pass through. There are many different tanking products available on the market, but the best ones are those that offer flexibility and long-lasting protection. Flexibility is important as it needs to be able to cope with both negative and positive water pressure to act as a vapour permeable layer which will still allow the substrate to ‘breathe’ while preventing water from being able to seep through.


As with any building work, preparation is one of the most important parts of the job. Before you apply any waterproofing coatings, you should ensure that the surface is clean and free of any flaking paint, dirt or debris. It’s also important that any cracks or holes are filled prior to treatment as these will otherwise become weak spots. These can be treated with a suitable waterproof compound.

The point where the floor and wall meet also needs special attention. Here, it’s recommended that you create a triangular ‘fillet’ at the intersection and seal it using a special durable plugging compound which will expand as it cures to form a permanent watertight seal with similar characteristics to concrete.


It’s a good idea to choose a product that can be brush or spray applied so you can quickly and easily coat larger areas, and one that can be applied to areas that are already damp. It’s important to cover the area thoroughly, so look for a product that is available in a choice of colours, (usually grey and white). Using different colours for each coat will make it easier to see any parts that have been missed, helping make sure you have applied the coating evenly across the whole area.


If you’re looking to save time and money, choose a product that can be used as a final decorative finish too. If you’re planning on using render or plaster over tanking slurry, it’s essential to refer to the manufacturer’s instructions, but it’s advisable to opt for cement based rather than gypsum plaster. As gypsum has hygroscopic properties (meaning it absorbs and retains water), it can fail in damp environments and facilitate the spread of mould. If dry lining is preferred, you should never use mechanical fixings that could penetrate and damage the tanking layer but instead, use a suitable dry ‘dot and dab’ adhesive.


Tanking is a project commonly seen within self-builds, but one that can seem a little daunting, so before you start it’s a good idea to do your research. Most product manufacturers have lots of useful information online, including application advice. Choosing complimentary products from one supplier is also a good way to ensure that all the different elements of the tanking process work effectively together.

Andrew Scanlon is applications manager at Everbuild (a Sika company)