Beyond storing the odd bottle of wine and an old tin of paint, cellars are often damp and unusable. If you want to make full use of your cellar or basement, you’ll need to keep it dry. Tanking systems provide an impermeable waterproofing coating to the walls and floor, helping to keep them nice and dry.
When it comes to waterproofing a basement with damp walls, you have two main options:
1) Install a cavity drainage system
2) Basement tanking
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What is basement tanking?
The term tanking has come to refer to below ground damp proofing where you form an impervious tanking in the basement to prevent water penetration. Cellar tanking or basement tanking refers to the application of a liquid waterproof coating (tanking slurry) to the walls and floor of a cellar. It’s used to treat damp walls by preventing water ingress and effectively making the walls permanently watertight.
Damp walls are common in cellars because they’re below ground. Water from the retaining earth can make its way through the walls and into the cellar. When this moisture passes through the wall, it can also carry salts and minerals with it. Damp walls are cold which creates issues with condensation and in turn, mould. Tanking slurry is designed to deal with these issues and is applied directly to damp walls. When the tanking slurry cures, it forms an impermeable waterproof barrier, preventing water ingress and damp.
It’s important to point out that unlike a cavity drain system, tanking slurry blocks water from entering your cellar rather than allowing it in and controlling it to an evacuation point.
The most effective way of tanking your basement or cellar is with tanking slurry. This is a specially formulated mixture designed to be applied to cellar walls to stop water ingress. It either comes pre-mixed or as a powder to be mixed on-site with clean water.
How does tanking slurry work?
Tanking slurries are a special blend of Portland cements, aggregates and chemical modifiers that work together to block the passage of water. Most tanking slurry contains an additional acrylic polymer that improves strength, bonding and abrasion resistance.
Tanking cellar walls
When it comes to tanking wet or damp walls, you need to prepare thoroughly before applying the tanking slurry. There are several steps you should take to minimise the risk of the tanking failing. We go into more detail about this below.
How to tank your cellar with tanking slurry
The key is to prepare the masonry surfaces thoroughly. In older homes, this can take a long time but preparation is crucial to a watertight system and well worth the effort.
The first job is to ensure that you can get to the full surface of all the walls (removing all shelves and other items) and that the floor is also clear. Then you need to hack off any plaster and render from the cellar walls along with any previous coatings (bitumen, paints etc) right back to the original masonry. Rake out any old mortar joints and any other loose material before finally making sure the walls are dust free.
Note: for tanking to work, the brickwork needs to be stable so that it can cope with the build-up of water pressure.
Once you’ve removed all materials from the walls, check if there’s any active water seeping from the wall. If there’s evidence of water seepage, you’ll need to tap off the water pressure where necessary. When tanking a cellar, we always suggest that the walls and floor are coated to create a waterproof box. The point where the wall and floor join can be a potential weak point in any system, we therefore recommend creating a reinforced joint.
How to create a wall floor junction or fillet joint
At the wall floor joint, chase out the floor to a minimum of 20mm x 20mm, cutting into the wall if possible. Flush out the chase and remove any debris. While this is still damp, apply one coat of tanking slurry 100mm up the wall and 100mm across the floor. While the slurry is still tacky, apply Fillet Seal over the tanking slurry and into the chase, creating a cove from the wall to the floor.
Mixing the tanking slurry
Most tanking slurries come as a powder and require mixing on site. Tanking has to be used within 30 -45 minutes once mixed as the product will become unworkable and will have to be disposed of.
Applying the cellar tanking
Cellar tanking products are designed for application onto damp substrates. If cellar walls are relatively dry, you should wet out the substrate fully with clean water, making sure it’s damp but with no standing or surface water before applying the tanking. Tanking slurry requires at least two coats. The first coat should be applied directly to the masonry, brick or concrete surface by brush in a horizontal direction. You should go down and across the wall floor joint and apply the slurry 100mm onto the floor.
Walls should be ready for a second coat in 2 – 24 hours. Before applying the next coat, you want the slurry to be touch dry and able to support a second coat without pulling off. The second coat should be applied over the top, using vertical strokes this time. The wall floor joint should also receive a second coat.
Tanking a cellar floor
It’s important to tank the floors of the cellar as well as the walls. The floor should be done after you’ve fully tanked the walls and you can follow the same method you did with the walls. When it comes to the finish, tanking coatings should always be protected and never left exposed. Apply a floor screed (this can be self-levelling). You can then tile or install other floor finishes on top of the screed if needed.
Tanking takes time to cure thoroughly – between 24 and 48 hours. The tanking on the walls will cure at different rates with wetter parts of the walls taking longer.
You may notice moisture as the slurry cures. This is called ‘sweating’ and is nothing to worry about. It’s a natural part of the curing process which normally occurs in the early drying stages and is caused by water vapour condensing onto cold surfaces. The level of sweating will vary according to how much ventilation is available and any type of heating used.
Finishing and decorating
After applying your cellar tanking to both the walls and floor, you can then look at finishes and decorating. You will need to add a breathable render before being able to decorate. After 24 hours, you can render over the tanking surface. Even if the tanking looks damp or darker in certain areas, a render can still be put over the surface as the whole area will dry together.
Because the substrate behind the tanking surface will never dry out, it’s very important that any re-decoration doesn’t act as a vapour barrier. Only vapour permeable materials such as trade emulsions and ordinary wallpapers should be used. Gloss paints, vinyl emulsions, together with vinyl and washable wallpapers should be avoided as these will ‘trap’ moisture behind the decorated surface, causing future problems. Any re-decoration within 12 months after the completion of the works should only be regarded as temporary.
Biocraft carries out basement tanking and cellar waterproofing projects throughout Berkshire, Oxfordshire, Hampshire and Surrey. For advice or to arrange a site visit, please call 0800 781 8358 or email [email protected].