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17 Jun 2025
Most cellars and basements start life as glorified storage rooms for forgotten Christmas decorations and that exercise bike you swore you'd use. But what if you could transform that damp, musty space into a proper usable room? The secret lies in basement tanking – a waterproofing method that's been keeping British basements dry for decades.
Basement tanking is essentially creating a waterproof barrier on your cellar walls and floor using a liquid coating that dries to form an impermeable seal. Think of it as giving your basement a waterproof jacket that stops moisture from seeping through the walls.
The process involves applying a specially formulated mixture directly onto damp masonry surfaces. Once it cures, you get a permanent waterproof barrier that blocks water ingress rather than just managing it. This differs from cavity drainage systems, which let water in but control where it goes.
Here's the thing – when your walls are below ground level, they're constantly battling against the earth that naturally holds moisture. This water doesn't just sit there politely; it actively pushes through brick and stone, carrying salts and minerals along with it.
The result? Cold, damp walls create the perfect breeding ground for condensation and mould. Not exactly the cosy den you had in mind.
You've got two main routes when dealing with a damp basement:
Cavity Drainage Systems - These systems work by allowing water in but controlling it to a designated drainage point. Think managed retreat.
Basement Tanking - This completely blocks water. Think of building a fortress.
For most homeowners wanting a straightforward solution, tanking wins hands down. It's direct, effective, and once it's done, it's done.
Not all waterproofing materials are created equal. While some folks might suggest bitumen coatings, these are better suited for small above-ground areas. For proper basement tanking, you need cementitious Waterproof Tanking Slurry.
KingfisherUK's Waterproof Tanking Slurry includes acrylic polymers that enhance strength and bonding, which is crucial when dealing with constant moisture pressure.
You'll also need some additional products:
This bit isn't glamorous, but it's critical. Rushed preparation is the number one reason tanking systems fail.
First job – get everything off those walls. We're talking shelves, old paint, render, bitumen coatings, the lot. You need to get right back to the original masonry. Yes, it's a pain. Yes, it's worth it.
Rake out loose mortar joints and ensure everything is dust-free. If you find cracks or holes, fill them with a quality filler before proceeding.
Older buildings often have salt contamination from years of moisture movement. If this sounds like your place, apply a salt neutraliser to prevent these salts from compromising your waterproofing later.
Once you've stripped the walls, look for active water seepage. If you spot any, you'll need to stop this water pressure first using a rapid-setting plugging compound.
The junction where the wall meets the floor is often the weakest point in any waterproofing system. Create a reinforced joint by chasing out a 20mm x 20mm groove where the wall and floor meet.
Tanking products are alkaline, so cover up adequately – including gloves, goggles, and a face mask, if necessary. Don't skip this bit.
Most tanking slurries come as powder that you mix on-site with clean water. Only mix what you can use within 30-45 minutes – this mixture doesn't last long once it starts setting.
Add water to your mixing vessel first, then slowly add the powder while mixing continuously. A high-torque drill with a plaster mixer attachment works best. Mix for three minutes after adding all the powder, then let it stand for a minute before mixing again.
Here's where it gets interesting – tanking products work better on damp surfaces. If your walls have dried out, wet them down first (no standing water though).
First Coat: Apply horizontally with a brush directly onto the masonry. Go down and across that wall-floor joint, extending 100mm onto the floor.
Second Coat: Wait 2-24 hours for the first coat to become touch-dry, then apply the second coat in a vertical direction. Please don't leave it longer than 24 hours between coats.
Pro tip: Use different coloured slurries for each coat so you can see exactly where you've been. Nothing worse than missing a patch and finding out the hard way.
Don't forget the floor – you want to create a complete waterproof box. Use the same two-coat method, ensuring that the reinforced wall-floor joint is overlapped.
Never leave tanking exposed long-term. Apply a floor screed (self-levelling works well) once everything's cured, then add your final floor finish on top.
Allow 24-48 hours for the material to fully cure. Don't panic if you see moisture appearing during this time – it's called "sweating" and it's completely normal. It's just water vapour condensing on cold surfaces as the material cures.
Wetter areas will take longer to cure than drier spots, but that's expected.
Once everything's cured, you can start thinking about finishes. You'll need a breathable render before decorating – at least 10mm of three parts sharp sand to one part cement.
After the render's dried, you can add dot-and-dab plasterboard or multi-finish plaster. Just remember – never paint directly onto tanking.
Since the substrate behind your tanking will never fully dry, use only vapour-permeable decorating materials. Stick to trade emulsions and standard wallpapers. Avoid gloss paints, vinyl emulsions, and washable wallpapers – these trap moisture and cause problems down the line.
Basement tanking costs depend on the size of your cellar and the extent of the damp problem. Most products are priced per square metre, making calculations straightforward.
As a rough guide, quality tanking slurry covers between 6.5-10m² per 20kg bucket, depending on your substrate. For a typical cellar with four walls (2m high x 5m long each), you're looking at around 13 buckets for the walls plus 8 for the floor.
Don't forget to account for additional costs, including preparation materials, joint sealers, and finishing products. While it's not the cheapest home improvement project, it adds genuine value and usability to your property.
Skipping the prep work – This always comes back to bite you. Do it correctly the first time.
Using the wrong products – Bitumen might seem cheaper, but it's not suitable for full basement tanking.
Leaving too long between coats – Stick to the 24-hour maximum rule.
Poor ventilation during curing – Good airflow helps with the curing process and reduces sweating.
Using non-breathable finishes – Your tanking needs to breathe, so choose decorating materials accordingly.
If you want to utilise your basement space effectively and are prepared to do the job properly, tanking offers an excellent long-term solution. It's more straightforward than cavity drainage systems and, when done correctly, provides decades of reliable waterproofing.
The keyword here is "properly." This isn't a weekend bodge job – it requires careful preparation, quality materials, and attention to detail. But get it right, and you'll have a dry, usable space that adds real value to your home.
Your cellar doesn't have to remain a damp storage area. With the right approach and materials, that forgotten basement could become your new favourite room.
