Brief Guide: How to Tank a Cellar

For many properties that have a cellar, these are spaces with only limited use – store a few boxes, maybe keep a few bottles of wine on a rack or leave the tins of paint from the decorating.  But this doesn’t need to be the case and it is possible to transform a cellar into a usable space.  To do this, you need to keep it dry and to achieve this, you need to use a tanking system that creates a waterproof coating on walls and floors. Common Furniture Beetle.

There are two main ways to waterproof a basement, especially if it has damp walls.  You can install a cavity drainage system or you can use cellar tanking.  Tanking is a term that has created some confusion recently as cavity wall membranes and above ground damp proofing has also come to be included in the term.  So here we examine what exactly tanking is and how it can make your cellar usable.

What is cellar tanking?

Cellar or basement tanking is the process where a liquid waterproof coating known as tanking slurry is applied to both the floor and walls of a cellar.  It is used to deal with damp walls to prevent water ingress and effectively makes the walls watertight on a permanent basis.

The reason that cellars often have damp walls is that they are below ground.  Water from the surrounding earth can reach the walls and be absorbed, then appearing in the cellar.  The moisture can also bring salts and minerals that cause stains and other problems.  Damp walls, in turn, are cold and this leads to problems with condensation and also mould.  

Tanking slurry is aimed at dealing with these problems and is applied directly to the damp walls.  It then cures and creates the waterproof seal, stopping the absorption of water from the surrounding earth.  Tanking slurry blocks water from entering the cellar, as opposed to cavity drain systems, which allow water in and then control where it goes.

How does tanking slurry work?

Tanking slurry is one of the most effective ways to deal with cellars.  It is also known as cementitious tanking but both names apply to the same product – a special mixture that is designed solely to stop water ingress.  It comes in either pre-mixed for or as a powder than can be mixed on site with clean water.

Some people recommend using a bitumen coating to tank a cellar but this isn’t suitable for the entire space.  These coatings are better suited for small above ground areas and for external waterproofing.

Tanking slurries blend together cements, aggregates and chemical modifiers to block the passage of water.  Some also include acrylic polymer to improve the strength and make for stronger bonding to the walls as well as adding abrasion resistance.

How to tank cellar walls?

Key to the success of cellar tanking is the preparation work that is required.  Masonry surfaces needed to be prepared thoroughly and in older homes, this might involve a lot of work but is worth it in the long run.  All items on the wall need to be removed to get complete access then all plaster and render is removed.  Any previous coatings are also removed so that the original masonry is exposed.

Mortar joints and any loose material must be cleared before any further steps are taken.

Should the room have experienced high salts levels, then a salt inhibitor may be used to stop them causing future problems.  Once the walls are clear, any signs of seepage will need to be checked for to ensure this can be dealt with before tanking takes place.  

A reinforced joint may also be required at the weak spots where walls meet the floor to ensure waterproofing.  This will involve cutting around 20mm into the wall, cleaning away debris and applying a coating of slurry 100mm up the wall and across the floor.  A special sealant is then added while the slurry is still tacky, creating a cove from the wall to the floor.

How To Mix and apply the slurry?

Always wear protective gear when using slurry including gloves, a face mask and goggles.  When in powder form, don’t allow the slurry to come into contact with skin and avoid inhaling it.  When mixing on site, only mix enough to use within a 30-45 minute period as, after this, the product will be unworkable.

Every product will have its own instructions on how to mix with most recommended mixing at slow speed with a high torque drill and a plaster mixer.  Mixing usually takes place for around three minutes after all the powder has been added to get a batter-like consistency.

Cellar tanking should be applied to damp substrates so if your walls are dry, you will need to dampen them before applying the slurry.  Normally, the product requires two coats with the first straight onto the masonry in a horizontal direction.  It can take anywhere from 2 to 24 hours before the walls are ready for a second coat and it should be completely dry before you do this.  Don’t wait any longer than 24 hours to add the second coat.

Tanking the floor

Floors are as important to tank as walls and once the walls are finished, you should then tank the floor.  Follow the same method as with the walls and apply two coats.  Once you have finished the second coat and it has dried, you should apply a floor screed and then any tiles or other floorings on top.  You shouldn’t leave the tanking exposed.

You may notice some moisture as the slurry cures – this is normal.  It is part of the process as water vapour condenses on cold surfaces.

Decorating

You need to add a breathable render to the walls before being able to decorate and leave at least 24 hours after the second coat before doing this.  You can do this even if there are still some damp or darker patches in some areas.

You should use around 10mm coat of three parts sharp washed sand to one-part cement or specialist plaster to coat the walls.  Don’t apply paint directly to the tanking.  Always use vapour permeable materials such as trade emulsions or wallpaper – no gloss paint, vinyl emulsions or washable wallpapers are advisable.

What is The Cost of tanking?

The cost of tanking depends on the size of your cellar, the type of substrate and the extent of the damp problem.  If you want to know more about the cost, work involved and any other questions, you can contact an damp expert TapCo Homedry and we can arrange a visit to give you a comprehensive quote.