3 Best Ways of Tanking Walls Below Ground Level

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Tanking walls below ground level, can be extremely important if you want to make use of a basement or cellar. whether you intend to fully convert the room, or just use it for storage, tanking is a way that you can completely damp proof the space.

tanking walls below ground level

Rooms under ground level are far more prone to water ingress. This is because they are below the properties damp proof course. Also, they will not usually be fully protected from damp during construction. This is especially true in older properties.

The water enters the building due to hydrostatic pressure. Essentially water from the ground surrounding the basement or cellar builds up. It then finds its way through small gaps and cracks in the basement wall and floor causing damp.

This means if you intend to use a below ground room, you will need to make the room watertight. There are several ways to achieve this. The one you choose will depend on two main factors:

  1. What will the room be used for?
  2. How bad is the water ingress?

In many cases, if the level of damp is minimal, you will probably be able to use a simple tanking system. However, in some cases you could have a significant amount of damp, or even flooding. In this type of scenario, it is highly likely you will need to use a more sophisticated, below ground tanking system.

3 Ways of waterproofing below ground level

As we mentioned previously, the system you use will depend on the level of damp. Below are your 3 main options when it comes to waterproofing and tanking walls below ground level.

  1. Masonry paint – This is cheap and might be the best option if your basement is not that damp. Also, if you just intend to use the room for basic storage or something like a utility room. Then this might be a good enough solution.

  2. Tanking slurry – Another option that will provide more protection, is a tanking slurry. This is a cementitious slurry that can be applied to the walls and floor of your basement. Once applied it forms a waterproof barrier, this will completely stop water from penetrating through the walls and floor of your basement.

  3. Plastic damp proof membrane and cavity drainage – Your final option is the most complicated and will be the most expensive. This involves installing a waterproof membrane and a drainage system. The entire waterproofing system will be completed with a pump that removes the water collected by the drainage system.

As you can see, the option you choose will really depend on the final use of the room. As well as the extent of the damp. The first option is only useful if you intend to add a small amount of protection to a room that is already not that damp. This will also be a room that does not have a high level of decorative finish.

The second two options can be used for any type of room. All the way from storage, to a working room in your house that you spend time in. This could include things like:

  • Bedroom
  • Office
  • Childs playroom
  • Sitting room
  • Kitchen
  • Etc

The below ground tanking option you choose here, will usually depend on the extent of the damp problem. The difference in price between these two options will be quite significant. Tanking slurry is quite affordable and can costs as little as £5 per m2.

A membrane and drainage system on the other hand, will be significantly more expensive. This will also require some level of expertise to install. So, you will have additional labour costs for this type of installation.

 In many cases a tanking slurry will be enough. However, you should consider a membrane and drainage system if you have a lot of damp and moisture coming through the walls at multiple points. If there is a lot of hydrostatic pressure, this could cause a tanking slurry to fail over time.

Applying masonry paint

Applying a masonry paint is quite a simple task. However, there is a right way and a wrong way to do it. Anybody can pick up a paint brush and apply a coat of paint. The difference between a professional painter and somebody doing this DIY is usually in the preparation.

If you don’t prepare the surface before you paint, then this is going to affect the quality of the finished job. Essentially, if the surface is poorly prepared, it is far more likely to have problems with the paint peeling flaking and cracking. This will also affect the paints integrity as a waterproof barrier. As a result, this could mean the paint doesn’t last as long and the damp problems could continue.

Before you start to apply the paint, you should thoroughly clean the surfaces. this will include removing any loose dust and debris. Also, if there are any cracks, holes, or damaged pointing, these should all be repaired before painting.

Finally, your first application should be a water diluted coat. The reason for this, is that most surfaces in a basement or cellar will be porous materials, such as bricks, stone, or concrete. These types of surfaces will draw water from the 1st coat that is applied. Simply add around 20% water to the existing masonry paint for your first coat. Following this you can add one to two coats of 100% masonry paint to finish.

Applying tanking slurry

Applying a tanking slurry will also require you to prepare the surfaces thoroughly. This will involve everything that you did for masonry paint preparation. So, you will need to clean the surface, remove any loose material and debris. As well as filling any cracks and holes in the surface.

Additionally, you should wash the surface thoroughly with clean water. most tanking slurry manufacturers recommend that you do this one day prior to applying the slurry. Then a second time around an hour prior to application.

The next step will be to add a salt neutraliser to the surface. This will stop salts from passing through the tanking slurry once it is applied.

Finally, you can mix and apply the tanking slurry with a masonry brush. This will require two coats, one applied horizontally, and one applied vertically. You can read our full tanking slurry case study here.

Tanking membrane and drain system

Your final option is the tanking membrane and drainage system this is the most complicated option available. In most cases this will require the help of a professional.

This will involve adding a membrane to the walls and possibly the floors of your basement. The membrane is attached by placing it over the surface of the wall. Once in place, holes are drilled through the membrane and it’s attached to the wall with specialised rubber plugs.

This process forms a completely waterproof barrier, that the water runs behind. At the bottom of the membrane a drainage tray is installed that will collect the moisture running down the back of the membrane. Once the water is in the tray it is then transferred to a tank which contains a pump. Once the water is in the tank it is pumped away from the basement. This usually leads to an exterior drain.

As you can see this system is slightly more complicated. Due to this, it will require more specialist knowledge to install. This means, going the DIY route could lead to problems, including the finished job being defective.

It is also worth noting, that most companies who install this system offer quite long guarantees (sometimes up to 25 years). This means if you have any problems, they will be fixed free of charge.


As you can see, when it comes to tanking walls below ground level you have a few options. The one you choose will depend on your own personal situation. The main considerations will be how damp the area is. As well as its final use.

Two of the options provided (painting and tanking slurry) are quite simple. If you are reasonably competent at DIY and can follow simple instructions, then you should not have much problem doing these yourself.

If on the other hand, you have a more serious damp problem and you want to create a room with a high quality finish. Then a membrane and drainage system might be your best option. However, this is going to cost much more than the other two options. Plus, unless you have some building experience it is unlikely that this is a DIY job.