Tanking slurry is a great way of creating a waterproof barrier on internal walls and floors. However. It does not always provide the best possible finish.

If you are tanking inside your home, in a room that will be used as a living space, then it is highly likely you want to apply a smooth finish after tanking has been applied.

Most properties will have a smooth plastered finish in most rooms, even those decorated with paper. It is quite reasonable to assume that people would plaster over a tanking slurry once it has been applied.

You can plaster over tanking slurry. However, you should use a dilute SRB to prime the surface before applying a plaster finish. It is advised, to use a 1:1 SRB and water mixture over the finished tanking slurry.

Wait until the primer is tacky (not completely dry), then apply the plaster finish.

The main problem with plastering over tanking slurry, is the slurry is non-porous. This means that it does not allow suction, and the wet plaster cannot penetrate the surface of the tanking slurry. The primer solves this by adding an intermediary layer that the plaster can key too.

If a primer is not used, it is far more likely that the plaster will come away from the wall once dry.

Stopping damp issues before tanking

Whilst tanking can provide a damp proof barrier, it should not be used to just cover up damp issues. This may work in the short term, but if the source of damp remains, you are simply covering up the problem and trapping moisture in the walls.

If water builds up in the wall, it will apply more pressure to the tanking slurry. Eventually this pressure could cause the slurry to blow and come away from the wall.

Whats more, the trapped moisture can cause other issues, such as spalling in bricks due to freeze thaw cycles. Over time this can cause serious structural damage, due to deterioration of the brickwork.

Finally, there is always a chance that the excess moisture will simply find a way around the tanking. Unless you tank the whole house, it is likely that damp will find another way in.

Essentially, you need to locate the problem before the tanking is applied.

In most cases, damp entering from outside will be a form of penetrating damp. If the problem is near ground level, it is also possible that rising damp is the issue.

To learn more about penetrating damp click here. This article covers the subject in more detail, including how to identify penetrating damp, as well as potential solutions.

For a more detailed explanation of Rising damp click here

Once you have identified and fixed the root cause of damp, a tanking slurry is a great way to add additional internal protection.

Also, after damp issues are fixed, it is common for a small amount of moisture to remain in the wall. This moisture can still penetrate through, along with hygroscopic salts, that can ruin the decorative finish inside. A tanking slurry is a good way of stopping this issue.

Alternatives to plastering over tanking slurry

There are several alternatives to tanking slurry itself. For example, a plastic membrane could be applied to walls. You may also use a lime based render. Both are potential options, and both come with their own pros and cons.

The main advantage with a tanking slurry, is its ease of application. As a DIY damp solution, tanking slurry is the most simple to apply. If you can follow the mixing instructions and work a brush, you will not need any professional help (unless you would prefer to outsource the work)

Installing a membrane or a render on the other hand, are much more technical. Both could probably be done by a competent DIY’er, but they require far more expertise than painting a slurry onto your walls.

There are also alternatives to applying plaster straight on to the tanking slurry. This will usually involve applying a plaster board and skimming over. Again, this can be done in a variety of different ways.

You can see a few options below:

• Dot and dab boards over the tanking slurry and skim. This is a popular option and is relatively simple, boards are fixed to the tanking slurry with an adhesive and plaster is skimmed over the boards.
  
  However, this is not 100% straight forward. Due to the non-porous nature of the tanking slurry, it is not advised that a standard plasterboard adhesive is used. You can read more about dot and dab over tanking slurry by clicking here.
  
  
• Applying timber batons to the wall and fixing the plaster boards. This is another good option, although some might claim, that fixing batons to the wall could compromise the tanking slurry.
  
  To avoid this, you can inject silicone into the drilled holes. This will seal any holes in the slurry caused by plugging and screwing the batons.
  
  
• Building a stud wall with a small cavity between the tanking slurry. This is likely to be the least popular option, as it will result in the most loss of space. However, in some cases it could be the best option.

For example: if you are tanking a single skin brick wall, a stud wall could provide extra advantages. Such as the ability to add extra insulation.

Conclusion

So, to conclude you can plaster over tanking slurry. However, you must make sure the surface is primed correctly before applying plaster.

There are also a few other things that you should consider.

Firstly, you need to fix the root cause of the problem, this is very important for a long term damp solution.

And finally, consider the other methods of finishing. Plastering can be a good way, but it might not always be the best solution. Always weigh up the pros and cons before deciding which method to use.

Damp coming through a floor is quite common. Especially with solid concrete floors in your property. Some common places you may find this issue are:

• Downstairs rooms with solid concrete floors
• Garages and sheds
• Basements and cellars

These are all areas where water can penetrate through the concrete and result in damp on the surface.

Tanking slurry can be used to stop damp coming through a floor. In most cases it is a very effective solution. However, if the surface is not prepared correctly and the tanking slurry is applied poorly, then it could fail at a later date.

Why does damp come through a concrete floor?

An issue with the existing DPM (damp proof membrane) is the most common reason for rising damp in a concrete slab. In some cases, a DPM may not have been installed at all.

A damp proof membrane is a waterproof barrier, it is installed below the concrete to stop water from rising into the slab. This is usually made of a durable and completely water resistant polyethylene sheet.

The material is usually intended to last the lifetime of a building, but as with anything, it can degrade slightly over time. Plus, there is always a chance that the membrane suffered some form of damage during the building process, which has caused it to fail later.  

One potential solution when your DPM fails, is to dig up the concrete slab and insert a new DPM. However, this is a big job, and you will need to completely remove the existing concrete slab to add a new DPM.

In most cases this is a job for a professional. Once concrete is fully cured, ripping it up is no small task. Expect this to be a lot of work and expense.

How to use tanking slurry on your floor

The good news is that something like tanking slurry is far more simple. You will not need to dig up the existing floor to replace the DPM. Instead the tanking slurry will form a waterproof barrier on the surface.

Before you apply tanking slurry to your floor, it is important that you prepare the surface thoroughly. There should be no dust and debris on the surface, and if you are applying the slurry to smooth concrete, it is advised that you use a primer.

Priming the surface will enable the tanking slurry to get a good key. This will stop it from lifting and cracking.

The best option here is a SBR primer (Styrene Butadiene Rubber) this will prime the surface and allow a better adhesion for the tanking slurry.

It is recommended to use a mixture of 1 part SBR to 2 parts clean water. This will create a good key. It will also be dilute enough to allow silica from the slurry to penetrate the concrete.

Applying tanking slurry to your floor

Tanking slurry is relatively easy to apply. It usually comes in powder form, so you will need to mix the slurry yourself.

You should mix your tanking slurry in a clean bucket with clean water. It is also advised that you use some form of mechanical mixing. A good option here is a drill with a mixing paddle attachment. You can see a good example of a mixing paddle by clicking here – amzn.to/3jhwO4f

The problem with mixing tanking slurry by hand, is it will be very difficult to achieve a fully mixed and smooth finish. By using a mixing attachment and a drill, you can easily get a nice smooth, lump free finish.

Once your tanking slurry is fully mixed you can start to apply it to the floor. To do this, simply pour some of the slurry onto a section of the floor from the bucket, next spread the mixture with a brush. You can use a masonry paint brush, or if you prefer to stand up a soft sweeping brush is also a good option.

You will need to apply two coats of the tanking slurry to achieve the best results. Each coat should be applied in different directions to achieve the best finish.

For example, if your first coat is brushed lengthways, then the second coat should be applied across the width of the room.

Generally, the second coat can be applied after around two hours. At this point the first coat should be dry to the touch. Before you apply the second coat you should spray a fine mist of water onto the first coat.

Following the second coat your floor should be fully covered with the tanking slurry. Once the job is finished you should leave it for 24-48 hours, this will allow the mixture to completely go off. At this point the floor should be fully damp proofed. This will mean previous damp problems are no longer an issue.

Conclusion

Tanking slurry can be a great way to waterproof a floor. It is also very effective on walls and is an affordable way of adding a waterproof barrier to a room.

Its ease of application and low cost make it very popular DIY damp solution. It does not really require any technical knowledge. If you can follow simple instructions on the container and operate a whisk and a brush you should be good to go.

To learn more about tanking slurry you can read our full tanking slurry case study here.

In this article, we walk through a case study of tanking a small outbuilding (floors and walls). This includes the full process from start to finish. As well as our favourite tanking slurry to use.