Can you dot dab on tanking slurry?

In this article, we will be looking at whether it is possible to dot dab over tanking slurry. This will include the potential problems you may face. How you can overcome these problems. As well as three good alternatives, that will provide a very similar finish, without having to dot dab boards.

If you are just looking for a quick answer then yes, you can dot dab over tanking slurry. However, it does come with some potential problems. This is mainly due to condensation. To avoid these issues, you should consider using insulation boards. Also, several tanking slurry manufacturers, recommend using a dry adhesive, instead of a standard board adhesive.

Potential problems with dot dab over tanking slurry

Tanking slurry can be an excellent way of waterproofing walls and floors. Once applied, it creates a waterproof barrier. If applied correctly, this barrier will completely stop problems such as penetrating damp.

In many cases your tanking slurry might be somewhere like a garage. Here the finish might not be as important. In this case, it may be enough to just paint straight over the tanking slurry and job done. However, in some cases you may want a better finish such as plastered walls.

If this is the case, you may be wondering the best way to achieve this. Obviously, if you’re reading this article right now, you’ve probably considered dot dabbing boards on top of your tanking slurry.

This is a viable option. However, it is not without potential issues. These issues will usually come from one of two forms of condensation.

1. Water is unable to escape from the board adhesive – Because tanking slurry is water resistant, it does not allow moisture from the board adhesive to escape. If the moisture has no way to escape, this can cause condensation to build up behind the board.
   
   As the moisture has nowhere else to go, it is likely to soak into the boards as this is the only porous surface. This can penetrate through and cause damp and mould. It can also cause mould to build up behind the board.
   
   
2. Condensation build up on the dabs – The second issue you might get is condensation forming on the finished side of your plaster board. This will usually form on the dabs, leaving damp patches.
   
   The reason this happens is the dabs are cold. Because of this they act as a due point. If the board is fixed to an external wall, the cold will transfer through the board adhesive. This will mean that the areas you have applied the adhesive will be far colder than the rest of the wall. As a result, all the condensation will settle on these cold patches.

Ways to avoid these problems

Because we know the main issue here is condensation, this does make finding a solution easier.

Condensation happens for a couple of reasons:

1. Excess moisture due to poor air circulation and ventilation
2. Dew points. These are cold surfaces that moisture is drawn to (our board adhesive)

The first thing you can do, is make sure the room has good ventilation and moist air is being removed. This is not a direct fix for our dot dab problem, but it is just a good idea in general.

Next, we need a way for moisture to escape from the board adhesive, behind the newly fitted boards. A good way to achieve this is to leave a gap at the bottom of the board. Simply pack the board 20mm off the ground when fixing.

By allowing a gap below the board, you will allow air to circulate. This will create a way for the moist air to escape. After the adhesive has had sufficient time to dry and all the moisture has escaped, the gab can be covered by skirting boards.

So far, we have eliminated the moisture from behind the board. However, we still have a cold surface and those dabs may still act as a dew point. As we mentioned previously, a good way to reduce condensation is to use good ventilation in the room itself.

This can be achieved by installing air vents. Possibly an extractor fan and even just opening windows and doors can help with air circulation.

The other thing you can do is use insulated plaster boards. Insulation boards will reduce any cold that is able to seep through the dabs. Depending on how cold the wall is, will help you decide the thickness of insulation board you require.

Standard insulation boards are 12.5mm plasterboard and the insulation is between 25mm – 50mm. This means the entire board will range between 37.5mm and 62.5mm.

When insulated plasterboards are used, this will stop cold being transferred through the dabs. Because of this it will create a more even temperature on the surface of your board. The result of this is you won’t get damp patches forming.

Alternative ways to board over tanking slurry

The previous section covered the best way to avoid any damp issues, when installing boards with a standard board adhesive. However, this is not your only option.

Below are 3 different ways you could consider installing boards over your tanking slurry:

Alternative 1 – Building a stud wall

One objection you may have at this point, is that you will lose space by adding a stud wall. This is true to a certain extent. However, all things considered you wont really lose that much.

Let’s assume you use CLS timber at 75mm plus a 12.5mm plasterboard. Then lets also assume you leave a cavity gap that is the same distance from the wall as board adhesive. In this scenario, you are only coming out 25mm further, than if you dot dabbed with a 62mm insulation board.

Also, if the wall is relatively small, you may get away with using smaller timber. For example, you could use some 2X2 sawn timber, that will shave off 25mm. This would be the same thickness as using dot dab and 62mm insulation board. Plus, you don’t need to lose the insulation. This can be installed in between the studs.

Alternative 2 – Batton the wall

This is another timber solution and could work well if you have a flat surface to work off. Simply use some 25mm x 50mm sawn and treated timber and attach these directly to the wall.

One thing to consider here is how you will fix the timber. Because the wall was damp, drilling holes could compromise the effectiveness of your tanking slurry. Here you have two options. Firstly, you could inject some silicone into the drilled holes. Or secondly, you could just use a strong grip adhesive and not use screws at all. Or maybe a combination of the two.

If saving space is a big issue, then this could be a good option. You could again use an insulation board. However, to really maximise the saved space, you could also consider moisture resistant, or foil backed plasterboards. The only problem here is you will lose the insulation benefits

Alternative 3 – Using foam adhesive to fix your boards

This option is often recommended by tanking slurry manufacturers, as an alternative to standard board adhesive. This does not require any timber and is essentially very similar to installing via dot dab.

The main difference is the type of adhesive you use. Instead of a standard dot dab, you will use a specialist plasterboard foam. A popular brand that provide this product is Soudal. This company specialises in foams, fillers, and adhesives. Their plasterboard adhesive foam does get a lot of positive feedback.

The application is very similar to using something like expanding foam. You simply attach the nozzle and squeeze the trigger to release the foam. The foam is added to the board in the same way you would add standard dot dab. Once the foam is on the board simply place it in position and wait for the foam to go off.

One thing to remember with this method, is the foam does expand. So, you will need to either fix or wedge the board in position. If you just leave it the foam will expand and move the board. This will result in uneven boards and a poor finish. You may even need to remove the boards and start again.

Conclusion

As you can see, dot dab over tanking slurry is an option. The key thing to remember, is you may need to make some adjustments to get the best results. The biggest issue you are likely to face is with condensation.

This can occur in different ways, on either side of the board if you don’t take extra precautions.

Also, there are other ways to board over tanking slurry. You don’t have to use a standard dot dab. We have mentioned three good alternatives in this article. Each one comes with its own benefits.

What makes all of the methods even better, is they are all relatively easy to install. Even if you are not a professional tradesman. Most can easily be achieved with some basic DIY knowledge.