How to Tank Damp or Wet Walls

Damp walls are cold walls, fact! Water and moisture that is allowed to pass through a wall onto it’s inner surface will inherently maintain that area at a cooler temperature than it’s surroundings. When this this allowed to happen it can cause condensation build up and black mould growth.

Additional when moisture passes through a wall it can also carry salts and minerals with it. When these salts reach the inner decorated surface that’s normally covered with plaster or paint it will completely ruin painted and wallpapered surfaces and also eventually blow the plaster off the wall.

When a walls surface is below the damp proof course, partially below ground or is used to retain earth, it will always be damp due to the fact that it’s surroundings naturally contain water and moisture.

What is Tanking?

The traditional method of dealing with this was to batten the inner wall surface out and panel over the top of it. This would be ok for a while but due to the moisture presence, eventually the wooden paneling would itself become damp and then rot.

As technology has evolved, this eventually gave birth to tanking. Tanking is effectively a liquid coating that bonds to damp masonry and then cures to form a waterproof barrier.

Tanking Walls for Damp Proofing – Choice of Methods

The is essentially three main methods for tanking wet walls; liquid bitumen coatings, cement-based slurry and air gap membranes;

• Liquid bitumen coating – painted onto the cleaned brickwork, stone, block or render – difficult to apply to old, damp salty masonry, prone to separation from the wall and render coats due to incomplete curing. Best for non-critical small jobs, or external coating below the damp proof course.
• Cement based slurry – applied by soft brush onto damped, cleaned brickwork, stone, block or render – easier to apply because the cement base wants to amalgamate with the masonry and mortar. Not a true vapour barrier, so needs to be finished with ‘breathing’ materials. Ideal for partial below ground situations or exterior coating on foundation walls.
• Air gap membranes – not strictly ‘tanking’ in the same way as liquids – it is a dimpled plastic sheet fixed onto brick, stone, block or render with plastic plugs to form an air gap cavity. These Membranes can have a plain surface, for battening or a Mesh surface for direct plastering or plaster boarding. Ideal for cold, single leaf walls and for lining rooms with cold solid 9 inch walls.

Tanking Slurry and Cementitious Slurry – How to Apply

• Remove any old plaster from the walls surface. This also includes any loose debris, dust paint or render. Additionally remove any other items that shouldn’t be there. Once the walls surface is free and clear of dust and any other rubbish wash it down thoroughly with fresh water
• Now, ensure that there is no active water seeping from the wall. If there is then you will need to create some relief holes and also a drainage solution. This can be a tricky job and will probably require expert advice
• Now apply a Salt Neutraliser to the surface to prevent any salts from compromising your new tanking layer. Salt neutraliser is typically a clear liquid that neutralises the salts that are found in masonry, plaster and render. If the plastered surface has not been too badly damaged by salts then in some cases it can save it by neutralising and that remain. In any case, it is a good idea to add as it will essentially ″future proof″ and potential issues. When applying, apply in two coats ensuring that you wet the surface with fresh water between each coat
• As the surface of the wall will be a little damp at this point, now is a good time to fill any holes with mortar, sand (make sure that the sand is washed, ask at your local builders merchants) and cement. Mix in a 4:1 ratio to ensure the correct consistency. It is also a good idea to add a waterproofer or plasticiser product such as Renderproof to the cement mix as this will aid in the cement bonding with the sand and prevent moisture passing through
• Next dampen the wall again and apply a coat of tanking slurry or solution to the walls surface. Tanking slurry’s will either come premixed or in a powder form. If in a powder form you will need to mix it with water (refer to manufacturers specifications for specific amount of water) until it resembles a soup like consistency. Once mixed correctly apply it to the wall with a soft paste brush using horizontal strokes



• Once you have applied the first coat leave until it is tack free and then apply a second coat over the top, this time applying it using vertical strokes. By doing this you should then cover any holes or patches that you missed with the first coat
• Finally, leave your second coat to cure thoroughly. Once totally dry the surface can then be replastered with a renovating plaster, painted over with a microporous paint or traditionally rendered with sand and cement

Air Gap Membranes, Damp Proof Membranes and Plastic Lining Membranes – How to Install

Aside from tanking there are other methods of water proofing and damp proofing walls such as using a plastic lining membrane (also known as air gap membranes or damp proof membranes) to line a wall and provide a barrier between it and a walls surface.

Depending on how much water or moisture you have coming through your wall, using a lining membrane may be a better solution than simply tanking.

Here follows a brief explanation on how to install linings – for a full run down on the process see our lining damp walls project.

• Firstly remove any gypsum plaster that may be left on the walls surface (cement and sand render is ok to stay). Also remove any other items that shouldn’t be there such as wall plugs, wooden plugs and timbers
• As in the tanking method above, check if you have any water seepage coming through the wall. Additionally also check if there has been a history of any flood damage. If you do have water penetrating the walls surface you will need to install a drainage solution such as a traditional drain or a sump and pump to remove the excess water
• The next task is then to fix your chosen lining membrane to the walls surface. There are many options available on the market today. There are two different types available, one is for use if you are battening and boarding over the top to create a surface to decorate on to and the other is a Mesh Membrane that features a fine mesh that can be directly plastered onto or can take plasterboard adhesive for dot and dab plasterboard fixing
• With your finishing method established, fix the membrane onto the walls surface. Normally special plugs will be supplied or can be purchased. If you are battening and plasterboarding over the surface Kontact Membrane is best as the heads of the fixing plugs feature pre-drilled holes ready for no.12 self tapping screws so the battens can be screwed straight on



• If you are also experiencing damp and moisture issues on the floor then this may also need covering with a membrane. Once it has been lined you can then cover and finish using floating tongue and groove chipboard or you can also apply a concrete screed. When there is physical water seeming up you will need to bond the wall and floor membranes together to create a ″sealed system″. Note: When water is present you will also have to incorporate a drain or sump and pump.

If you have a damp issue in walls or floors, then tanking is a great option to go for to stop any further damage to any surfaces. One of the best features of any of the tanking options is that they can easily be done on a DIY basis.