Tanking slurry is a cement-based waterproofing coating used on suitable mineral backgrounds to form a barrier against water ingress. In the right design it can be very useful, especially in below-ground work. But it is not a universal treatment for every damp wall. Before specifying a coating, the moisture source, structure, background, joints, floor connection and intended use of the room all need to be understood.
What is tanking slurry?
Cementitious tanking slurries are waterproofing compounds applied to masonry, concrete or compatible cement-based backgrounds. In structural waterproofing terminology, tanking is generally a form of Type A barrier protection: a waterproof layer is used to resist the passage of water.
The Property Care Association (PCA) homeowner guidance describes tanking products as barriers to water ingress and notes that successful systems depend heavily on preparation, design and application. This is why a proper specification is more than simply choosing a tub of slurry and applying two coats.
When tanking slurry may help — and when to investigate first
Below-ground masonry
A cellar or basement wall where a designed Type A barrier system is suitable for the structure and intended room use.
A random damp patch upstairs
Check plumbing, rain penetration, roof details and other possible sources. Tanking the visible patch may only conceal the symptom.
Condensation and mould
Surface condensation is an air-moisture and temperature problem. Ventilation, heating, insulation and moisture generation may be more relevant than a tanking coat.
Low-level salts or damaged plaster
Establish whether ground moisture, a DPC issue, external levels or another source is involved before deciding how the wall should be rebuilt.
Wall-to-floor water ingress
The junction cannot be treated as an afterthought. Continuity between wall and floor waterproofing is a key part of the design.
Significant basement water pressure
Where failure would cause serious damage or the room is intended as habitable space, waterproofing design should be treated as a specialist discipline.
Diagnose the moisture before choosing the treatment
Approved Document C requires walls, floors and roofs to provide appropriate protection from harmful effects of ground moisture, precipitation and condensation. Those are different moisture mechanisms, and they do not all have the same remedy. See the government’s Approved Document C guidance.
A moisture meter reading or a photograph on its own does not automatically identify the mechanism. Persistent, widespread or below-ground moisture deserves a proper assessment before finishes are stripped and replaced.
Type A, Type B and Type C waterproofing are not the same thing
BS 8102:2022 recognises different approaches to below-ground waterproofing. Homeowners do not need to become waterproofing designers, but knowing the distinction helps when comparing quotations.
Barrier protection
A waterproof barrier is applied internally, externally or within the structure. Cementitious tanking can fall into this category.
Structurally integral protection
The structure itself is designed and constructed to resist water, commonly using reinforced water-resistant concrete systems.
Drained protection
Water is managed behind a cavity drainage membrane and directed to suitable drainage, often including a sump and pump.
The PCA’s waterproofing guidance for homeowners explains these three approaches. A tanking slurry page should therefore not imply that Type A is automatically the correct system for every basement.
Why preparation and continuity matter so much
A waterproof coating is only as useful as the background and detailing supporting it. Existing paint, weak plaster, loose material and contamination can prevent reliable adhesion. Cracks, corners, pipe entries and wall-to-floor junctions are also common weak points because the waterproof layer has to remain continuous through changes in direction and material.
Establish the structure and water source
Understand what the wall and floor are made from, where ground level sits and how water could reach the area.
Expose a suitable background
Unsound decorative finishes and weak material normally need to be removed as required by the selected system.
Repair and detail vulnerable points
Cracks, corners, service penetrations and floor junctions need the system-specific treatment rather than simply more slurry brushed over them.
Apply the specified waterproofing build-up
Mixing, coat number, thickness, timing and curing should follow the current manufacturer data for that exact product.
Use compatible follow-on finishes
The render, plaster or protective finish over a waterproofing layer must be compatible with the system. Do not assume standard gypsum plaster can always be applied directly.
For an example of why product-specific instructions matter, Sika’s UK technical page for its Damp Proofing Slurry lists defined substrates and uses and identifies it as a polymer-cement waterproof coating for Type A shallow basement structures. That does not mean every slurry has the same specification.
The wall itself has to be suitable
One detail often missed in simplified “how to tank a wall” articles is that waterproofing changes how water pressure acts on a structure. PCA guidance for existing below-ground structures notes that cementitious systems can hold back significant water pressure, but the substrate must be strong enough to accept the resulting stresses. Lightweight or unsuitable construction may require a different approach.
That is one reason I would not recommend choosing a tanking method from photographs alone where there is meaningful below-ground water ingress. Structural condition and waterproofing design can matter as much as the final plaster finish.
What happens to the wall after tanking?
The final finish depends on the waterproofing system. Some cementitious systems receive compatible renders, protective coats or other specified finishes. The sequence matters because puncturing, incompatible materials or premature finishing can compromise the system.
If damp has damaged the existing plaster, the correct order is generally to resolve or design for the moisture mechanism first, establish the waterproofing or damp-control build-up, and only then reinstate the internal finish. For the plastering side of that process, see my plaster repair guide and skim or replaster guide.
What a useful tanking quotation should make clear
- The diagnosed or suspected source of moisture.
- Whether the affected construction is above or below external ground level.
- The exact waterproofing product or system being proposed.
- What existing plaster, paint or weak material will be removed.
- How wall-to-floor junctions, corners, cracks and penetrations will be detailed.
- Whether the floor is included and how continuity will be achieved.
- The manufacturer-required coat build-up, curing and protection.
- Which render, plaster or decorative finish is compatible afterwards.
- How ventilation and condensation will be considered in the finished room.
For larger basement or cellar waterproofing projects, the PCA recommends involving appropriately skilled waterproofing specialists. Its structural waterproofing guidance explains why below-ground waterproofing is a specialist design task.
Tanking slurry FAQs
Will tanking slurry cure every damp wall?
No. Tanking is barrier waterproofing. Condensation, plumbing leaks, penetrating rain, defective external details and other moisture problems may require different remedies.
Can tanking slurry be used on an internal basement wall?
Some cementitious systems are designed for internal application to suitable basement masonry or concrete. Suitability depends on the structure, water conditions, substrate and complete waterproofing design.
Can you plaster directly over tanking slurry?
Not as a universal rule. Follow the selected waterproofing manufacturer’s specification for protective coats and finishing materials. Compatibility and timing are product-specific.
Is tanking the same as a damp-proof course?
No. A damp-proof course is intended to resist moisture rising through masonry. Tanking normally refers to a waterproof barrier over a wider wall or floor area, particularly in below-ground waterproofing.
Is tanking better than a cavity drainage membrane?
Neither is automatically better. They manage water differently and the correct choice depends on the structure, water risk, intended use, maintainability and design requirements.
Do I need a waterproofing specialist?
For significant below-ground water ingress, basement conversions, habitable below-ground rooms or situations where waterproofing failure would be costly, specialist waterproofing design is sensible.
Authoritative references
Need the plaster finish repaired after a moisture problem?
Send wide photographs, close-ups and your postcode. Where the moisture source or waterproofing design needs specialist diagnosis, that should be established first; once the background and build-up are suitable, I can discuss the plastering and finishing work.
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