What Is Tanking Slurry?

Tanking slurry is a cementitious waterproofing compound applied to masonry surfaces — typically basement walls and floors — to create a rigid, waterproof barrier. It's one of the most accessible waterproofing methods for DIYers and is widely available from builders' merchants.

It works by bonding directly to the masonry and forming a dense, low-permeability coating. Applied in the correct thickness and on a properly prepared surface, it can hold back low to moderate water pressure. This guide covers the complete process from start to finish.

Tools and Materials You'll Need

  • Tanking slurry (cementitious waterproofing product)
  • Clean water
  • Stiff brush or angle grinder with wire brush attachment (for surface prep)
  • Hammer and cold chisel
  • Hydraulic quick-setting cement (for active leaks)
  • Waterproof fillet mortar
  • Mixing bucket and drill with paddle mixer
  • Stiff-bristled masonry brush or rendering trowel
  • Gloves, goggles, and dust mask

Step 1: Identify and Stop Active Leaks First

Tanking slurry should never be applied over actively flowing water. Before anything else, inspect the walls and floor for cracks or holes where water is visibly seeping. These must be plugged with hydraulic quick-setting cement, which sets in minutes and stops active water ingress. Follow the manufacturer's instructions — typically you mix a small amount to a putty consistency and hold it firmly in place until it sets.

Step 2: Prepare the Surface Thoroughly

This is the most critical step. Tanking slurry will not bond to a dirty, weak, or painted surface.

  1. Remove all existing paint, efflorescence (white salt deposits), loose render, and contaminated plaster
  2. Hack off any hollow or soft areas of plaster back to solid masonry
  3. Rake out mortar joints to a depth of about 15mm to give the slurry better mechanical adhesion
  4. Clean the surface thoroughly with a stiff wire brush to remove dust and debris
  5. Dampen the surface with clean water before application — the surface should be damp but not saturated

Step 3: Form the Cove Fillet at Wall-Floor Junctions

The junction between the wall and floor is a critical weak point. Before tanking, apply a cove fillet of waterproof mortar — a curved, triangular bead running along the base of every wall. This eliminates the sharp 90-degree angle where stress cracking commonly occurs and ensures the tanking forms a continuous waterproof lining. Allow it to partially set before proceeding.

Step 4: Mix the Tanking Slurry

Follow the manufacturer's water-to-powder ratio precisely. Typically, you add the powder to the water (not the other way around) and mix with a paddle mixer to a smooth, lump-free consistency. Mix only what you can use within the product's working time — usually 20 to 30 minutes. Do not add extra water to extend workability as this weakens the product.

Step 5: Apply the First Coat

Using a stiff masonry brush or trowel, apply the first coat in a firm, scrubbing action to work the slurry into the surface and mortar joints. Work from bottom to top. Ensure complete coverage with no missed spots. A typical first coat thickness is 2–3mm.

Step 6: Apply the Second Coat

Allow the first coat to become firm but not fully dry — usually 3 to 6 hours, depending on conditions. Apply the second coat in the opposite direction to the first (if you brushed horizontally first, brush vertically now). This cross-hatching technique reduces the risk of pinholes. Total finished thickness should typically be at least 3–5mm — check the product datasheet for the minimum specification.

Step 7: Curing and Aftercare

Keep the tanking damp during curing — mist it with water if conditions are warm or dry. Avoid foot traffic or finishing work for at least 24–48 hours. Allow the full cure period (often 7–28 days) before applying any render, dot-and-dab boarding, or decoration on top.

Common Mistakes to Avoid

  • Skipping surface preparation — the most common cause of failure
  • Applying over paint or existing sealers
  • Applying in one thick coat rather than two thinner coats
  • Allowing the first coat to dry completely before the second
  • Not forming the cove fillet at wall-floor junctions
  • Using the product in freezing conditions (most products require temperatures above 5°C)