What Is Waterproof Rendering and When Should It Be Used?

We should consider the purpose, application, and environment in which it is to be used, more especially in below-ground situations where hydrostatic pressure becomes a key factor.

The objective of any waterproofing system is to create a dry, usable internal space. Waterproof rendering plays a critical role in achieving this, particularly for structures built below external ground level, where groundwater ingress is a threat.

This process is often referred to as “tanking”, although that term more accurately applies when all walls, floor surfaces, and soffits are completely sealed using waterproof materials to form a watertight “tank.” Waterproof render acts as a barrier, preventing groundwater from entering the structure, especially where hydrostatic pressure is present due to the water table.

By contrast, damp proofing systems—like our specialist PTL two-coat render—are typically used above ground level to manage residual moisture from soil and moisture vapour.

Waterproof rendering is essential in:

• Basements and cellars
• Earth-retaining walls
• Any below-ground structure that is exposed to persistent or periodic water table problems.

The waterproof system selected depends on the structure, existing conditions, and the outcome of an expert survey from a CSSW (Certified Surveyor Structural Waterproofing) surveyor, or Waterproof Design Specialist (WDS)

Preservation Treatments uses a variety of systems, tailored by our experienced/qualified surveyors after careful assessment. Here are the most common types:

1. Pre-Bagged Waterproof Renders

• Waterproof additive is pre-mixed in the bag.
• Can be single-coat or multi-coat (up to 3 coats), depending on manufacturer.
• Typically achieves a final thickness of 18–20mm.

2. Fast-Setting Pre-Bagged Renders

• Available in one-, two-, or three-coat systems.
• Final thickness: 18–20mm.

3. Site-Mixed Waterproof Renders

• Made on-site using specialist sand, cement, and waterproofing admixtures.
• Usually applied in four coats:
  1. Spritz coat (first barrier + creates key for bonding)
  2. Base render coat
  3. Second spritz coat.
  4. Final top render coat
• Each layer is mixed to a specific ratio and includes waterproofing admixture.

4. Waterproof Slurry Coats

• Thin application: two coats achieving 3–4mm in total.
• Best for smooth, consistent surfaces in good condition, (ideally re-enforced concrete)
• May require a levelling render coat (2:1 ratio, 6–8mm thick) if substrate is uneven.

5. Waterproof Screeds for Floor Slabs

• Typically uses materials from items 1 or 2.
• Applied at approx. 40mm thickness.
• If using item 4 materials on floors, slab must be in good condition.
• Note: slurry coats are not suitable as wearing surfaces.

Waterproof rendering falls under Type A (Barrier Protection) as defined by BS 8102:2022. This standard covers structures made from concrete or masonry that do not inherently resist water ingress. Therefore, a barrier system—applied internally or externally—is necessary, often used alongside land drainage for maximum effectiveness.

Providing they are fully trained and experienced in applying waterproof systems correctly.

This includes:

• Proper surface preparation
• Correct repair of any damaged substrate
• Using the right materials, ratios, and additives
• Understanding cure times between coats
• Applying with care to avoid failure due to weak bonding or voids

However, it is important to note that most builders cannot provide long-term guarantees together with insurance-backed guarantees, which we at Preservation Treatments can offer for added peace of mind, together with longevity within the waterproofing industry since 1972.

Waterproof rendering is not just a version of damp proof render—it is a fundamentally different system used in specific below-ground scenarios to resist water under pressure, not just moisture vapor. Choosing the right system and having it applied correctly is crucial to the long-term performance of your building.

If you have a project, you would like us to look at, or you would like more information about our specialist waterproofing services, our qualified surveyors are here to help.