Tanking
Waterproofing

Tanking is the term used for waterproofing systems applied to below-ground structures — foundations, retaining walls, basement slabs, and any concrete element that will be in contact with the ground or subject to hydrostatic pressure. It's a completely different discipline from roof or deck waterproofing, and the stakes are higher: once a structure is backfilled, fixing a failed tanking system means excavation.

What is tanking, exactly?

The name comes from the concept of making a structure watertight like a tank — preventing water from entering from any direction. Where a roof membrane deals with water running over it, a tanking system must resist water pushing against it from the outside under hydrostatic pressure.

This distinction matters because it affects both material selection and installation technique. A membrane that performs well on a flat roof may be completely unsuitable for a below-ground application where it needs to resist ground water pressure without delaminating from the substrate.

Where tanking is used in Auckland

Tanking is required wherever a structure is in contact with ground moisture or where water ingress would cause damage or structural problems:

• Foundation walls — residential and commercial, particularly on sloped sites or where groundwater is present
• Retaining walls — concrete retaining structures that will be backfilled with soil
• Basement slabs and walls — both new builds and remediation of existing leaking basements
• Tilt-slab construction — the base of tilt-slab panels and the footing connection
• Subterranean car parks and service areas
• Planter boxes and podium decks — any structure where soil is placed over a concrete element

Materials used for tanking

Several material types are used for tanking applications, each with appropriate uses:

Sheet-applied membranes (Bituthene, Grace Ice & Water Shield)

Self-adhesive or torch-applied bituminous sheet membranes are commonly used on foundation walls. They're applied to the primed concrete surface and provide a continuous, high-performance waterproof layer. Bituthene (manufactured by GCP Applied Technologies) is one of the most widely specified products in New Zealand for this application.

Liquid-applied membranes

Polyurethane or bituminous liquid membranes applied by roller or spray. Better for complex geometry, penetrations, and areas where a sheet membrane can't be easily applied. Can be used alone or as a detail coat under a sheet system.

Cementitious waterproofing

Rigid or semi-flexible crystalline systems applied to the concrete surface. Used where access for membrane application is limited, or as a secondary system behind a sheet membrane.

The tanking process: timing is everything

The most critical aspect of tanking is timing. The membrane must be applied at the right point in the construction sequence — after the concrete has cured sufficiently and been prepared, but before any subsequent construction activities that could damage the membrane or before backfilling occurs.

On a typical foundation tanking project:

1. Concrete cure and preparation — surfaces are assessed for honeycombing, form tie holes, and cracks. These are repaired and ground smooth before any membrane is applied.
2. Priming — concrete surfaces are primed to ensure membrane adhesion. Moisture content is checked — applying a membrane to green or damp concrete is one of the most common causes of delamination.
3. Membrane application — sheet or liquid membrane is applied continuously across all surfaces, including the transition from slab to wall.
4. Detail work — all penetrations (pipes, tie holes, construction joints) are independently detailed before the field membrane.
5. Protection board — a drainage cell or protection board is placed over the membrane before backfilling to protect it from damage.

When tanking has failed

Signs of failed or inadequate tanking in an existing structure include:

• Efflorescence (white salt deposits) on internal concrete or blockwork walls
• Visible water ingress after rain or in winter months
• Damp patches on internal wall linings or floor coverings
• Rust staining from corroding reinforcement
• Mould growth on internal surfaces

Remediation of failed tanking on an existing structure is significantly more complex and expensive than a new installation — sometimes requiring excavation to access and re-apply the external membrane, or the use of internal remedial systems where external access isn't possible.

Getting it right before backfill is always the better option.