What Is Tanking? A Guide to Below-Ground Waterproofing in Auckland

If you're building a basement, converting a below-ground space into a habitable room, or dealing with a leaking retaining wall in Auckland, you'll encounter the term “tanking”. It's one of the most critical — and most misunderstood — aspects of building construction. Get it wrong, and you face years of dampness, mould and expensive remediation. Get it right, and your below-ground spaces remain dry and usable for the lifetime of the building.

This guide explains what tanking is, when it's required under the NZ Building Code, how the different systems work, and what you should expect from a professional tanking membrane installation in Auckland.

Tanking Explained: The Basics

Tanking is the process of applying a continuous waterproof membrane to the external (or sometimes internal) surfaces of below-ground building elements — basement walls, floor slabs, retaining walls and foundations. The membrane creates a complete barrier between the building structure and the surrounding soil, preventing groundwater, soil moisture and hydrostatic pressure from forcing water into the building.

Think of it as wrapping the below-ground portion of your building in a waterproof envelope. Just as your roof membrane keeps rain out from above, the tanking membrane keeps groundwater out from below and to the sides.

When Is Tanking Required in Auckland?

Under the New Zealand Building Code (Clause E2 — External Moisture), any building element that is in contact with the ground and forms part of a habitable or occupied space must be protected from moisture penetration. This means tanking is required for:

• Habitable basements — bedrooms, offices, media rooms, living spaces below ground level
• Retaining walls with buildings behind them — where the retained earth is higher than the interior floor level
• Underground car parks — both residential and commercial
• Below-slab areas — where the water table is at or near the floor slab level
• Lift pits and service ducts — critical infrastructure areas below ground

Auckland's climate makes tanking particularly important. The city receives over 1,200mm of annual rainfall, and many suburbs sit on clay-heavy soils that retain moisture and create high water tables. Hilly terrain like the North Shore, Remuera and Titirangi means retaining walls are common, and each one is a potential pathway for water if not properly tanked.

Types of Tanking Membrane Systems

There are two main approaches to tanking: positive-side (external) and negative-side (internal). We use both, selecting the appropriate system based on access, ground conditions and project stage.

External (Positive-Side) Tanking

Applied to the outside face of the wall before backfilling. This is the gold standard for new construction. The membrane is protected by the earth itself and blocks water before it reaches the concrete. Systems include torch-on bituminous membranes, self-adhesive sheet membranes and liquid applied membranes.

Internal (Negative-Side) Tanking

Applied to the inside face of existing walls. Used primarily for remediation of leaking basements where external access is not possible. Cementitious and epoxy-based coatings bond to the concrete and resist water pressure from the soil side. While effective, it places the waterproof barrier on the wrong side of the structure — meaning the concrete itself remains wet.

The Installation Process

A professional tanking installation follows a systematic process. For external tanking on a new build, the sequence typically involves:

1. Surface preparation — the concrete walls are cleaned, any honeycombing or defects are repaired, and sharp edges are rounded to prevent membrane puncture
2. Primer application — a bituminous or epoxy primer is applied to ensure proper adhesion of the membrane to the concrete
3. Membrane installation — the tanking membrane is applied in overlapping strips with all joints, penetrations and terminations carefully detailed
4. Protection board — a rigid protection board is installed over the membrane to prevent damage during backfilling
5. Drainage layer — a drainage board or gravel drainage layer is installed to channel groundwater away from the membrane and reduce hydrostatic pressure
6. Backfill and compaction — carefully controlled backfilling to avoid membrane damage

Tanking Costs in Auckland

Tanking membrane costs in Auckland range from $80–$180 per square metre depending on the system, access, ground conditions and complexity of detailing. A typical residential retaining wall of 30m² might cost $2,400–$4,500 for the tanking membrane installation. Full basement tanking for a new build with 150m² of wall area could range from $15,000–$27,000.

While these are significant investments, the cost of remediating a failed tanking system is typically 3–5 times higher due to the excavation, building removal and reinstallation required. Getting tanking right the first time is always more cost-effective.

Why Use a Licensed Building Practitioner for Tanking

Tanking membrane installation is classified as restricted building work under the Building Act 2004. This means it must be carried out by a Licensed Building Practitioner. Using an unlicensed installer is not only risky — it's illegal, and it can invalidate your building consent and insurance.

As LBP #BP181755 with Roofing R4 & R5 endorsements, we have the qualifications and experience to ensure your tanking system is installed correctly, meets NZ Building Code requirements, and is documented for council inspection. Contact us for a free assessment of your below-ground waterproofing needs.