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:
- Surface preparation, the concrete walls are cleaned, any honeycombing or defects are repaired, and sharp edges are rounded to prevent membrane puncture
- Primer application, a bituminous or epoxy primer is applied to ensure proper adhesion of the membrane to the concrete
- Membrane installation, the tanking membrane is applied in overlapping strips with all joints, penetrations and terminations carefully detailed
- Protection board, a rigid protection board is installed over the membrane to prevent damage during backfilling
- Drainage layer, a drainage board or gravel drainage layer is installed to channel groundwater away from the membrane and reduce hydrostatic pressure
- Backfill and compaction, carefully controlled backfilling to avoid membrane damage
Tanking Costs in Auckland
Tanking is quoted per project rather than by the square metre, because the variables outweigh the area by a wide margin. A retaining wall tanked externally before backfill is the simplest and cheapest case. The same area remediated internally on a basement that is already leaking, where the water is pushing against the membrane and there is no room to excavate, is a different job entirely.
What moves it: whether the work is external or remedial internal, how much groundwater pressure the system has to resist, whether excavation is possible and what it involves, the detailing at construction joints and service penetrations, and whether drainage layers and protection boards are needed alongside the membrane.
What is worth knowing before you decide on the specification is the cost of getting it wrong. Remediating a failed tanking system means excavating back down to it, sometimes removing built structure to reach it, then reinstating everything afterwards. It is a large multiple of the original installation. Below ground is the one place where the cheaper system is almost never the cheaper decision.
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.
Frequently Asked Questions
Do I need tanking for a retaining wall?
If the retaining wall has a habitable space, garage or storage area behind it (on the lower side), then yes, tanking is required by the NZ Building Code to prevent moisture penetrating through to the interior. Free-standing garden retaining walls without structures behind them typically do not require tanking.
Can you tank a basement from the inside?
Yes, internal (negative-side) tanking is possible and is often the only practical option for existing basements where external excavation is not feasible. Internal tanking systems use cementitious or epoxy-based coatings that bond to the concrete and resist negative hydrostatic pressure. However, external tanking is always the preferred approach for new construction.
How long does tanking membrane installation take?
A typical residential retaining wall tanking project takes 2-4 days. Full basement tanking for a new build can take 5-10 days depending on the size and complexity. Remedial tanking on existing basements may take longer due to preparation requirements and access constraints.
What happens if tanking fails?
Failed tanking can lead to water ingress, dampness, mould growth, structural deterioration and in severe cases, flooding. Remediation of failed tanking is significantly more expensive than getting it right the first time, which is why using a Licensed Building Practitioner with proven tanking experience is critical.
Is tanking covered by a warranty?
Yes. We provide a workmanship warranty on all tanking installations, and the membrane manufacturer provides a separate product warranty. The specific warranty terms depend on the system used and the application. As an LBP, our work is also covered by the Building Act’s implied warranties.