Building Below the Water Table
Basement construction is relatively uncommon in New Zealand compared to many countries — the combination of earthquake risk (which makes deep foundations more complex and expensive), expansive soils in some areas, and a culture of building on ground level rather than under it has historically limited basement use. But in Auckland and Wellington, where land costs are high and sections are tight, basements are increasingly being incorporated into residential and commercial buildings as a way to add habitable or storage space without increasing above-ground footprint.
Waterproofing a basement — preventing groundwater from entering through walls, floor, and the critical wall/floor junction — is a specialist area that requires careful design, appropriate product selection, and quality installation. A wet basement is a serious problem: it causes structural damage, mould growth, and loss of the usable space that justified building the basement in the first place.
Waterproofing Approaches
Basement waterproofing falls into two broad approaches: positive side (external) waterproofing, applied to the outside of the structure before backfilling; and negative side (internal) waterproofing, applied to the inside face of the structure. Positive side waterproofing is generally preferred where accessible — it keeps water out of the structure rather than just managing it once it’s inside — but requires the structure to be accessible for waterproofing before backfilling, which is straightforward for new construction.
Common positive-side systems include: sheet membranes (self-adhesive or torch-applied bituminous); spray-applied liquid membranes; and composite drainage board systems that protect the membrane and provide drainage. The system selection depends on site conditions, water table levels, and the structural form of the basement walls.
Construction Sequence
The sequence of basement construction — excavation, piling or sheet piling, base slab, walls, waterproofing, backfill — requires careful coordination. Waterproofing must be complete and inspected before backfill, which means the membrane contractor’s work needs to be sequenced into the overall construction programme rather than added as an afterthought.