Yes, applying a liquid membrane waterproofing system to a concrete basement floor is a job many capable DIYers can tackle — but only if the conditions are right and the preparation is done properly. Skipping steps or choosing the wrong product for your situation can lead to costly failures, so read on before you crack open a tin.
Understanding What Liquid Membrane Does
Liquid membrane is a brush-, roller-, or trowel-applied coating that cures to form a seamless, flexible waterproof barrier. On a basement floor it protects against moisture vapour transmission, minor water ingress, and damp rising through the slab. It is not a structural repair product — if your basement has active water leaks, cracking, or drainage problems, those issues must be resolved first.
Common types available in NZ include:
- Polyurethane-based membranes — highly flexible, good for slabs subject to minor movement
- Cementitious (cement-based) membranes — rigid, excellent bond to concrete, good for below-grade applications
- Bitumen-modified membranes — cost-effective, often used under screeds
- Epoxy coatings with waterproofing properties — very durable but less forgiving of movement
Products from Ardex, Mapei, and Sika are well-established in the NZ market and widely available through trade and hardware suppliers.
Do You Need a Building Consent?
Applying a waterproof membrane to an existing basement floor is generally not a restricted building work and typically falls under Schedule 1 exemptions — meaning no building consent is required for the membrane application itself. However, if you are converting a basement into a habitable space, adding drainage, or altering the structure, consent may be needed. Always confirm with your local council before starting, as requirements vary. You can find guidance at MBIE Building Performance.
Step 1 — Assess the Substrate
Before buying anything, check:
- Active water ingress: Water pushing through cracks or joints under hydrostatic pressure cannot be stopped by a surface-applied membrane alone. You'll need a waterproofing specialist.
- Moisture vapour: Use a plastic sheet taped to the floor for 24–48 hours. Condensation underneath indicates high moisture vapour emission — choose a product rated for damp substrates.
- Cracks and honeycombing: Fill with a suitable repair mortar and allow to cure fully before applying membrane.
- Contamination: Oil, paint, or curing compounds must be removed — usually by grinding or shot-blasting.
Step 2 — Prepare the Surface
Good preparation is the single most important factor in membrane performance. The concrete must be:
- Clean — sweep, vacuum, and degrease thoroughly
- Sound — tap the floor; hollow spots indicate delamination and must be repaired
- Open-textured — smooth, power-trowelled concrete should be lightly ground or acid-etched to improve adhesion
- Dry enough — check the product's moisture tolerance; cementitious membranes are more forgiving than epoxies
Pay special attention to cove details at wall-to-floor junctions. Most manufacturers require you to form a fillet (a small curved or angled bead of mortar) at this junction before applying membrane, as corners are a common failure point.
Step 3 — Prime if Required
Many liquid membrane systems require a primer coat to improve adhesion and reduce pinholing. Check your product's technical data sheet — this is not optional. Apply primer with a brush or roller and allow it to reach the correct tack before proceeding.
Step 4 — Apply the Membrane
| Application method | Typical use |
|---|---|
| Brush | Detail work, corners, pipe penetrations |
| Roller (medium nap) | Large open floor areas |
| Trowel | Thicker cementitious systems |
Key tips:
- Apply in two or more coats at right angles to each other, as directed by the manufacturer
- Maintain the wet film thickness specified — too thin and the membrane will underperform
- Embed reinforcing fabric (if supplied) into the first wet coat at all joints, cracks, and wall-floor junctions
- Allow each coat to cure to the manufacturer's specified time before applying the next
- Work in temperatures between roughly 10°C and 35°C and avoid direct sun on the wet membrane
Step 5 — Protection and Overcoating
Once cured, the membrane must be protected before laying any screed, tiles, or flooring. Check the product's overcoating window — applying a screed too late can reduce adhesion. If tiling, confirm the membrane is compatible with your tile adhesive system.
Safety Considerations
Some liquid membranes contain solvents or isocyanates (in two-part polyurethanes). Always:
- Read the Safety Data Sheet (SDS) before use
- Ensure adequate ventilation in the basement — use fans if necessary
- Wear appropriate PPE: gloves, eye protection, and a respirator rated for the product
- Follow WorkSafe NZ guidelines for working with chemical products in confined spaces
When to Call a Professional
Call a licensed waterproofing tradesperson if:
- There is active water ingress or hydrostatic pressure from groundwater
- The basement is being converted to a habitable room (this may trigger NZ Building Code requirements under clauses E2 and E3)
- You find significant structural cracking or slab movement
- The job involves plumbing or drainage alterations — these are restricted works requiring a licensed tradesperson registered with the Plumbers Gasfitters & Drainlayers Board
- You are unsure about any part of the process
A qualified waterproofer will also be able to provide a warranty on their workmanship, which matters if you ever sell the property. You can find experienced local waterproofing professionals on tradedirectory.co.nz.