Can I Apply a Second Brand of Liquid Membrane Over an Existing Failed One?

In most cases, applying a second brand of liquid membrane directly over an existing failed one is not recommended and can make the problem significantly worse. The short answer is: if the existing membrane has failed, the underlying cause needs to be identified and fixed first — and in the majority of situations, the old membrane must be fully removed before a new system is applied. There are limited exceptions, but they depend on the type of failure, the products involved, and the substrate condition.

Why a Failed Membrane Can't Simply Be Overcoated

A liquid applied membrane works by forming a continuous, bonded, flexible film over a substrate. When it fails — whether through delamination, cracking, pinholing, or loss of adhesion — that bond between membrane and substrate has broken down. Applying a new membrane on top of a failed one means your new product is bonding to something that is already compromised, not to the substrate itself.

Common problems that result from overcoating a failed membrane include:

• Poor adhesion — the new membrane bonds to the old one, which is already lifting or delaminating
• Trapped moisture — water that has already penetrated beneath the old membrane becomes sealed in, accelerating substrate damage (rot, corrosion, or concrete deterioration)
• Incompatibility — different membrane chemistries (polyurethane, acrylic, bituminous, epoxy) can react badly with each other, causing bubbling, softening, or complete failure of the new coat
• Masking the real problem — the root cause (poor drainage, movement cracks, inadequate falls, substrate failure) remains unaddressed

Under the NZ Building Code, waterproofing in wet areas is governed by clauses E2 (External Moisture) and E3 (Internal Moisture). Any waterproofing work must perform to these requirements. Cutting corners by overcoating a failed system risks non-compliance and can create significant liability, especially in rental properties subject to Healthy Homes standards.

When Overcoating Might Be Acceptable

There are narrow circumstances where a second membrane coat over an existing one could be considered:

• The existing membrane is intact and well-adhered but has simply reached the end of its service life (this is maintenance, not failure repair)
• The existing membrane has minor surface degradation (UV chalking, surface crazing) but is still bonded firmly to the substrate
• The membrane manufacturer explicitly states in their technical data sheet that overcoating with a compatible product is acceptable
• Both products are from the same chemistry family and the manufacturer has confirmed compatibility in writing

Even in these cases, you must follow the manufacturer's surface preparation requirements to the letter. BRANZ research consistently shows that surface preparation is the single biggest factor in waterproofing system longevity.

How to Assess the Existing Membrane

Before making any decision, carry out a thorough assessment:

1. Tap test — tap across the surface with a coin or knuckle. A hollow sound indicates delamination.
2. Pull test — try to lift an edge or a small section. If it peels away easily, adhesion has failed.
3. Check for moisture — use a moisture meter on the substrate beneath. Elevated readings mean water has already got in.
4. Identify the membrane type — check any original documentation, or consult a waterproofing professional. Mixing chemistries without confirmation is high risk.
5. Inspect the cause of failure — look for cracked flashings, inadequate falls, movement joints that have opened up, or substrate damage.

The Right Way to Re-Waterproof

In most failed membrane situations, the correct process is:

• Remove the old membrane completely — mechanically grind, scrape, or strip back to a clean, sound substrate
• Repair the substrate — fix any cracks, rot, or deterioration; re-establish correct falls if needed
• Address the root cause — fix drainage, movement joints, or flashing issues before applying anything new
• Prime correctly — use the primer specified by your new membrane manufacturer for that substrate type
• Apply the new membrane system — follow the manufacturer's coverage rates, number of coats, and cure times exactly

Products from Sika, Ardex, and Mapei all publish detailed technical data sheets and application guides — always read and follow these for the specific product you are using, as requirements vary between systems.

Compatibility: Always Check Before You Buy

If you believe overcoating is appropriate in your situation, contact both manufacturers directly — not just the retailer — and get written confirmation of compatibility. Do not rely on general advice. Membrane chemistry matters enormously:

You can find membrane products and surface preparation materials at Bunnings, Mitre 10, and PlaceMakers, but always source technical data sheets directly from the manufacturer's website.

When to Call a Professional

Waterproofing failures in wet areas (bathrooms, decks, balconies, roofs) can cause serious structural damage if not properly remediated. You should engage a qualified waterproofing tradesperson if:

• The failed membrane is in a shower, bathroom, or laundry (internal wet area)
• The area is a deck or balcony over a habitable space
• There is any sign of water damage to the structure beneath
• You are unsure of the membrane type or cause of failure
• The work may require a building consent — always check with your local council, as some remediation work may trigger consent requirements under the Building Act

For work involving plumbing connections, a licensed plumber is required — check the Plumbers, Gasfitters and Drainlayers Board for licensed practitioners. You can find qualified local waterproofing tradespeople on tradedirectory.co.nz.

Sources & Further Reading

• BRANZ — Weathertight and moisture research
• MBIE Building Performance — NZ Building Code clauses E2 and E3
• Sika NZ — Waterproofing systems and technical data
• Ardex NZ — Waterproofing products and application guides
• Mapei NZ — Waterproofing and tile installation systems
• Tenancy Services — Healthy Homes moisture and drainage standards