Applying liquid membrane too thinly in a wet area is one of the most common — and costly — waterproofing mistakes a homeowner can make. An under-thickness membrane may look fine on the surface but will almost certainly fail over time, allowing water to penetrate into the substrate, framing, and beyond. Understanding why dry film thickness (DFT) matters, and how to get it right, can save you thousands in remediation costs.
Why Dry Film Thickness (DFT) Matters
Dry film thickness (DFT) is the measurement of a cured membrane coating after all solvents or water have evaporated. Most liquid waterproofing membranes used in NZ wet areas — such as bathrooms, showers, laundries, and balconies — require a minimum DFT of around 1.0–1.5 mm, though you should always check the specific product's technical data sheet (TDS) for the exact requirement.
When membrane is applied too thinly, the cured film simply doesn't have enough body to:
- Bridge minor cracks or substrate movement
- Resist the hydrostatic pressure of repeated water exposure
- Maintain integrity at corners, junctions, and penetrations — the highest-risk zones
- Meet the performance requirements of NZ Building Code clauses E3 (Internal Moisture) and E2 (External Moisture)
What Actually Goes Wrong
A thin membrane can fail in several ways, often not immediately obvious:
Pinholing and Micro-Cracking
When applied too thinly, liquid membranes are prone to pinholing — tiny voids in the film that allow water to pass through. As the substrate moves slightly with temperature changes (very common in NZ's variable climate), a thin membrane can also develop micro-cracks it simply doesn't have the elasticity to bridge.
Delamination
A film that is too thin may not bond properly to the substrate, especially over fibrous reinforcing tape at junctions. Over time, water gets behind the membrane and causes it to lift or bubble — a process called delamination.
Water Ingress and Structural Damage
Once water bypasses the membrane, it saturates the substrate — typically fibre-cement sheet, moisture-resistant plasterboard, or a mortar bed. Prolonged moisture causes:
- Swelling and softening of sheet substrates
- Rot in timber framing and flooring
- Mould growth, which is a serious health concern and may be relevant to rental properties under the Healthy Homes Standards
- Corrosion of fixings and reinforcing
Failed Building Inspections and Insurance Issues
If waterproofing work is part of a consented renovation, a membrane that doesn't meet the specified DFT may fail a building inspection. Even without a consent, if a leak causes damage and an insurer finds the waterproofing was substandard, your claim could be declined.
How Much Membrane Is Enough?
As a general guide, most NZ-market liquid membranes (such as those from Ardex, Mapei, or Sika) require two or more coats applied at right angles to each other, with the second coat applied only after the first has fully cured. Always refer to the product's TDS for:
| Parameter | What to Check |
|---|---|
| Wet film thickness per coat | Tells you how thickly to apply each coat |
| Number of coats required | Usually 2, sometimes 3 at high-risk zones |
| Coverage rate (m² per litre) | Helps you calculate how much product you need |
| Curing time between coats | Critical — applying too soon traps moisture |
| Minimum DFT (cured) | The pass/fail benchmark |
A wet film thickness gauge (a simple, inexpensive comb tool available at most trade stores) lets you check thickness while the product is still wet, before it's too late to add more.
High-Risk Areas That Need Extra Attention
Certain spots in a wet area are far more vulnerable and often need an additional coat or reinforcing fabric:
- Floor-to-wall coves and junctions — movement is greatest here
- Around penetrations (waste outlets, pipes) — seal with compatible collar or bandage tape
- Internal and external corners
- Substrate joins and sheet edges
BRANZ research consistently identifies junctions and penetrations as the most common points of waterproofing failure in NZ homes.
Can You Fix a Thin Membrane?
If you catch it before tiling, yes — you can apply an additional coat, provided the existing membrane has cured and is still in good condition (no delamination, contamination, or damage). Check the product TDS for maximum recoat windows. If tiles are already laid over a failed membrane, remediation typically means a full strip-out, which is expensive and disruptive.
When to Call a Professional
Waterproofing in wet areas is not restricted work under NZ law in the same way that electrical or plumbing work is — but it is high-consequence work that directly affects the weathertightness and structural integrity of your home. If you're unsure about substrate preparation, product selection, or achieving the correct DFT, it's well worth engaging a qualified waterproofer.
For the plumbing connections in your wet area (waste outlets, supply pipes), note that this is restricted work under the Plumbers, Gasfitters and Drainlayers Board — it must be carried out by a licensed tradesperson.
You can find experienced, local waterproofing professionals on tradedirectory.co.nz who understand NZ Building Code requirements and manufacturer specifications.
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SiteWise
Licensed Building Practitioner (LBP)
