Checking wet film thickness (WFT) while applying a liquid waterproofing membrane is one of the most important steps you can take to ensure a watertight result. Without it, you're guessing — and an under-applied membrane is one of the leading causes of waterproofing failures in NZ homes. The good news is that checking WFT is straightforward once you know how.
Why Wet Film Thickness Matters
Liquid membranes — whether polyurethane, acrylic, or bitumen-modified — are designed to perform only when applied at a specific thickness. Manufacturers specify a minimum wet film thickness (usually in microns or millimetres) that, once cured, delivers the dry film thickness needed for waterproofing performance. Apply too thin and the cured membrane may crack, pinhole, or simply not meet the requirements of NZS 4858 (the NZ standard for wet area construction) or the NZ Building Code, particularly clauses E3 (Internal moisture) and E2 (External moisture).
For tiled wet areas like showers and balconies, getting this right matters even more — a failed membrane hidden under tiles is expensive to fix and may require a full strip-out.
What You Need: The Wet Film Thickness Gauge
The tool for the job is a wet film thickness (WFT) comb gauge — a small, inexpensive notched metal or plastic comb that you press into the wet membrane immediately after application. Each notch is a different depth; the last notch that comes out coated with membrane tells you your current WFT.
- Look for gauges at trade paint suppliers, waterproofing suppliers, or stores like Bunnings or Mitre 10.
- As a rough guide, basic comb gauges cost $10–$40 depending on range and material.
- Make sure the gauge range covers your target thickness — many liquid membranes require 500–1,000+ microns WFT per coat.
How to Use a WFT Comb Gauge: Step by Step
1. Know Your Target Thickness First
Before you start, read the membrane manufacturer's technical data sheet (TDS). It will specify the required WFT per coat and the number of coats. Products from Ardex, Mapei, and Sika all publish detailed TDS documents — always follow these, not just the label.
2. Apply Your Membrane
Apply the membrane with a roller, brush, or squeegee as directed. Work in manageable sections — roughly 1–2 m² at a time — so the membrane is still wet when you check.
3. Press the Gauge In Immediately
Within a minute or two of application (before any skinning occurs), press the comb gauge firmly and perpendicularly into the wet membrane. Hold it flat against the substrate — don't angle it.
4. Read the Result
Lift the gauge cleanly. Check which teeth are wet with membrane and which are clean. The highest notch that is wet gives you your WFT reading. If it's below the target, apply more membrane to that section before it skins over.
5. Check Multiple Spots
Take readings in at least 3–5 locations per coat, including corners, coves, and field areas. Edges and corners are where applicators most often under-apply.
6. Clean the Gauge
Wipe the gauge clean with a rag or appropriate solvent immediately after each use so it stays accurate.
Coverage Rate as a Cross-Check
A useful double-check is to calculate your coverage rate. The TDS will list coverage in m²/litre at a given WFT. Weigh or measure your membrane before and after applying to a known area, then compare against the spec. If you used significantly less product than the TDS suggests, you've likely applied too thin — even if your gauge readings looked okay.
| Check Method | What It Tells You | When to Use |
|---|---|---|
| WFT comb gauge | Thickness at specific spots | During application, every coat |
| Coverage rate calculation | Average thickness across whole area | After each coat |
| Dry film thickness gauge | Final cured thickness | After full cure (specialist tool) |
Common Mistakes to Avoid
- Checking too late — if the membrane has started to skin, the gauge reading will be inaccurate.
- Only checking the middle — always check corners, coves, and around penetrations (pipes, drains) where thinning is most common.
- Ignoring recoat windows — applying a second coat outside the manufacturer's recoat window can cause adhesion problems. Check the TDS.
- Skipping the reinforcing fabric — many systems require a reinforcing fleece or mesh at junctions. This is part of the system, not optional.
NZ Regulatory Context
For wet areas in NZ homes, waterproofing must comply with the NZ Building Code and typically NZS 4858. Waterproofing a shower or balcony generally does not require a building consent under Schedule 1 exemptions, but you should confirm this with your local council before starting — rules can vary. BRANZ publishes good guidance on wet area waterproofing best practice for NZ conditions.
When to Call a Professional
While checking WFT is a skill any careful DIYer can learn, the overall waterproofing job requires attention to detail throughout. Consider hiring a qualified waterproofer if:
- The area is large or complex (balconies, decks, wet rooms).
- You're waterproofing over a timber-framed substrate where movement is a factor.
- The project is part of a larger build or renovation requiring a Licensed Building Practitioner (LBP) — check lbp.govt.nz.
- You're not confident about substrate preparation, which is just as critical as application thickness.
You can find experienced local waterproofing tradespeople on tradedirectory.co.nz to either do the job or check your work before tiling.
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