AIW Technical Bulletin - WFT vs DFT = WTF?

Liquid membranes are extensively utilised throughout Australia in a variety of contexts, including internal wet areas, balconies, subterranean applications, and even as UV-exposed rooftop membrane systems facing intense UV conditions, which demand robust and durable waterproofing solutions.

In the ever-changing landscape of liquid membrane technologies, it can be challenging to determine the most suitable system or technology for specific applications. This article aims to shed light on Wet Film Thickness (WFT) and Dry Film Thickness (DFT) concepts, paving the way for a deeper discussion on membrane applications in a subsequent article.

• WFT = Wet Film Thickness
• DFT = Dry Film Thickness

Every liquid membrane has a specific solids content (typically between 40-80%), in simple terms this means that the remaining percentage of the membrane will typically evaporate during the curing process.

As an example, if a membrane is applied at a rate of 800 micron (0.8mm) per coat and is 50% solids, once the membrane has cured you will be left with a coating that is 400 micron (0.4mm). This seems quite technical and you're probably thinking to yourself "why do I need to know all this information, I'll just slap it on nice and thick and it'll be fine''- incorrect.

Many membranes I test for finished DFT (Dry Film Thickness) are well under the requirements and the applicators have stated they went real thick with the application.

Manufacturers should always specify the required DFT on their product's technical data sheet. This specification is essential because each membrane must pass certain standards, such as AS4858 for internal wet area use or AS4654.1 for external above-ground use. The sample thickness used for these tests must be the minimum DFT applied in real-world scenarios.

It's not uncommon in the market for manufacturers to test a product at a higher DFT (1.5-2mm) to achieve satisfactory results and on the technical data sheet state that it can be applied at lower DFT's eg: 0.8=1.0mm. This is not deemed as a compliant application of the membrane.

Whatever sample thickness was used for the test results must be the minimum DFT the product can be applied at as there is no data/backing to confirm the membrane would work at a lower applied thickness.

Now that we've covered off on how critical it is to achieve the correct WFT/DFT for liquid membranes, lets cover off on the easiest ways to help achieve this on site:

1. Wet Film Thickness Indicators/combs: Using a wet film thickness comb is a great way to confirm that you're applying the membrane at the correct thickness whilst it is wet, this approach also offers the advantage of allowing for adjustments; if the initial application is too thin, more membrane can be added promptly to prevent potential problems in the future.
2. Measuring the Application Area: A simple yet efficient strategy involves measuring the area where the membrane will be applied and calculating the product consumption per coat. For instance, if the technical data sheet of a membrane specifies coverage of 20m2 per 15L pail per coat, you can delineate areas of 20 square meters to ensure correct usage. Even with this method, it's still advisable to verify the wet film thickness using a gauge to ensure accuracy and consistency in the application.
3. Ultrasonic Film Thickness testing: Once the membrane has been applied and is fully cured, ultrasonic film testing can be undertaken. This is specialised equipment that can provide the DFT of the membrane in a non-destructive manner. Ultrasonic testing is typically effective on various forms of membranes and is commonly performed by waterproofing inspectors, certifiers, and similar professionals to ensure compliance and quality.

I hope this information enhances your understanding of the significance of membrane thicknesses and provides insight into how to effectively implement testing methods for compliance in your upcoming waterproofing projects.

Written by

Kieran Biber

AIW Technical Committee Member