Class Ratings of Waterproofing Membranes: What They Mean in the Real World

Class ratings for waterproofing membranes can sound like abstract numbers pulled from a standard – Class I, II, III – but on site, they have very real consequences for how membranes move, how bond breakers are detailed, and ultimately whether a wet area stays waterproof over the long term.

In this article, ARDEX Technical Services Advisor Chris Brady breaks down what the classes actually measure, how they relate to bond breaker widths, and how to choose the right combination for your next project.

Where do class ratings come from?

In Australia, class ratings for waterproofing membranes are defined in AS 4858. The key property they describe is elongation – in simple terms, how far a membrane can stretch before it suffers long-term damage.

The standard groups membranes into three main classes:
– Class I – Low flexibility, least stretchy.
– Class II – Medium flexibility.
– Class III – High flexibility, the stretchiest membranes.

These aren’t just labels; they refer directly to measured elongation values.

Elongation ranges for each class

When a membrane is tested, a strip of it is stretched to see how far it can extend before failure or unacceptable damage. The class rating is based on the percentage of elongation compared to its original length.

Typical ranges are:
– Class I – Around 10–59% elongation.
– Class II – Around 60–299% elongation.
– Class III – 300% elongation or more.

To put that into perspective, if you have a 10 cm wide strip of a Class III membrane, you should be able to stretch it out to about 30 cm before it reaches its limit. Class I membranes, on the other hand, will stretch much less before they become over-stressed.

What about liquids vs sheet membranes?

The same class system applies to both liquid-applied and sheet membranes. A Class III liquid and a Class III sheet membrane have both met the same elongation requirements – they just come in different formats.

That means when you see “Class III” on a datasheet, you can immediately expect a highly flexible system, whether it’s a roll-on product or a sheet that’s bonded to the substrate.

Bond breakers and class ratings

Class ratings don’t sit in isolation; they are directly tied to how wide your bond breakers need to be at corners and junctions.

Because less flexible membranes can’t stretch as far, they need the movement to be spread over a wider area. More flexible membranes can accommodate the same amount of movement in a tighter space.

A useful rule of thumb is:
– Class I membranes – Need the widest bond breaker, typically around 100 mm.
– Class II membranes – Usually require a medium-width bond breaker, around 35 mm.
– Class III membranes – Can work with a narrower bond breaker, typically 12 mm minimum.

In practical terms, that 12 mm width is similar to what you’d get from a neatly tooled silicone cove between the floor and wall in a bathroom. For less flexible membranes, you might instead use a wider transition tape or broader bond breaker detail.

Types of bond breaker systems

Two common approaches to bond breakers are:

– Sealant/bond breaker coves – For example, a bead of compatible silicone or sealant, tooled into a smooth cove at junctions.
– Bonding and transition tapes – Such as ARDEX STB tape and similar products, which bridge the junction and create a debonded zone beneath the membrane.

Whichever system you use, the important part is that the effective width of the bond breaker matches the membrane class. A narrow bond breaker under a stiff, low-elongation membrane won’t give the system enough room to move.

What happens if you get the bond breaker wrong?

Using the wrong width bond breaker for the class of membrane can have serious consequences.

If a Class I (low flexibility) membrane is detailed with a very narrow bond breaker:

– Movement at a junction or corner will be concentrated in a tight line.
– The membrane may be stretched beyond its elongation capacity.
– Over time, this over-strain can cause fracturing or cracking in the membrane at that point.

Once the membrane fractures, the waterproofing system is compromised – and even small cracks can allow water to track behind finishes and cause hidden damage.

By contrast, a Class III membrane paired with a suitable 12 mm bond breaker can accommodate typical building movements in a much tighter zone without overstressing.

Choosing the right class for the job

Class ratings are not about “good vs bad” – they are about suitability for the application.

Some general guidelines include:

– Exposed rooftops and podiums – Often need membranes that are tough, UV resistant and capable of handling occasional foot traffic. These systems may be closer to Class I or II, where rigidity and wear resistance are as important as movement.
– Tiled internal wet areas – Such as bathrooms and showers, will often use Class III membranes under tiles, where flexibility helps manage movement at junctions and around penetrations.
– Under tile or vinyl systems – Frequently rely on more flexible membranes to provide a protective layer beneath the finish, while the finish itself provides wear resistance.

It’s important to look at both the class rating and the material type of the membrane to confirm it’s suitable for things like UV exposure, foot traffic and the planned floor or wall finish.

Standards, datasheets and details

While podcasts, training and over-the-counter advice are useful, the final word on system design comes from:

– The Australian Standards, including AS 4858 for membrane classification.
– The product datasheets, which spell out suitable bond breaker types and widths for each membrane.
– The manufacturer’s CAD details and technical drawings, which show exactly how junctions, terminations and penetrations should be treated.

ARDEX data sheets clearly state which bond breaker systems can be used with each membrane, and the ARDEX CAD library provides standard details for common situations such as wall/floor junctions, hob terminations and balcony edges.

Training and technical support

Understanding class ratings and bond breaker detailing is a core part of ARDEX’s waterproofing training:

– Liquid membrane courses cover class ratings, bond breaker design and practical application techniques.
– Sheet membrane courses show how the same concepts are applied in roll or sheet systems, including internal and external corners, lap joints and terminations.

Spaces in sheet membrane courses are often limited and fill quickly, so early booking is recommended.

For installers who can’t attend training immediately, ARDEX Technical Services is available to help interpret standards, choose appropriate classes and bond breaker systems, and review details for specific projects.

Key takeaways

When you’re next selecting or installing a waterproofing membrane, keep these points in mind:

– Class ratings come from AS 4858 and reflect how stretchy a membrane is.
– Class I membranes are the least flexible and need wider bond breakers (around 100 mm).
– Class II membranes sit in the middle, with bond breakers around 35 mm.
– Class III membranes are the most flexible and can work with narrower bond breakers (around 12 mm).
– The same class system applies to both liquid and sheet membranes.
– Matching the bond breaker width to the membrane class is critical to avoid fractures and leaks.

If you’re ever unsure about which combination to use, refer to the membrane’s datasheet, check the relevant ARDEX CAD details, or call ARDEX Technical Services before you start. A quick conversation up front is far cheaper than chasing leaks later.

ARDEX Technical Hotline: 1800 224 070

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