Levelling Over Hebel: Managing a Soft, Porous Substrate

Hebel – an autoclaved aerated concrete (AAC) product – is widely used in Australian construction for walls and floors. It’s light, insulating and easy to work with. But when it comes to levelling and floor coverings, Hebel’s softness and porosity demand a different approach from standard concrete.

In this article, ARDEX Technical Services Advisor John Werlemann explains why levelling over Hebel requires special consideration, and outlines the key steps to building a robust system.

What is Hebel and why is it challenging?

Hebel panels are lightweight and highly insulating, which makes them attractive in modern construction. However, compared to conventional concrete slabs they are:

– Much softer and lower in compressive strength.
– Highly porous, behaving almost like a sponge.
– More susceptible to damage if point loads are concentrated.

Because of this, simply treating Hebel like concrete and applying a standard levelling compound is a recipe for problems such as cracking and debonding.

Think about what’s going on top

When designing a levelling system over Hebel, you need to consider not just the substrate, but also the final floor finish. Different coverings impose different demands on the levelling compound and the Hebel beneath.

Two examples that require particular care are:

– Vinyl and resilient floor coverings – These are thin and flexible, making them prone to telegraphing any movement or indentation in the substrate. If the levelling compound or Hebel is too soft or cracks, it is likely to show through.
– Direct stick timber flooring – Timber moves with changes in humidity. If the timber pulls or pushes on the levelling layer and that layer is not strong or flexible enough, it can cause the leveller to fail and even damage the Hebel surface.

Tiles, by contrast, distribute loads more evenly and are generally more forgiving – but you still need an appropriate system under them.

Priming Hebel correctly – two coats for a reason

Because Hebel is so porous, thorough priming is crucial. If you don’t prime adequately, the Hebel will suck water out of the levelling compound, potentially:

– Weakening the leveller.
– Causing poor surface strength or dusting.
– Leading to premature cracking.

ARDEX technical literature recommends a two-coat priming system using ARDEX P 51:

– First coat: diluted 1:3 with water – this low-viscosity mix penetrates deeply into the Hebel, saturating the pores.
– Second coat: diluted 1:1 with water – this seals the surface and provides an ideal base for levelling.

This sequence turns the highly absorbent Hebel into a more controlled substrate, helping the levelling compound cure properly and achieve its designed strength.

Adding flexibility with ARDEX E 25

To better cope with Hebel’s softness and any movement in the system, ARDEX often recommends modifying certain levelling compounds (such as ARDEX K 15 or K 12) with ARDEX E 25.

E 25 is a polymer dispersion that, when used as a mixing liquid in place of some or all of the water, gives the levelling compound:

– Increased flexibility.
– Improved bond strength.
– Enhanced performance in high-stress situations, such as direct stick timber or in-floor heating over Hebel.

In effect, the modified leveller becomes more compatible with the Hebel substrate and better able to accommodate small dimensional changes without cracking.

Minimum thickness and load distribution

Because Hebel is relatively soft, maintaining a suitable levelling compound thickness helps spread loads and protect the substrate. ARDEX generally recommends:

– A minimum of around 5 mm of levelling compound over Hebel to reduce the risk of fracture.

Going too thin can leave the leveller vulnerable to cracking under localised loads, particularly beneath resilient floor coverings and timber.

Common mistakes to avoid

John highlights a few recurring issues seen on problem jobs:

– Inadequate priming – Skipping or reducing primer coats leads to weak, chalky levelling surfaces.
– Using unsuitable levelling compounds – Not all products are rated for use over Hebel; some may be too rigid or too weak without modification.
– Ignoring the final floor finish – Systems that might be acceptable under tiles may be unsuitable under vinyl or timber.

Each of these mistakes can result in cracking, indentation, telegraphing through finishes, or even damage to the Hebel itself.

Top tips for levelling over Hebel

For a successful installation:

– Do your homework – Read ARDEX Technical Bulletin TB105 and the relevant product datasheets before starting.
– Prime correctly – Use the recommended two-coat P 51 priming system.
– Select an appropriate levelling compound – Confirm that it is suitable for use over Hebel.
– Use ARDEX E 25 where required – Especially under direct stick timber, vinyl or in-floor heating.
– Respect minimum thicknesses – Maintain at least 5 mm of leveller over Hebel.

As always, if you’re unsure whether a particular system is suitable for your Hebel project, contact ARDEX Technical Services for guidance.

ARDEX Technical Hotline: 1800 224 070

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