How to install crazy paving with 20mm mortar?

keshe5

Hi,

I’m laying crazy paving next to a driveway.
I’m laying it on new concrete, but I will need to put around 20mm of mortar to get it to be level with the existing driveway (taking into account the 20mm thickness of bluestone paving). It is currently around 40mm before the existing concrete/driveway.

How thick can tile adhesives be spread/laid? Like the Davco SMP EVO in the crazy paving tutorial video? Or is there another mortar better suited?

Thanks

Kath

EricL

Hello @keshe5

Welcome to the Bunnings Workshop community. It's sensational to have you join us, and thanks for sharing your question about crazy paving.

Most modern tile adhesives are not designed to be used thick as a levelling bed they’re primarily for bonding once the surface is already close to finished height. They’re tested and rated for fairly thin beds on relatively even substrates. For example:

• Standard tile adhesives are typically formulated for a 3 mm thin bed, maybe up to about 6–10 mm in bonded thick-bed situations on a fairly flat surface.
• Some manufacturer's technical guides specifically recommend max 10 to 12 mm bed thickness for adhesives under stone/tile.
• Pushing adhesive much thicker than the manufacturer’s data sheet risks cracking, shrinkage, debonding and weak bond because it isn’t formulated as a structural levelling mortar.

Crucially, adhesives cure by chemical reaction (cement-based) or dehydration (premix mastics) if you go thicker than they’re designed for, they cannot cure properly through the depth you’ve put down.

For your situation, building up roughly 20 mm over concrete and then bedding 20 mm-thick bluestone you want a proper thick bedding mortar / screed rather than a typical tile adhesive. I propose the following options:

1. Poly-modified thick bed mortars / screeds (Australian Builders 20kg Tile And Paver Screed)
   These are purpose-designed thick bed mortars or stone bedding mortars that are formulated for 10 to 50 mm beds for tiles/stone. These give the strength and flexibility you need under heavy natural stone. These can be trafficable and bond well to concrete when mixed and applied correctly. I recommend adding BondCrete for improved adhesion.
   
2. Traditional sand cement mortar
   A mix like 4:1 or 5:1 sand to cement, laid 20 to 40 mm thick, is a very common trade method used for paving and stone on concrete. Worked up to level and compacted with a rubber mallet, it provides a reliable, load-bearing bed. Again, I recommend adding BondCrete for improved adhesion.
   

In conclusion you can’t reliably make up 20 mm of level with typical tile adhesive alone, adhesives are usually limited to around 10 mm thick once compressed under the tile/stone and are not intended to be used as structural bedding.

For your crazy paving on a new concrete slab, I recommend using a purpose designed thick bed mortar / screed (or traditional sand-cement mortar), then bed the bluestone into it. This will be far more durable and won’t risk bond failure or cracking due to excessive bed thickness. My best advice is to make sure that your new concrete has cured properly and is no longer holding trapped moisture.

Please note that if your area is larger than 2m x 2m, I propose engaging the services of a professional as they will have the knowledge and experience to handle larger areas where screed or mortar has been applied.

Please remember to wear personal protection such as gloves, goggles and mask when working on your project.

Let me call on our experienced members @Nailbag and @Dave-1 for their recommendations.

If you need further assistance, please let us know.

Eric