Tiling onto a concrete patio base and levelling it

[Unregistered]

Hi all,

I'm hoping someone can either reassure me or provide some guidance on tiling outdoors.

So we're in the middle of a building project and using porcelain tiles to run through the kitchen, through bi fold doors and on to a patio to create a continuous surface.

The builder has put slot drains in against the bi fold and along the centre of the concrete slab.

He's done the slab in more than one pour, so there are low spots, and changes in level, but it's around 100mm thick. It's also pretty muddy now. The levels need to come up by around 10-20mm across around 10sqm.

Ordinarily, I'm guessing you'd use a self levelling compound and then tile over, however outside, I'm concerned about frost getting under the tiles and/or under any layers that are added on top of the concrete. I'm trying to avoid a scenario where in a couple of years' time, all the tiles outside start to blow due to frost.

So my question is - can you use some form of mortar/self-leveller to correct the levels, before adding the tile adhesive? The tiles are 600mm x 600mm x 8mm, so would be especially prone to cracking. Also is there a special grout/adhesive/preparation the builder should be using?

Finally, there is a hot tub being put on the patio after it's finished, which will weigh around 2.5 tonnes.

Of course the worst case scenario is the builder breaks the slab up and does it in a single pour to the right level.

Would massively appreciate any advice you can give me!

Thanks,

Ed

 

[Ajax123]

There are certainly smoothing compounds and adhesives for outdoor use. It sounds like your substrate is a bit of a mess though. You shouldn't bridge the joints in the slab as these are effectively joints. So you need to resolve that bit.

2.5 tonnes is actually not that much. Based on it being say 2.5m x 2m (numbers chosen for ease of maths) that equates to 5 square meters. That means you have about half a tonne per square meter. This calculates down to around 5kN/m2 static loading. The standard concrete grade for this type of application would be minimum gen 3 with pp fibres. This is a 20kN/mm2 concrete so you are at 25% of its loading capacity. The adhesive and tiles are generally considerably stronger.

 

[Dougs Third Go]

I'd be waiting until Spring, most adhesives, levelling systems, don't perform if used in temperatures below 5deg C.

 

[ED0]

Thanks both for the responses.

Can you recommend the smoothing compounds/adhesives for outdoor use?

Also, where the two slabs join isn't a straight line - the first slab was feathered off to nothing over around 1 m, the second pour was then feathered back over the top. I doubt the builder used a fibrous mix either - should I rip it out and re do it properly?