How to create slope on a pre-existing concrete slab to lay tiles?

Nuno

Hi!

To save some $ we are exploring if we can do the tiling on a new outdoor concrete slab in metro Brisbane.

The slab will be new, but by the time of tiling will have cured for at least 30 days. The size of the slab is 7.25m wide x 5m long and it will be a level surface to allow posts and building a roofed deck above it. The 5m length is what needs to fall away from the house.

Our builder has advised that if we are going to tile, it needs to have a slope away from the house to allow water drainage. I need to double check this, but I think we need a fall of 1:100 per metre (10mm per m) which gives a 50mm drop from the house to the end of the patio.

How do you do this to ensure a smooth gradual slope with no dips in the middle over such a big area? eg do we pour it in sections?

Do we need to screw in some wooden supports to the sides to hold in the cement (and potentially form a guide for the slope)?

What type of concrete should we use (given we are amateurs and won't be moving as quickly as a pro)?

I've read another discussion (https://www.workshop.bunnings.com.au/discussion/246438) which has some helpful tips about first creating a slurry and using bondcrete.

Also, given that it's going to still be quite a new slab, I've read some how-to's about something called an uncoupling membrane to mitigate against tiles cracking as the slab continues to settle. Is this needed in the Brisbane climate since the slab isn't really going to be exposed to much temperature variation? It may also just be a North American product as I haven't been able to find it here, or it's called something else.

Let me know and thanks in advance for advice!

EricL

Hello @Nuno

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

If the slab has not yet been poured, I propose speaking to your concrete specialist and informing them of your plans regarding tiling. They can incorporate the fall that you have just mentioned to your slab which will technically save you from having to pour another level of concrete on your slab that has the fall in it.

If the slab is already in place, I can only suggest engaging the services of your concrete specialist. Pouring a slab that is 1 meter by 1 meter or 2 meters max is still D.I.Y manageable. But anything bigger will start to become difficult as you will need knowledge and experience to manage large concrete pours. Timing and cement mixture is critical as any mistake in the mixing and drying process can cause the slab to split or dry off with micro fractures.

If you are putting in a fall such as 1:100 The concrete specialist will have to build custom form work that has the fall integrated into it. This will allow them to level the surface with the fall built in. The concrete specialist will also have the knowledge of how to anchor the new concrete surface to your existing slab.

Can it be poured in sections? This will be at the discretion of the concrete specialist. To accommodate movement, I suggest using silicone on the gap of the tiles where the slab expansion joints are located. I also suggest using rubber modified tile adhesive to compensate for any movement in the slab.

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

If you have any other questions we can help with, please let us know.

Eric