Wonky concrete - Preparing sub-base on uneven slab

[WonkyConcrete]

Hi everyone,

I need some expertise on building up and tiling over some shoddy concrete slabs.

So I had an idea: Build an extension, with the same tiled floor inside and out.

The interior tiled floor leads up to a bi fold door track, which has a slot drain butted up against the exterior face. Then there's a c.1.5m strip of concrete slab. Then another slot drain, then another c.1.5m strip of concrete, then lawn.

The first piece of concrete varies around 2" from end to end. However the second piece of concrete wasn't even shuttered ??? nope, they "used the edge of the lawn instead". Genius. Suffice to say that there's around a 4-6" tolerance at best, across this piece, which puddles, runs into the lawn, etc.

The next thing to say is that there's no DPM anywhere.

I've read the advice on here, and it appears I should have the following to tile outside:

1. Hardcore,
2. DPM,
3. Adhesive,
4. Tile,

What I currently have is:

1. Mud, with bits of hardcore squashed into it
2. Concrete, varying in thickness (a bit of a guess) from 100mm - 300mm

Questions:

1. Can I build-up the level of one or more slabs?

I looked at using a frost proof self leveller with crushed granite, but am not convinced it would last long. It's also extremely expensive for the volumes I'd need.

I noted that you can use a wet-lay method for tiles outside, on muck. My concern is a scenario where the frost gets onto the slab and blows the tiles (or what they're fixed to) in a few years time.

2. Should I bite the bullet and hire a breaker?

I'm tempted to rip out the worst of the two slabs (the one furthest from the house). I could also use an industrial floor grinder, to get the first slab within some kind of acceptable tolerance although I'd still need over 10mm of adhesive in places

I also have 3 x new concrete pads to build for sheds, a path, etc so I could just re-shutter it, order some readymix, and smash it out in one go.

3. Break-into the surface to give a key, and reduce the height so that I can get a decent 100mmm pour on top of what's there, using an SBR slurry, then more concrete (shuttered properly)?

4. A combination of the above, with a Ditra matting, to decouple the surfaces in case of movement/frost/etc.

The tiles we have are around 13mm thick porcelain, 600x600. I've used Ditra matting indoors everywhere.

5. What about a DPM - should I have one?

6. Any other options I've not thought of?

This is my project for the summer. I want to do this once, and do it right.

Any advice would be hugely appreciated, especially around the methodology and the specification of the concrete/slurry/mixes that I should be using.

Thanks!

[rxbren]

No just no do not use porcelain tiles outside they will be deadly when wet there not suitable, they will crack, break, come loose and cause all manor of issues
You need to use proper slabs, sbr, sand and cement laying bed plus a suitable depth subbase

[WonkyConcrete]

OK, well thanks for the comments.

I've heard of plenty of people doing this, and there's a page on this site that talks about it...

http://www.pavingexpert.com/ceramic_01.htm

The area will be well drained, clean, and sheltered by the overhang of the building. The tiles are frost proof and slip resistant.

It's kind of the whole concept of the build, so I'd like to find a solution that uses the same floor inside and out if possible!

[rxbren]

Learn something new everyday I've o Ly ever came across it once where the ho.eowner had used i door tiles hence al the problems i listed ill hae a poper read through that when i het a chance

[WonkyConcrete]

Phew - you had me worried there.

I guess it's as much a question about concrete as it is tiling - can you build concrete up in thin layers, without it delaminating?

[GB_Groundworks]

yes you scale/scabble the concrete to create rough texture the n you can concrete ontop normally 80mm minimum layer

[seanandruby]

For thinner applications use grano', same preparation as Giles suggests, it can be applied from 6 ml where a thin cover is needed.

[WonkyConcrete]

Thanks Giles & Sean - much appreciated.

With the Grano, I'm guessing that's as per: http://www.pavingexpert.com/mortars.htm

So would I screed that flat within a few mm and then use a tile adhesive after it had set?

Still concerned about the lack of DPM - is that needed to keep everything stable?

Cheers guys

[seanandruby]

Yes that's it wonky. Don't need dpm on top of existing concrete, just scabble it, clean it and camp it down before laying, maybe prime it with ant.

[WonkyConcrete]

Thanks Sean. Do you know why they specify dpm under the concrete on the http://www.pavingexpert.com/ceramic_01.htm page? If it's needed then maybe using a ditra mat on top of what I have would help in this respect?