Tiling onto UFH concrete slab

[SuperPav]

I suspect I know the answer here but having never done it before, I'm about to start tiling the utility and kitchen which is part of a wider open plan 70sqm area.

 

It's being tiled in 15mm limestone, and I'll be using either Keraflex maxi S1 or Keraquick S1 adhesive (I usually prefer working with the slower stuff, but am willing to go for the fast set if it's needed for the stone).

 

 

Now, originally, I hadn't even considered any form of decoupling mat. However, the slab has shown some stress fractures. 

Slab is 150mm self-leveling concrete with reinforcing mesh and UFH tied to mesh. It's been laid about 6 months ago so is fully dry/cured. Cracks have appeared at doorways and other "jutty out" places, all cracks are <1mm and some are just hairline. However, they appeared at various points in the first 3-4 months (rather than all at once) but seem to have now stopped. The cracks are not linear across a threshold etc. so I can't just lay an expansion joint over it (and frankly trying to avoid any expansion joints as the "doorways" are open plan 2.0m openings.

 

Now that I'm about to lay the tiles it's dawned on me that I need some form of crack control. Initially I thought I'd get away with a thin (e.g. Mapei waterguard) decoupling membrane, by just patching it over the cracked areas, and tiling over them with the flexible adhesive. However, the more I read about it, the more people just nonchalantly say "you need to ditra mat it" or equivalent (in my case the cheapest thing I can get is Mapei Um35 which is basically the same thing). 

 

Problem is, it's another £700 (+ whatever the extra adhesive costs) which, as everything else it seems, was not planned for and which unfortunately at the moment we don't have so it would mean at least one room not getting plastered.

 

Even if I had to replace 5 sqm worth of tile in the future it would still be cheaper than £700 as I'd be doing it myself. I obviously don't want the tiles to crack and at 15mm they're not as thick as some limestone tiles, but before I get rolls of the gold dust membrane, can somebody reassure me that it's worth it (or alternatively have any experience of tiling onto a cracked, albeit solid, concrete slab with just patches over the cracks or no decoupling at all)

 

[SuperPav]

[Andehh]

Considered a  flexible adhesive or equivalent? If its cracked & taken the strain out of the slab it shouldn't move much more?

[Jilly]

Wait til @nod replies as he’s the expert. We didn’t use a decoupling mat as it reduces the efficiency of the floor potentially. Ours is a smaller area and no screed cracks but used expansion gaps in the doorways and you don’t notice them if you use good ones (cheap ones looked crappy) 

[Temp]

We (only) used flexible adhesive and had cracks propagate through the stone. Only in one place at a doorway but annoying.

[Mike]

6 hours ago, SuperPav said:

the more I read about it, the more people just nonchalantly say "you need to ditra mat it" or equivalent

If there are cracks, that's what I'd do. Look for one - such as Mapeguard UM 35, Ditra, Durabase CI++ - that can be used as both a decoupling membrane and as a crack-isolation membrane (suitable for bridging minor cracks in the subfloor that's otherwise sound).

 

I'd also be installing expansion joints at doorways and the recommended intervals. As Jilly says, they don't have to be obvious.

Edited August 22, 2023 by Mike

[SuperPav]

thanks everybody, UM35 it is then!

 

Although in my head I still can't get round the fact that 20 years ago they wouldn't have had any membranes and people have been tiling onto slabs for god knows how long. So what's changed? Why can't you just tile on something like a lime mortar which inherently is very soft/flexible, and worst case is a tile goes loose (rather than cracks) and you just re-lay it on some fresh lime mortar...

[Pocster]

2 hours ago, SuperPav said:

thanks everybody, UM35 it is then!

 

Although in my head I still can't get round the fact that 20 years ago they wouldn't have had any membranes and people have been tiling onto slabs for god knows how long. So what's changed? Why can't you just tile on something like a lime mortar which inherently is very soft/flexible, and worst case is a tile goes loose (rather than cracks) and you just re-lay it on some fresh lime mortar...

It’s a bit like everyone use to plasterboard a shower enclose and then tile it . You wouldn’t ( shouldn’t ! ) do that now. It’s a bolts n braces approach I think . If you didn’t use decoupling mat and flexible adhesive and tiles crack - you’ll kick yourself . If you do and they still crack - then at least you tried …..

Edited August 23, 2023 by Pocster

[Mike]

6 hours ago, SuperPav said:

Why can't you just tile on something like a lime mortar which inherently is very soft/flexible

If you are laying tiles of traditional size and thickness, then you could. There are some on the floor above mine laid just like that - very wonky (it's a timber deck) but not overrun with cracks after nearly 200 years.

 

Preferably you'd lay them over a sand bed laid on gravel or shingle - the traditional 'decoupling membrane' that's been used for centuries.

 

4 hours ago, Pocster said:

So what's changed?

Modern rigid construction methods & thin large format tiles would be two key factors. Modern decoupling membranes are the response.

 

[DerbyLad]

On 23/08/2023 at 17:38, Pocster said:

It’s a bit like everyone use to plasterboard a shower enclose and then tile it . You wouldn’t ( shouldn’t ! ) do that now. It’s a bolts n braces approach I think . If you didn’t use decoupling mat and flexible adhesive and tiles crack - you’ll kick yourself . If you do and they still crack - then at least you tried …..

 

On 23/08/2023 at 17:38, Pocster said:

It’s a bit like everyone use to plasterboard a shower enclose and then tile it . You wouldn’t ( shouldn’t ! ) do that now. It’s a bolts n braces approach I think . If you didn’t use decoupling mat and flexible adhesive and tiles crack - you’ll kick yourself . If you do and they still crack - then at least you tried …..

Hope you don’t mind me going off post, but what do you mean by shower enclosure comment? We have skimmed plasterboard and intend to apply a tanking solution before tiling. Would that sound correct?

[Pocster]

6 minutes ago, DerbyLad said:

 

Hope you don’t mind me going off post, but what do you mean by shower enclosure comment? We have skimmed plasterboard and intend to apply a tanking solution before tiling. Would that sound correct?

What I meant in times gone by pb a shower enclosure and then simply tiling over it . Slightest of grout failure and the pb soaks up the water and the wall becomes unstable .