Ceramic tiling onto cracked concrete hearth

[_hunter]

Hi -

I'm trying to lay a flush tiled hearth for a new fireplace. It's likely to be decorative rather than used for fires, but so far it has been built to regulations for solid fuel use.

To get it flush with the new floor, the hearth has been raised with a sand and cement slab over a concrete slab. This top mortar layer has cracked badly with a lot of shrinkage cracks (see photos), although it was laid with an SBR Bond slurry underneath so it's pretty solidly stuck down and there isn't any vertical movement or hollowness. It was laid 4 weeks ago, and I am planning to wait the full 6 weeks before laying any tiles.

Is it likely to be ok to just tile with a flexible adhesive? Or is the cracking too bad, that I should either take the slab up (I think the SBR is going to make this very difficult IMO), or forgo it ever being used as a fireplace (not overly fussed by this) and use a thin decoupling membrane to be on the safe side with all these cracks?

I've also considered adding a layer of HardieBacker board, but the minimum thickness is 6mm which will now push the tiles above the floor level and also it's still not meant to be used under a fire.

 

[Dave]

Take that up and SLC it. 👍

 

[_hunter]

OK. Is SLC ok for use in a fireplace hearth? Something like this: Flexible Self Levelling Compound SLC 1550 (Fibre Flex) 3-50mm - https://totaltiles.co.uk/flexible-self-levelling-compound-slc-1550-universal-fibre-flex-3-50mm.html

 

[_hunter]

Take that up and SLC it. 👍

I've tried taking it up and it's completely solid ... it's not going to come up easy.

Would it work to have a thin layer of SLC over the top, say 1-2mm, and then tile on that?

 

[Dave]

I've tried taking it up and it's completely solid ... it's not going to come up easy.

Would it work to have a thin layer of SLC over the top, say 1-2mm, and then tile on that?

if its cracked like that it should , how deep is it ?
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I've tried taking it up and it's completely solid ... it's not going to come up easy.

Would it work to have a thin layer of SLC over the top, say 1-2mm, and then tile on that?

sorry just seen pic of depth, use a crack mat and tile on top.

 

[_hunter]

if its cracked like that it should , how deep is it ?

It's the SBR bond slurry has stuck it down very solidly. I've tried chiselling under the joining edge (at floor level) and into the cracks but it doesn't shift. I can't see it coming up without taking an SDS to it.

This is actually the second time this slab has been laid, as the first time it cracked - without SBR - and came up almost by hand. Which is why we tried again with an SBR slurry, but the mortar cracked again, but it's now stuck pretty damn hard to the concrete underneath.

The mortar layer is only about 20mm thick, just to the level of the old floorboards that the ply is sitting on. The SBR slurry was generous!

 

[Dave]

It's the SBR bond slurry has stuck it down very solidly. I've tried chiselling under the joining edge (at floor level) and into the cracks but it doesn't shift. I can't see it coming up without taking an SDS to it.

This is actually the second time this slab has been laid, as the first time it cracked - without SBR - and came up almost by hand. Which is why we tried again with an SBR slurry, but the mortar cracked again, but it's now stuck pretty damn hard to the concrete underneath.

The mortar layer is only about 20mm thick, just to the level of the old floorboards that the ply is sitting on. The SBR slurry was generous!

Was it laid with yellow sand or sharp sand ?

 

[_hunter]

Was it laid with yellow sand or sharp sand ?

Builders sand - this: Tarmac Building Sand - Major Bag | Wickes.co.uk - https://www.wickes.co.uk/Tarmac-Building-Sand---Major-Bag/p/132008

My only reservation of using a crack mat is I guess it's not non-combustable?

 

[Dave]

Builders sand - this: Tarmac Building Sand - Major Bag | Wickes.co.uk - https://www.wickes.co.uk/Tarmac-Building-Sand---Major-Bag/p/132008

My only reservation of using a crack mat is I guess it's not non-combustable?

its not going to catch fire under the tiles..