Slate tiles not sticking down

[Chris Blackwell]

Hi all, would really appreciate any advice you can give with a bit of a problem!
I've just about finished laying a slate patio, but haven't yet grouted.



The tiles are 600x600x~12mm. I've laid them on a bed of mortar (4:1 sharp sand/cement with a splash of plasticizer) on top of well cured concrete slab. I coated the backs of the tiles with a SBR/cement slurry before bedding them down.

Everything seemed fine at first but now i've noticed that a dozen or so tiles have become loose, and just lift off the bed. The bed is well bonded to the slab, just not to the tile.

This patio gets full sun in the afternoon, so my guess is that the recent cold mornings followed by hot sun in the afternoon has caused enough expansion to de-bond them.

I'd rather not have to hack up the bed and re-lay them, so i'm wondering if I can bond the tile to the bed? As a test i lifted one, marking its orientation, cleaned the back and painted the back and the bed (generously) with SBR slurry, and dropped it back in place. It seems to have stuck fast, but i don't know if this will work for the long term. I would expect that the cement in the slurry should form a decent chemical bond and once grouted it should be fine.

Any advice would be appreciated
Cheers, Chris

 

[jay]

:welcome:

Think you would be better off to lift and relay the lot otherwise you will be sitting on a time bomb sorry not the news you wanted

good luck

 

[diamondtiling]

Hi Chris, did you wash the dust off the back of the slate first?

 

[Chris Blackwell]

@Jay, Do you have any comment on my method?
If i need to relay them i will, but i don't want the same to happen again!

 

[Aston]

1. how did you prepare the concrete prior to laying the sand and cement?? did you wet it down, did you use a slurry prior to laying the sand and cement? the concrete may have sucked the moisture out of the mix!

2. you say you did a 4-1 mix but was the mix semi dry or wet, ie bit thick than mortar? how thick was the sand and cement bed?

 

[Chris Blackwell]

Hi Chris, did you wash the dust off the back of the slate first?

No i didn't, but i did generously paint the back of every tile with an SBR/cement slurry. When i lift the loose tiles i can see the SBR has dried and bonded nicely to the tile.

 

[Chris Blackwell]

1. how did you prepare the concrete prior to laying the sand and cement?? did you wet it down, did you use a slurry prior to laying the sand and cement? the concrete may have sucked the moisture out of the mix!

The concrete slab was given a good soaking in the area where we worked and then kept damp as we worked, no slurry was used on the slab and the mortar is bonded to the slab well. The mortar is solid, not crumbly as would be the case if it had been sucked dry. It is only the tile-mortar bond that is breaking

2. you say you did a 4-1 mix but was the mix semi dry or wet, ie bit thick than mortar? how thick was the sand and cement bed?

The mortar was mixed to a similar consistency i use for brick laying, smooth and buttery, so it'll hold a shape but not slump.

 

[Aston]

how long had the concrete been down?

 

[Chris Blackwell]

how long had the concrete been down?

i forget when i laid it, 3 or 4 months, could be more.

 

[Knackered Knees]

When I thick bed stone, I prime the concrete slab, then whilst that is drying wash my stone to remove dust, then notch trowell the concrete slab with adhesive, then apply my sand cement bedding and rake it to thickness and tamp down, then notch trowell the back of my stone and hammer into place with a rubber mallet, picking the stone up and checking coverage as I go.

In days gone by before thin bed adhesives I would soak all my stone in a large tub of water, wet the slab down with cement slurry then place my sand cement screed on top sprinkle a generous layer of dry cemnt on top and take my previously water soaked stone and hammer into place with a rubber mallet.

The problems your having may be due to not presoaking the stone first and most of the moisture has been sucked out of the slurry when placed on the back of the stone, as well as not wiping all the dust off prior to fixing.

 

[Chris Blackwell]

Thanks everyone for your input, its appreciated

@Knackered Knees - The advice i was given by the tile supplier was to use an SBR slurry on the backs of the tiles. My understanding was that the SBR wouldn't adhere well to a wet tile (SBR being a rubber compound). Also i see your point about wiping the dust off, however the liberal coating of SBR was more than enough to take care of the dust on the tiles as they weren't massively dusty.

I guess the "why" is sort of academic, lesson learned; ask for advice before not help afterwards!

It may be the case as Jay said that the best option would be to lift and relay them, but would anyone care to offer an alternative, or pass comment on re-bedding them on a slurry? To be clear this is my patio, and i'm not a tiler, just a diy-er, so if i can avoid relaying them that is completely preferable!

thanks again

 

[White Room]

Do you have solid coverage under the slate with the sand/cement.

 

[Chris Blackwell]

Do you have solid coverage under the slate with the sand/cement.

Yes, i'd say 90% or better coverage on all the tiles

 

[DHTiling]

The slurry coat method is more for work on dry pack laying.. imo as it gets a better hold, i think the wet bed has hindered the bond with the slurry coat.

If the bed is solid and well adhered as you say, then stick the tiles back down with Adhesive..

 

[Chris Blackwell]

If the bed is solid and well adhered as you say, then stick the tiles back down with Adhesive..

That does seem like the best solution. I couldn't use adhesive originally as there was too much variation in the levels on the concrete, but if i lift the tiles and treat the mortar like a screed, then i should be able to relay onto that without too much difficulty.

So before i do that what should i do to prep the mortar and tiles, and what adhesive should i use?

I'm guessing i should wait until the mortar is a week or two old so that its nice and solid and then it'll need sealing, but what with?