Riven Slate: use sand/cement or adhesive - is there a definitive answer

[Jim Rickards]

Hi - my builders bedded 10-12 mm brazilian riven slate on a sand cement mix. The base below was secure concrete laid a few weeks before. This was a new kitchen floor job, brand new extension.

I don't think they've done a very good job because many sound hollow when walking on them and a few were loose within days. The grouting is uneven in thickness and there are some very lippy bits. The chippy came in to do the skirting board and he said it was very uneven.

I'm expecting some uneveness because it's the nature of slate, same with the grouting, but I am worried about the hollow sounds and loose slate. The builders who built the extension also laid the tiles, so specialist tilers were not used.

So my first question is, is it normal to use a sand cement mix to bed the slate? Or should adhesive have been used? Is there any definite answer to that, because I've read conflicting views.

My second question is, how can it be fixed it long term.

And my third question is, am I within my rights to withold some money until it's fixed, or to get someone else in (a professional tiler) to sort it out and take that money off the final bill. It was a big extension and this is only one part of the job.

Sorry for the long post. Just wanted to explain it thoroughly.

[CJ]

How long ago was the base laid?
At a rough estimate, 1mm per day drying time. So 65mm screed.....65 days drying time

It also sounds like he has laid the slate in a dot/dab fashion.......which is definately a no no.

You can use sand & cement to lay slate........but it does take a bit longer to dry.......how soon after were they walked on?

The lipping is part and parcel of riven slate.......you can take a hammer and bolster to any really high edges and chip them off.

Was the floor sealed????

[Jim Rickards]

Hi - thanks for your quick reply.

Re lipping - I thought as much, and not really a problem with it. Will do what you say on the couple of very proud edges.

The base had a couple of weeks to dry so I think that shouldn't have been a problem and we didn't walk on the slate for 3-4 days at all, then only lightly.

Could there have been a problem with the proportions in the mix. It sounds like the mix has dried out without adhering to the underside of the slate.

The slate was sealed using quality stuff, penetrating mattstone, but they didn't clean the slate properly before doing it. (That's another story!)

[Jim Rickards]

1mm per day drying time. So 65mm screed.....65 days drying time

Just read that bit, more carefully. They didn't wait that long, that's for sure.

[Oli]

The base not having time to dry out properly may well have contributed to the problem, as CJ said, the recommended drying time is 1mm / 1 day so it should have been left for 65 days, not 14, i doubt it will have properly dried out in over 1/4 of the time it should have been left.

[CJ]

It definatley sounds like they have dot/dabbed the slate.......(hollow sound)

The screed not left long enough to dry..............that could cause OTHER problems very soon. The floor is STILL drying out, and very slight shrinkage will result in the tiles de-bonding from the screed..........so it COULD possibly get worse.

There is a green screed adhesive which should have been used in this instance.
BAL Adhesives - Products - green screed

Sorry to pee on ya chips (as the saying goes)

[theyomper]

If laying on a sand/cement screed I would have slurried the back of the slate, cement/water mix to help with adhesion to the screed.

[lucius]

I dont think anyone has read the original properly it was laid on a concrete base not a screed so 1mm a day dont come into it.If the slate was in an area of heavy traffic it should be laid in thick bed addy and if you realy wanted to be sure any where thats the way it should be done only problem it is expensive on addy. Theyomper got it right always slurry the backs when using s&c as a bed i actualy slurry with addy they never move then but the real answer is use someone who knows what they are doing.
Lucius.