advice on substrate prep on an old concrete subbase to receive stone .

[jonnyc]

I am quoting to lay a reclaimed limestone chequerboard floor over an existing base that is assumed to be concrete once the existing stone tiles have been removed by main contractor.
there will be no underfloor heating involved but i would ideally like to lay some sort of membrane between the original base and my new stone to help mimimise possible problems in future.
the reclaimed stone is a one off floor @ 300x 300mm with the white tiles being approx 20mm and the black 40mm.
i have no problem with the different thickness as i can bed up the thinner stone in a thickbed adhesive which would accomodate this depth and effectively based on a level sub base i would have 25mm bed under white tile and 5mm under black.
although the finished floor height is not an issue this bedding thickness does thow up a potential problem about what membarne i can lay underneath.
i have to discount ditra because they only guarantee 10mm max bed adhesibve over their mats.
you could have 100mm thick stone and 5mm bed and thats ok but not 20mm stone and 25mm bed!!!
insulation board would be ideal as this will take the chill off the floor but is really a separating membrane.
i thought it was and have always used instead of ditra over plywood and under elctrical heat mats.
are there other alternatives.
this is a very prestigious job and i would like to be able to offer some good advice to secure the conrtract

 

[Peter]

Does a half fat decoupler like Mapei Mapetex allow the heavier bed?

 

[jonnyc]

will need to look at this tomorrow.
thanks for heads up

 

[Spud]

what size area is the floor Jonny ?

 

[nybor62]

could you not lay the stone using sand and cement and a slurry mix, do away with membrane . has ther's no under floor heating i dont see it being a issue , imo . just a thought

 

[jonnyc]

kitchen area is about 75qs mts but area i will bed stone is only approx 60sq mts as will just bed stone past line of plinth.

 

[Ken Bruty]

Just looked at your website Jonny, very nice work

 

[jonnyc]

could you not lay the stone using sand and cement and a slurry mix, do away with membrane . has ther's no under floor heating i dont see it being a issue , imo . just a thought

I could just lay the stone in a mix of washed sand/ white cement and flex adhesive all mixed together with a thin flex adhesive on back of stone and on subfloor to make a nice sandwich.
the proportions would be 10.00 kg slow set flex adhesive/10.00 (99(9kg white cement/75 kg washed plastering sand ( 3 bags )
I have used this method without fail for twenty years when fixing old reclaimed stone but i will never give a guarantee.
by this method you can solid bed stone from 5mm to 25mm .
thee is enough flex with plastering sand and flex adhesive in mix to get stone down to 5mm bed at extreme.

 

[jonnyc]

well i managed to catch dural customer services befire they closed about the ci mat and they are still saying that they will guarantee their mat with a varied bed of adhesive between 5-25mm.
so thats is sorted as i can solid bed stone with this variance no problem.
just leaves the problem of finding a thick bed adhesive to 25 mm that will not slump .
i used to use pci carrament but of course this is not readily available over in uk now i think although i have heard there is someone who might be bringing it in.
if anyone knows who would be grateful or alternatively another product that does the same job.
needs to be ok for between 5-25mm bed under min 20mm stone without slumping and preferably rapid set

 

[Ken Bruty]

I use Weber set thick bed up to 25mm good anti-slump but not rapid unfortunately.