Looking for advice on slate floor on anhydrite screed.

[hmtiling]

It's a recommendation by the British stone federation. It may be in the latest bs5385, but it's not in my copy (2009)

I know the stone federation do. But pretty sure it's not BS

 

[Bill]

So all this talk of recommendations actually means diddly squat?

 

[peteablard]

So all this talk of recommendations actually means diddly squat?

So you wouldn't recommend decoupling then?

 

[Bill]

So you wouldn't recommend decoupling then?

I am just wondering what happened to all the lovely stone floors I have laid without decoupling? Don't recall having any problems. I know there is nothing wrong with using one but sometimes there is no need so long as you use the correct adhesives.

 

[hmtiling]

I would STRONGLY recommend using one with a heated gypsum screed as in my experience they crack more often than they don't. With regard to screeds in general then it comes down to how well they been put down and unless I've done something myself I can't vouch for that and will use a decoupler when heated and stone.

 

[Ajax123]

I know the stone federation do. But pretty sure it's not BS

Hmmm!! I know I read it and it was actually one of the schluter guys that pointed me in the right place for the clause. However you appear to be absolutely correct. I cannot now find it in my copy either. that is really really frustrating... maybe I dreamed it.

Still I would always recommend decoupling natural stone from heated screed. The reason is simply due to the different coefficients of linear thermal expansion which can place stress on the natural fissures in stone leading to fracturing. This I see most often on marble and travertine but it still holds true for other stone.

 

[Spare Tool]

I'm just doing a bathroom in a house with wet heated trav floors through most of the ground floor, no uncoupling and there's more cracks than Blackpool promenade in July 🙂

 

[SJPurdy]

There are some weak stones available that crumble/split when subjected to the stresses created by thermal cycling, using an uncoupling layer under these may reduce the probability of them failing. If I was still tiling then I would pass on the advise that an uncoupling membrane is recommended by the stone fed for fixing (all) stone tiles over a heated floor. Personally if tiling onto a concrete floor then I can't see that it is necessary with slate as I've fixed hundreds of m2 in the past and have not had any fails however as it is an anhydrite screed then I would recommend using one as described above (stuck down with an anhydrite compatible adhesive onto the sanded, vacuumed floor with primer as recommended by the adhesive manufacturer; and then a cement based adhesive over the top to fix the slate) because then there can be no contact between the cement and anhydrite screed (so the membrane will have two purposes).
I have seen slate fixed over ditra crack but that was because no movement joints used where they were needed! I have also seen porcelain tiles crack when fixed directly over anhydrite (primer barrier then cement based addy) this because the water temp being passed through the screed was not correctly regulated and far too hot (not even raised slowly in temp after fixing the tiles). So make sure movement joints are used if needed, check the water temp regulated correctly and raise it's temp slowly up to the user temp when the floor fixing is finished and adhesive cured (I can't remember how long to leave it and how many degrees per day are recommended).

 

[Time's Ran Out]

I am just wondering what happened to all the lovely stone floors I have laid without decoupling? Don't recall having any problems. I know there is nothing wrong with using one but sometimes there is no need so long as you use the correct adhesives.

With or without UFH!

 

[hmtiling]

There are some weak stones available that crumble/split when subjected to the stresses created by thermal cycling, using an uncoupling layer under these may reduce the probability of them failing. If I was still tiling then I would pass on the advise that an uncoupling membrane is recommended by the stone fed for fixing (all) stone tiles over a heated floor. Personally if tiling onto a concrete floor then I can't see that it is necessary with slate as I've fixed hundreds of m2 in the past and have not had any fails however as it is an anhydrite screed then I would recommend using one as described above (stuck down with an anhydrite compatible adhesive onto the sanded, vacuumed floor with primer as recommended by the adhesive manufacturer; and then a cement based adhesive over the top to fix the slate) because then there can be no contact between the cement and anhydrite screed (so the membrane will have two purposes).
I have seen slate fixed over ditra crack but that was because no movement joints used where they were needed! I have also seen porcelain tiles crack when fixed directly over anhydrite (primer barrier then cement based addy) this because the water temp being passed through the screed was not correctly regulated and far too hot (not even raised slowly in temp after fixing the tiles). So make sure movement joints are used if needed, check the water temp regulated correctly and raise it's temp slowly up to the user temp when the floor fixing is finished and adhesive cured (I can't remember how long to leave it and how many degrees per day are recommended).

10 days then 5degrees a day I believe