I've been asked by a developer that I do some work for to do 18m2 large format slate onto a kitchen floor.

The floor is part cement screed and part floorboards. The developer is talking about replacing the floorboards, as they are rotten with 18mm ply. This will, apparently, level the floor. He, also, wants electric UFH (matting) laid.

My understanding is that isolation membranes can't be used with 'lectric ufh , so I am trying to work out the alternatives so that I can provide my quotation.

Option 1 - Replace floorboards with 12mm ply and then glue & screw 6mm cement boards leaving 10mm between screed to fill with silicon, i.e. expansion gap. Lay ufh, then self level prior to sticking the slate.

Option 2 - Replace floorboards with Wedi, lay ufh, self level then stick.

Option 3 - Self level screed and ply, lay ufh, self level then stick.

Option 4 - I'll leave this for you guys to recommend alternatives??

Am I on the right track? Can anyone point me in the right direction, please?

Thanks in advance.

You can use Ditra with electric UFH.......But you also have a break in substrates and i would put a movement joint at this point......

If you contact schluter.uk direct they can advise which one to use for slate and UFH...

Contact Schlüter UK - Schlüter-Systems

Thanks Dave,

When I spoke to my local CTD re: Ditra, they said that it can't be used with electric ufh. I'll email Schluter as per your advice.

Are you suggesting that I place a movement joint where the 2 substrates meet when tiling the slate? Will I need this if I lay a Ditra membrane, then ufh, then slc?

Yes!! Any movement joints need to carried from the substrate through to the tiling.....

This is info from schuter site....


Schlüter-DITRA is also recommended for uncoupling of radiant heated floors consisting of thin electric heating mats. Schlüter-DITRA may be installed either above or below the heating mat. However, the uncoupling function is more effective if Schlüter-DITRA is installed above the heating mat.

Wicked!! Thanks Dave,

So in summary, lay the UFH, then Ditra on top, so no need for SLC?
Just studying the Schluter site now.

I would still use slc because you need a nice flat base to stick the ditra too....

I have no experience of Ditra, so apologies if this question is rather "blonde".......

Do I just fix the Ditra over the area where the substrates meet, or do I need to cover the whole floor?

For an uncoupling installation to work you will need to cover the whole floor.......BUT i personaly don't think you will need it with slate.....travertine/limestone then yes but slate is quite dense so i don't think you will need it on this occassion.....

Ply the wood section..level the concrete side to match....the ufh..install in 2 sections..one for concrete , one for wood then install movement joint in the tiling right over the exisiting break in substrates.....the reason i say use 2 mats is so that the stress from the timber side dosen't effect the ufh....so as long as the mat is not over the joint then no probs.....

Nice one!

So last question, I think.....

A pre-fabricated movement joint or line of silicon?

Silicone( cold joint) would look untidy on rough slate IMO..deffo preformed joint....

Okay, in keeping with builders and developers, I've just found out that it's now going to be travertine (approx 600 x 400 x 15mm).

So to re-cap, replace rotten floorboards with 18mm ply, fix Ditra, lay UFH, chuck down SLC, then wack the trav down (not forgetting the preformed movement joint)?

Have I got that correct?

Developer says he doesn't want a movement joint, do I insist, or just advise the risk and tell him that I will not take any responsibility for future issues?

I would put the ditra on top of the ufh/leveller then lay stone directly to membrane...