Ditra mats on a newly installed screeded floor

[Oddbod1]

Does anyone recommend the use of a Ditra mat for a screeded floor with wet under floor heating? What is the purpose of this mat and is it an unnecessary expense?

Also does it lay straight down on top of the floor or need gluing down? If the latter how many mm should the glue be?

 

[Doggit]

The mat seperates the UFH and the (normally) floor tiles, and allows them to expand at different rates, so stops the tiles cracking. The mat will get glued down to the screeded UFH, and then the tiles go on top.

Check out the Ditra website

 

[Oddbod1]

I'm assuming it must have really low u values otherwise it would just be another barrier to heat coming through from the screeded floor? Do the majority of builders always install this i.e. its not just a gimmick then? Seems quite expensive at about £220 odd per 30m2 roll.

 

[Doggit]

Yes it has a low U value, but no, it's not a gimmick, but there are cheaper versions out there. But I will admit that I haven't needed to use one yet, so you may need to find someone more experienced from here on in.

 

[Oddbod1]

what glue is needed to stick down the mat?

 

[Doggit]

Don't quote me, but as you use tile adhesive on the top, I think you can use it to fix the mat to the screed as well.

 

[mointainwalker]

I don't know why you would consider using this on a screeded floor. It is worse than pointless as far as I can see ( extra cost/time ) as there is no problem in gluing tiles directly onto the screed.

I used something similar to this when tiling onto plaster-board walls in a wet-room* and this is the type of application* mostly referred to on the web-site.

 

[Oddbod1]

I don't know why you would consider using this on a screeded floor. It is worse than pointless as far as I can see ( extra cost/time ) as there is no problem in gluing tiles directly onto the screed.

I used something similar to this when tiling onto plaster-board walls in a wet-room* and this is the type of application* mostly referred to on the web-site.

What about ground movement then on freshly installed screed and the potential for tiles to crack

 

[mointainwalker]

Firstly you have to wait for screed to dry out before laying , which will be 6-8 weeks, so forget the "freshly-installed" screed. Secondly I haven't read of cracking occuring *, presumably because expansion rates screed/tile are very similar.

* Except when tiled hard up to a wall i.e. no expansion gap.

 

[Oddbod1]

So that I full understand, are we saying that any settlement that occurs over time reduces or increases the height of the screed by a few mm?

 

[Doggit]

Any shrinkage in the height will be infinitesimal; it's more about giving it time to dry and cure properly. But how thick was the screed. A lot of people quote 1mm per day.

 

[Oddbod1]

Around 65-75mm thickness, maybe less in other areas.
I'm more concerned that it does not "expand" and rise in height (if such a concept exists), esp with the wet underfloor heating pipes etc installed because the tiles etc will be flush with the rear and front door thresholds and I will not be able to cut the doors.

 

[Doggit]

No worries of that; screed shrinks as the water dries out. Once it's dry, it can expand sideways slightly (hence the use of a ditra mat), but won't expand upwards.

 

[Oddbod1]

If we have an 11mm tile, what would your suggested allowance be above the screed as I need to tell my front door installer when he comes to measure up. I would have thought 11mm tile, 6mm adhesive(?), 3mm ditra mat, 3mm glue for ditra mat and 2mm for primer? 30mm to be on the safe side to allow the steel door to open?

 

[Doggit]

Half a mill for the primer (if that). Done tight, you could get away with 2mm for the first adhesive layer, and then about 4 for the tiles, but if you talk to the tiler (and he's a good one) then he'll keep an eye on things.

What type of room is it. Have you thought about a wooden floor - much warmer when the heating's not on, and will come out much lower.