Advice on heated screed with terracotta - ditra mat?

[thegwads]

hi all
wanted a bit of advice on a floor i am quoting on. lovely big area on a new build, around 70 square metres.
piped underfloor heating, which was screeded over last week. apparantly the screed has fibres in it which make it flexible. Now, the heating will probably not be run before the initial tiling start date, which is 16th feb. I have recommended ditra matting to be laid, as i am positive there will be a certain amount of movement on the new floor while it firstly dries out properly, and secondly as the heat is turned on and then fluctates etc.
i have recently tiled a travertine floor with underfloor heating with which the customer declined the ditra mat. went back 3 weeks later to tile the bathroom walls, and a stress crack on the concrete floor had transferred itself through to the tiles above, and a crack was formed.
my questions are these:


is there a flexible screed in anyones experience that truly copes with the contraction and movement in a new heated floor?

is ditra matting the answer?

will standard flexible adhesive do the trick?


any opinions greatly valued

[pjc]

a decoupling mating is a good idea :Pete

[Dave]

The fibres add strength but unless a modifier was used in the screed then not flexible imo..

The UFH should be turned on before tiling commences and then cooled down..this will allow for lateral stress to take it's worst stress on the floor..

I wold deffo use an uncoupling membrane with piped ufh ...


How long will the screed have been down by the time you tile it..?


p.s. Is it a pumped screed...deffo cement based screed..?

[thegwads]

the screed was pumped in the other day, quick job by all accounts. and deffo has flexible properties to it, so i have been told

i am just very apprehensive of doing it without the ditra mat. but i can see customers point of view, works out at an extra 500+ quid for that area!!

apparently the heating cannot be turned on for a couple of months, need to find out loads more. my initial feeling is run it to top temp gradually, then turn it down. let the floor do it's moving as much as poss, and then ditra mat it just for safety sake.
the tiles are not cheap, and the last thing i want is in 5 months cracks appearing cos the floor has failed due to the moving substrate

[Dave]

Another quick question ....it isn't a gypsum screed then..?

[whitebeam]

I'd be checking that an't a gypsum screed

[thegwads]

and if it is a gypsum screed?

i will stick a quick photo up of it not long after it was laid (2 days ish) if i can work out how to

looks like it has flexible properties if you know what i mean - definiote kind of rubber bit too it. not a standard concrete screed if you catch my drift. i will find out the exact stuff tomorrow

but all screeds aside, is a ditra mat not the answer anyway to be on the safe side? or will some screeds not accept that either

[Dave]

Ditra or similar is deffo the answer...just need to know screed type as well.....prep wise..

[thegwads]

i will find out and get back to you if that is cool!

if it is some super dooper screed which will not cause cracking then should be fine without the ditra - but i cannot see how it will not contract and move as it dries etc.....

[beanz]

If they insist on not using the Ditra, i'd be tempted to walk away. Wouldn't want to have to replace all those expensive tiles at my own expense! The thought of laying the tiles without turning the heating on is a bit of a worry too...

[whitebeam]

Schluter have recomended to me that if the heat is'nt turned on to start with you can lay ditra but the heat must be left for a couple of weeks and then a few degrees a day

[thegwads]

been on site today - the screed in sand cement, with fibres mixed in for strength. chap who does the floors has never had a cracked one yet, and his usual tilers go straight on top, no worries.

been to topps and another tile place and while i was there asked them. they both said just use flexi, will be fine.

real quandry now. the underfloor will not be on for a while, plumber needs to come back and connect it all up, and the screed needs drying out naturally by all accounts.

so, further thoughts massively appreciated, also regarding priming the floor, and special prep work for the terracotta, which is pre-sealed on the face.
have heard before, due to dusty nature of terracotta can be a good idea to damp down the backs before laying

many thanks in advance

[wetdec]

Dont you trust me or what we did this on the phone ............LOL

[pjc]

whatever you do do not damp the backs, what terracotta are they ?

[thegwads]

well spotted wetdecs!!!!
yep - spoke, but i know i would be more comfortable with it, but finance etc.......

tiles are from fired earth, pre sealed, originally £70 a metre, half price in sale, chunky buggers they are!!!

[doug boardley]

what they've saved on the tiles will pay for the ditra imo

[lucius]

P.J.C why not damp the backs?
Lucius

[pjc]

i spent years trying to keep them dry will start all sorts of problems what terracotta are they ?

[thegwads]

Fired Earth Online

pretty sure these are the puppies! but presealed so i have been told

[thegwads]

Fired Earth Online

correct link
bugger me, 80 a metre normally!!!