What went wrong...?

[zoemac]

Sorry I think my use of the word powdery is a red herring.

I can peel off bits which is what makes me think its the SBR. There is a dusty layer ontop of the SBR but think this is from the tile adhesive which seems to be very chalky and brittle. I've added a new photo.

Under floor heating was commissioned before tiling - it was run on low for 8 hours on a couple of days.

 

[Spud]

From what I have read it sounds like you had laitence on the floor and it was still wet when it was tiled , I don't know of any screed where you can just brush the laitence off with a broom all the floors I have encountered have needed mechanical scarifying to remove the surface layer , running the heating on low for a couple of days wouldn't have achieved anything in regards commissioning the screed or help with force drying it

 

[Gazzer]

Another thing ! You say the floor heating was turned on but was it on when it was tiled ?

 

[judge]

Chalky and brittle adehsive doesnt sound to good either !

 

[Ajax123]

I just lifted a tile again, and I have added some photos to the Facebook album (link above) the floor underneath feels a little damp despite the underfloor heating being on - is this normal, or is this a sign that the screed was not dry enough when it was tiled.

no it is not normal if the screed is properly dry. The dampness is a clear indication that moisture has either gotten in or more likely was there in the first place

The adhesive is very firmly stuck to the tile, but not at all to the floor. The floor is covered with a white substance which is dusty in places and peels in others. I think this is the remains of the SBR.

this sounds like either Laitance or ettringite. Either way it is not good

Does the fact that I can peel/scrape off bits of SBR mean that it skinned rather than soaked in?

yes

When I scrape away the SBR the floor underneath looks good and clean - no other sign of residue or laitance. It is rough to the touch so I would have thought it'd be an ideal surface to tile onto.

i cannot see the pics from the link above but have answered your questions based on the information supplied. If the adhesive is powdery and brittle this would indicate moisture and possible ettringite. The tiles will unfortunately be unlikely to be save able and will need to be replaced

 

[Ajax123]

I think laitance is the problem here
i have seen exactly the same problem on 150m2 of granite with ufh
the floor was not scrabbled,laitance removed
it was exactly as you say,wet underneath and no adhesion to sub floor
white patchy areas,chalky its laitance

What you are describing does not sound like Laitance. It sounds more like segregation which is a screed defect. You should not scabble screeds. They should be mechanically abraded which usually means sanding.

 

[zoemac]

Thanks folks - it looks like possibly the floor was still damp despite having 2.5 months to dry some of which with heater and some of which with under floor heating commissioned. Also sounds like SBR could have been too thick and formed a skin.

Given the info from the screed supplier and the way the floor was prepped I just can't see that it laitance (but I could be wrong). It seems more likely that despite the SBR a reaction between the gypsum screed and the cement adhesive occurred which is why the adhesive seems really powdery and brittle.

Does that sound right to people?

Guess I just have to suck it up - unless anyone has any other suggestions?

So the plan of action would be - take up all the tiles, run the underfloor heating full belt for a bit to ensure it is super dry and then prep and seal the floor again and relay new tiles with a gypsum tile adhesive.

What do you think?

Can't afford to do this at the moment - so will be living with a cracked floor for a while. But at least I know what I now need to do

 

[Ajax123]

If you have run the heating for a couple of days on low then it has not been properly comissioned.
the correct procedure takes two to thee weeks.

this might help http://www.gypsol.co.uk/4_Underfloor_heating.pdf

 

[Ajax123]

If this was done by your builder or by a tiler then they should be footing at least some of the bill....

 

[zoemac]

No was left to dry for a while - probably 2 weeks and then was run on low for a short while and then underfloor heating not used till November