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I have a 75mm lafarge gyvron calcium sulphate flow screed floor that was put down over 100 sq m on the ground floor of my self build in December 2008. The kitchen/diner has a flat roof which only became watertight Feb 14th 2009.

When the screed was laid the house was watertight except the kitchen, which ended up with water on top of the screen from december to mid Feb. The official drying time for 75mm screed is 90 days (1mm per day up to 40mm and then 1mm per 2 days for the remainder) but this is from watertight.

The floor has to be levelled by about 7mm before the tile https://www.tilersforums.com/forums/tile-adhesive/ is laid. Our tiler did attempt to level about 6 wks ago but was not aware of any special requirements for calcium sulphate screed. The PVA was applied in about a 1:5 (I now understand that it should be applied neat to form a barrier between the calcium sulphate and any cement based levelling compound to prevent reaction).

The levelling compound dried and cracked all over the ground floor and had to be taken up. Strangely however, one batch of compound adhered to the floor better than the remaining - however the rep has been out and taken samples and not come back to us. On going back to the screed company I have now been told that the underfloor heating must be in operation before the tiles can be laid to ensure that the screed has settled.

The underfloor heating has now been on a week (since 15th May). As we have a ground source heat pump, water underfloor is at approx 38 - 40 degrees and would have started up gradually due to taking time to warm up - so basically, its not as we have blasted 70 degrees through the screed from start up, so all should be good. The rooms feel warm, not damp, no cold spots, all watertight.

Apologies for the long post but that's the background. My questions are:

1. Without spending loads of money on a special hydrometer testing for calcium sulphate screed floors to check the moisture content (standard meters don't work with calcium sulphate) - I feel that we have been watertight for 3 months plus with the heating on a week and the screed has been down for 5 - 6 months - surely it has to be dry now?

2. We are having 600mm porcelain tiles over the 100 sqm ground floor that has to be levelled by about 7mm. We have a great tiler but he hasn't tiled to calcium sulphate before - any recommendations on using PVA as barrier and then appropriate levelling compound to be used prior to https://www.tilersforums.com/forums/tile-adhesive/? Would you trust the pva barrier and then use cement based leveller (which is the main reactant) or go for a calcium sulphate leveller and then apply https://www.tilersforums.com/forums/tile-adhesive/ to that?

Any help would be greatly appreciated. I'm a new poster to tilers forum but when searching for tiles often ended up here and have found lots of useful information (I've read so many of Gary's posts now that I feel like 'family'!). Kitchen is fitted on 29th June so I need to get something moving soon and want to make sure that it's going to work this time - for definite.

Thanks in advance to all you experts out there - I would appreciate the benefit of your experience,

Cat :54:
 
OP
W

wetdec

Using an anhydrite screed is not a problem provided its treated correctly.

When the screed is dried out properly( to 0.5% moisture), remove any latency with a scraper, wire brushes or sanding then hoovering up all dust.

If you have a wet under floor heating system installed increase the temperature in incriments until full heat is reached then work backwards, this procedure helps in the drying process of the screed and aids curing.

When dust free use your https://www.tilersforums.com/forums/tile-adhesive/ maufacturer recomended sealer to seal the floor, this protects the screed and should be done in opposing directions to ensure coverage. It is important that guidence is saught from the https://www.tilersforums.com/forums/tile-adhesive/ manufacturer as the bond to screed is their territory.

Screeds move even after drying due to expansion and contraction especially when using wet under floor heating and can cause tiled surfaces to crack from latteral deflection. To combat this cracking an uncoupling membrane such as Ditra (1) or Dura-Ci membrane should be fixed before tile.

Cut to size and laid using a flexible tile https://www.tilersforums.com/forums/tile-adhesive/ the membrane is easily tiled over. The membrane acts to protect the screed from moisture from above so should therefore be sealed properly. The channels in the membrane aid air movement over ufh and during https://www.tilersforums.com/forums/tile-adhesive/ drying.

(1)Please check Ditra for their fixing instructions

There are always questions re gypsum / anhydrite screeds, hope this goes some way to answering some of them. :thumbsup:


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Ajax123

TF
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Hi Ajax - Thanks for the contact - will give you a call as I really need to get this floor down now. Based in Midlands 15 mins from Bham NEC.

Appreciated. Cat

That's bleedin typical as I was in Solihull and Coventry today -could ave called - never mind. Give me a shout tomorrow and I will do what I can to help.
 
OP
D

doug boardley

just to add on for people that are not familiar with laitence, up north, we call it cream or fat, it's the watery residue that rises to the surface after a concrete pour, it has no structural integrity at all.
 

Ajax123

TF
Arms
Reaction score
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Cat

Is your tile finished now? How did it go?
 

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