The dreaded Anhydrite/freeflow screed

[Gazzer]

As an ongoing contract i have been tiling a property with screed that i was told was Freeflow screed. No problems up till now. The screed was dry and any laitance was removed and then 2 coats of primer (90 degrees to each other) were laid. Dural Ci was laid with SPF adhesive then the Travertine laid on top of the dural again using SPF.
The problem is now, the next stage of the job is ready for tiling....or should i say, the owner is wanting it tiled. The floor is still very wet, this is known by the colour and feel. I do have a cheap moisture tester and this said the same.
I have told them of all my concerns and they understand but still they need the job doing. I wont be tiling until i am happy the screed is fully dry, my problem now is finding the appropriate literture that i could print off and show them. Something in plain English would be ideal, do any of you know any links that i may be able to get this from?

Also what is the best way of drying a floor?
I have said that open doors and ventilation is the best, second would be a dehumidifier. They are saying a space heater !, I feel this could make thinks worse by making the room sweat and maybe crack the floor if heat was to high and too close .

 

[top tiler]

dehumidifer i would say is the way forward a space heater makes the room sweat as you have said yourself .we have only done 2 floors on anhydrite screed the first was a showroom in summer preped as you did no brobs the 2nd was a house in winter floor was dry when we tiled it then over the christmas shutdown they left the de hum on it overflowed went under the tiles and the whole floor come up will not tile on this stuff any more not worth the agro that comes with it

 

[grumpygrouter]

Doesn't the durabase work the same way as Ditra Neale, in that you can use it on new screed as long as it is solid. The construction allows the moisture to get out and doesn't lock it in?

 

[top tiler]

grumpy what do you stick the durabase down with?

 

[Gazzer]

Yep just spoke with client and explained i wont take the risk. He is understanding and after talking with Mapei i feel i have made the right decision.
Just have to wait for the floor to dry out before i can start.:mad2:

 

[Gazzer]

Doesn't the durabase work the same way as Ditra Neale, in that you can use it on new screed as long as it is solid. The construction allows the moisture to get out and doesn't lock it in?

Having just got off the phone with Mapei tech they say floor must be dry, if not the screeded floor can "turn to mush" (their words not mine).

My way of thinking was that the water would eventually come out on way or another and could debond the adhesive holding the Dural Ci.

I prefer the safe option and let the floor dry, i like to sleep at night.

 

[grumpygrouter]

grumpy what do you stick the durabase down with?

Thats a good question. I have not had to tile a wet screed yet so have not had to think about it. I believe (though don't quote me) that Schluter say your normal choice of adhesive for the substrate is sufficient.

 

[Gazzer]

I would also like to point out that this area to be tiled was in the open for a few weeks before the roof of the conservatory was put on. Although they did put up a make do roof out of polythene, it collapsed with teh bad winds and heavy rain. This left the floor flooded with over 20mm of water all over.

 

[grumpygrouter]

Can't you treat it like a swimming pool Neale?

 

[top tiler]

thats the problem! you would have to grind the floor put 2 coats of sealer on it then the matting so you have sealed the floor? anhydrite is a diffrent animal to normal screed its just like tiling on to plarster but softer and it soakes up all the moisture in the air like tiling in a big spounge

 

[Gazzer]

I know its a pain in the ****:mad2:

 

[Branty]

Having just got off the phone with Mapei tech they say floor must be dry, if not the screeded floor can "turn to mush" (their words not mine).

My way of thinking was that the water would eventually come out on way or another and could debond the adhesive holding the Dural Ci.

I prefer the safe option and let the floor dry, i like to sleep at night.

Thats a new one on me.
As far as Im aware, your floor has to have less than 0.5% residual moisture to tile onto, and 2% if you're going to use an uncoupling system (you will need a moisture reader for that).
The reason for this, is the same as it is for any substrate with miosture in it, screed, render, plaster etc... shrinkage. With your pourable screeds you get a lot of moisture, epscially when exposed to the elements.

The adhesive debonding from the bottom of the matting, is what it is supposed to do (hence uncoupling membrane). With 0.5% residual moisture a good single part can deal with any minute shrinkage that may occur. With 2% residual moisture, if the screed shrinks then cracks, it will cause the adhesive to debond from under the matting, where the crack is located, but that crack and the stresses of that crack will not transfer to the tile field.
As for mush!

 

[Ajax123]

Having just got off the phone with Mapei tech they say floor must be dry, if not the screeded floor can "turn to mush" (their words not mine).

My way of thinking was that the water would eventually come out on way or another and could debond the adhesive holding the Dural Ci.

I prefer the safe option and let the floor dry, i like to sleep at night.

Anhydrite does not turn to "mush" when wet. What happens is that any moisture in the screed once the anhydrite has fully chrystallised to gypsum (generally between 24 and 72 hours) is surplus to requirements and must be removed. If it is left in the screed it sits at the gaps bewteen the chrystalls effectively lubricating the joints. This causes a reduction in the strength of the screed usually of approximately 30%. As the screed is generally 30N compressive it is still pretty strong even when wet. A typical sand cement screed achieves around 20N compressive when fully compacted (full compaction is rare) If the anhydrite screed is kept permanently saturated it can swell slightly but even this will not cause any serious disruption for a long time. Temporary wetting simply leads to a wet screed which then needs drying out again. On drying it returns to its full strength. A decoupling membrane can normally be laid at 2% on anhydrite although the anhydrite manufacturers might be nervous about this level of moisture. At least one manufacturer has also approved the use of Epoxy DPM's on anhydrite subject to it meeting certain criteria.

 

[bighen]

Definitely wouldn't use a space heater. This drives the moisture back into the fabric of the building.
If your client is adamant that they require tiling asap would recommend using a liquid dpm covered by a 2 pack latex prior to tiling.
Hope this helps

 

[Gazzer]

Definitely wouldn't use a space heater. This drives the moisture back into the fabric of the building.
If your client is adamant that they require tiling asap would recommend using a liquid dpm covered by a 2 pack latex prior to tiling.
Hope this helps

not really the job was in October last year :lol:

 

[CJ]

not really the job was in October last year :lol:

May STILL be damp-ish :lol::lol:

 

[bighen]

I was in charge of a 1 mill pound contract a few years back. It was under a JCT 80 contract and during the build period the weather had done nothing but rain.
The slab was saturated and even months after the roof being installed the RH was still 98 - 99% despite the doors being left open during daytime.The RH would drop during the day but overnight would rise again. We recommended that a liquid DPM be used prior to the final floor installation. After many meetings with the clients and the architects, an extension of time was finally granted re the floor as the client did not wish to pay extra for the DPM. The client waited a further 6 months for the slab to dry to the level permissible for the finish floor to be laid.

I understand that your original post was back in October of last year, but a saturated floor requires time to dry naturally especially during the inclement months. Unless as posted by Ajax earlier you are prepared to install a liquid DPM!

 

[Ajax123]

Definitely wouldn't use a space heater. This drives the moisture back into the fabric of the building.
If your client is adamant that they require tiling asap would recommend using a liquid dpm covered by a 2 pack latex prior to tiling.
Hope this helps

I agree that space heaters are of little use unless the area being dried can be sealed. They do work very well in conjunction with dehumidifiers but again the area needs to be sealed so one sort of compliments the other. By far the best way to dry an anhydrite screed is of course to use an underfloor heating system. This coupled with good ventillation (and I don't mean windows open on trickle venting I mean windows and doors open to blow some draght through) will speed up drying significantly. The biggest issue I come across with anhydrites is the depth they are spec'd. They should eb as thin as possible (min depth 35mm) but architects love screed to be 75mm deep. This does nothing for the drying time at all. If no underfloor heating is available I tend to suggest 1 of 3 courses of action, forced heating using hot air blowers with natural ventillation (reasonably quick and easy but needs care), cocoon drying (specialist application) or a liquid DPM subject to strict rules (not available for all calcium sulphate screeds) -or of course just wait for the floor to dry naturally which at 75mm. Forced drying is something which is vastly under used in the UK. It happens all the time in Germany, France and Holland where anhydrites are much more popular.