anhyrdrite screeds

[Branty]

Tiling to Anhydrite, is a lot like tiling to finish plaster, with more worries. I.E it wetter, thicker, and has a shinier finish.
Removing the laitance and the shine is essential. Probably more so is checking the moisture content. As Cliff says, You can do all the prep you like, but if there's to much moisture in the screed, the adhesive won't bond.

Your risidual content has to be below 0.5% before you can fix tiles direct to it. I believe you can use ditra if the risidual moisture content is 2%. In both these instances you will need a meter reading if you want to be sure.
It's all well and good your builder saying it's been down for four weeks, if the first two, it was exposed to the elements. Get him to do a moisture reading.

 

[JOHNNYCLARKE]

getting a bit confused now.
Mapei and Weber both said I could put ditra down with a rapid flex quickset even if screed only 2 weeks old.
Now presumably screed is still not completely dry.
So Mapei said take skin off top and apply primer G which i think is accrylic.
So any moisture left in screed should go down and sideways if it cannot get up through primer.
Is this correct?

 

[Gazzer]

Is this of any use ?
Broken Link Removed
broken link removed

 

[Branty]

getting a bit confused now.
Mapei and Weber both said I could put ditra down with a rapid flex quickset even if screed only 2 weeks old.
Now presumably screed is still not completely dry.
So Mapei said take skin off top and apply primer G which i think is accrylic.
So any moisture left in screed should go down and sideways if it cannot get up through primer.
Is this correct?

The wat I see it, is its all about the moisture. If there's too much in the screed then that has to affect the bond to the ditra no matter what adhesive you use.
If you can get some reading that says its less than 2%, then you can use ditra.

 

[JOHNNYCLARKE]

just been to look at job now with a diy dampmeter and it feels and looks damp still after 24 days down.
Screed is apparently 40 mm thick which should mean 40 days to dry.
Dont know what to do as committed myself after talking to the mapei tech guy on friday who said would be ok.
If i put this accrylic primer G down will this stop any moisture from screed coming up or just some?
I thought if it really seals in screed then any moisture would move sideways and up wall which would be fine would it not!

CLIFF IF YOU LOOK AT THIS I WOULD REALLY APPRECIATE MORE ADVICE OR CAN CONTACT ME DIRECT.
I AM DUE TO START MONDAY BASED ON MAPEI INFO BUT NOW IM CONFUSED.

THANKS JOHNNY

 

[Branty]

The only problem I see Johnny is the moisture could affect the primer coating.

 

[JOHNNYCLARKE]

Branty
You mean you think primer might not stick to screed.

Have you had experience with this yourself.
My problem is that I put in my quote that I could put ditra down if screed had been down for five days or over but not fully dry. This is fine for normal screeds for underfloor heating and client gave me go ahead and paid for ditra for 180 sq mts.
But I dont know if this is ok on this anhydrite screed.
Ive only had four of these out of maybe 300 in last four years!!The builder was told he should let screed dry 1 mm a day but ignored it because he has laid on this screed before early and had no problems and did not tell me .

It was only when I found out what screed was down last week did I contact screed firm and told should wait.
The checked with Mapei and Webber who both said ok to put ditra down but neither mentioined the 2 % min moisture and I have only heard this now.

I could say they should wait but this is 7k of work over next two weeks and I am fully booked for months afterwards.

 

[Branty]

Johnny I think you need to get some advice from Schluter.
I have fixed to anhydrite in the past. The first one came up, since then I always got a meter reading. One of below 0.5% means its ok to tile onto.
I don't think there's a difinitive figure when it comes to using a decoupling membrane. I think TTA put a figure out of 2%, but not sure.
Give Schluter a call, see what they say.
The problem with this screed over a normal sand and cement screed, is the huge volume of water.
Try giving BAL tech a call tommorrow and see what they say.

 

[JOHNNYCLARKE]

Branty
Thanks for advice .
I will check with schluter tomorrow of course.
Could not get them over weekend.
Have put start off for tomorrow till see what everyone says.

Johnny

 

[Sully]

Branty,

I don't want to hijack Johnnys post but my query is along similiar lines.

Went to see a new build extension job yesterday and the flooring was at an early stage. I could see concrete between concrete beems on the floor and the house owner advised that building regs had specified a 'floating floor' due to the soil / trees near the house. Over this will be piped underfloor heating working of the megaoflow system and this will be covered in 'screed'. I asked the householder to get me more details from the builder. Is it likely to be anhydrite screed? Or put another way, when / why do they use an anhydrite over a sand & cement. Total area is 40sq/m.

Secondly on a sand and cement screed - is it minimum 4 weeks drying or 1 day per mm. I also need to spec a 75mm screed on another job so just want to be sure that it's not 75 days drying time.

Andrew