Question about ceramic tiles straight on to anhydrite screed without priming?

Hi all,
I'm currently going through an extension build. It's 56 sqm to be used as a kitchen / utility / dining area.

We had poured screed put down on the 4th August. It had something added to it to promote quick drying (could be walked on in 48 hours).

It has wet UFH underneath it.

The screed people came back 4 weeks ago and 'buffed' the top surface.

Before the tiler arrived at the weekend we thoroughly hoovered the whole floor area to remove dust.

The tiler came at the weekend and fitted 600x600 porcelain tiles. He used Kerakoll Bio Gel No Limits adhesive.

I was Googling to find out about when it was safe to put the UFH back on and came across information about "priming" floor before tiling it. Our tiler hasn't done this.

He has done a cracking job of the tiling (he's a really good tiler with a great reputation) and I've spoken to him about priming and he says that he's done hundreds of these floors and never had a problem.

After more research I've found that Kerakoll do recommend that the floor be primed.

I just wondered how worried I should be? Has any of you experienced tilers laid floors without priming? How necessary is it? (i.e. is it just a case of "without priming you *may* have a problem" or is it a case of "without priming within two weeks all the tiles will pop")??

I just want to know how worried I should be? I'm not planning on putting the UFH back on for another 3 weeks and then I'll just gradually increase the heat over a week or two to prevent thermal shock.

Thanks for advice / help on this.

It depends.

Ask the tiler for a guarantee if he has the reputation and confidence in the installation.

Swwils said:

It depends.

Ask the tiler for a guarantee if he has the reputation and confidence in the installation.

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That's a good idea. My mate is running the job (extension) and so I am confident that if a problem was to arise then he would facilitate getting it sorted for me. But, I'll maybe ask him for a written guarantee that it will be fine. That way it'll be on him to help get it sorted if the worst happens. Thanks for the great advice

Also for information the reason you prime is multifold. Some surfaces you must prime, normally to reduce porosity(absorbency) of the background to prevent the formation of air bubbles, or to minimise risk of chemical reaction with cement-based adhesives and lastly to add surface strength to friable (weak) surfaces. Also most of the time it can also aid waterproofing.

Hence why "it depends" for a calcium sulphate or anhydrite screed it's almost always a yes, depending on the adhesive.

The ultimate aim is to have a stable surface. Since yours has had some considerable time since laid then buffed and then vary carefully cleaned up you have a better chance of a stable surface compared to say a very fresh screed thats not been buffed and was dusty. The removal of laitence is also a factor, so as you can see it's a large and detailed topic.

Customers should be encouraged to ask these questions, preferably before the jobs starts!

Swwils said:

Also for information the reason you prime is multifold. Some surfaces you must prime, normally to reduce porosity(absorbency) of the background to prevent the formation of air bubbles, or to minimise risk of chemical reaction with cement-based adhesives and lastly to add surface strength to friable (weak) surfaces. Also most of the time it can also aid waterproofing.

Hence why "it depends" for a calcium sulphate or anhydrite screed it's almost always a yes, depending on the adhesive.

The ultimate aim is to have a stable surface. Since yours has had some considerable time since laid then buffed and then vary carefully cleaned up you have a better chance of a stable surface compared to say a very fresh screed thats not been buffed and was dusty. The removal of laitence is also a factor, so as you can see it's a large and detailed topic.

Customers should be encouraged to ask these questions, preferably before the jobs starts!

Click to expand...

Yep, I was told that it was going to be sealed but, by the time the tiler turned up (a couple of weeks later) I'd forgotten all about it. I've been doing all the electrics / garden etc myself so was really busy when he arrived and started work. Had a quick chat with him, made him a brew and then went my separate way and forgot all about the sealing. Just assumed he would do whatever was necessary to do a good job. Fingers crossed we get away with it

What about the depcoupling membrane?

I thought wet ufh on an anhydrite Screed needed priming and a decoupling membrane.

I’ve got both on mine, and all has been well.

I removed the laitance which is (I told) very important. Like you said break the heat in gently.

Mr Chibs said:

What about the depcoupling membrane?

I thought wet ufh on an anhydrite Screed needed priming and a decoupling membrane.

I’ve got both on mine, and all has been well.

I removed the laitance which is (I told) very important. Like you said break the heat in gently.

Click to expand...

There's no decoupling membrane either? Just tiles down on top of the screed? I'd not heard of a decoupling membrane until you mentioned it. I've just had a Google and it mentions decoupling membranes on Anhydrite if it's expected to get wet. Hopefully this means a lot of dampness, like a bathroom?

Ours is a kitchen / utility / dining room. Maybe there's enough dampness in the kitchen/utility to warrant one?

I'll definitely get a guarantee!! Thanks

How deep is the screed?

Very likely an uncoupling membrane is essential in this one. It will very much depend on the screed depth. I have assumed it's thin.

Sand cement screeds should be laid at a minimum depth of 65mm when floating. This I am assuming is insulated on Celotex etc so is floating. The thinner the screed is the more propensity it has to curl and the weaker it is in tension so the more it is likely to crack.

Rapid drying screeds are simply sand cement screeds with an additive which is usually a plasticiser which reduces the water content. To hydrate fully cement needs less water than you might think and a good deal of the water in traditional screeds is there for workability purposes. Typically a rapid drying screed will fry at around 25mm per week however they should be cured first for 7 days following installation.

This is more important in these types of screeds because the loss of water when there is already not much in the period during which the chemical reaction is at its fiercest can lead to drying shrinkage cracking.

This may not be immediately visible and often manifests itself more when the heating is commisioned.

The underfloor heating MUST be comissioned prior to installing any type of covering on any type of deep screed. An uncoupling membrane and a green screed adhesive do NOT replace the need for good practice.

Regardless of the type of sand cement screed the underfloor heating should not be comissioned for a period of 28 days following installation. This is based on the rate of gain of strength associated with screeds and testing etc. which are carried out and therefore declared at 28 days age.

Comissioning sooner than this could exacerbate cracking and will almost certainly lead to cracking especially given the presumably shallow depth. It is not always about moisture I'm afraid.

So check the depth, report back. Luckily it sounds like you have not yet commissioned the UFH and had a good amount of time after the screed was laid.

Thanks for the info. The screed isn't sand/cement it's the pouring screed. It was a depth of about 65mm. You're right, the base was concrete, then plastic sheeting, then insulation, then more plastic sheeting, then the pouring screed.

The UFH was on but very low and only after 3 weeks of the floor being laid. At no point did it go over 25 deg and I turned it off (power off) a couple of days before the tiles were laid to ensure the floor was just at room temp.

Planning on putting the UFH back on after the tiles have been down for 4 weeks. And just going to increase temperature gradually as I've read about the risk of thermal shock. I'm new to screed/UFH etc and I'm just the 'customer' - I'm not doing any of this work myself. I was relying on the contractors to take care of all these details.

Just hoping we get away with how it's been done as I don't think my wife's nerves could cope with having to bring all the floor up again.

Did the screed crack at all?

Looks like you have a good depth of screed and decoupling would be less needed.

I wouldn't fret, it's not worth it. Even 'perfect' installations can go wrong and it leaves alot of head scratching.

Swwils said:

Did the screed crack at all?

Looks like you have a good depth of screed and decoupling would be less needed.

I wouldn't fret, it's not worth it. Even 'perfect' installations can go wrong and it leaves alot of head scratching.

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No the screed didn't have any cracks at all. It was perfect. Thanks for the reassurance. It's much appreciated.

I have worked in a few houses where the tilers didn't bother using any primers. I suspect it is because they can't be bothered to wait for the primer to dry.

I have seen them tile over newly plastered plasterboard walls with absolutely no primers in shower enclosures. There is no point mentioning ettringite. They get paid, they leave.