floor tiling soon after screeding.........

[DHTiling]

Which make of Anhydrite is it..?

 

[Ajax123]

Now he has decided on heating mats. Screed is anhydrite 40mm apparently ready to tile on in 3 days? Is that right and would the heating mat above make a difference? Apart from priming the screed would you have any other prep before laying the mat?

ready to tile on in three days - I don't think so!! Would love to know who has passed this gem on. The depth is fine. I assume now that there is no heating within the screed hence the use of heat mats instead. The standard drying rate in good conditions for anhydrite is 1mm per day up to 40mm depth so you are looking at 40 days. You can accelerate this by force drying the screed from 7 days old. Without underfloor heating this is acheivable by using space heaters and dehumidifiers or a specialist drying technique which would speed it up a little further. This will probably somewhere near half the drying time but this is variable based on site conditions. The weather at present is far from good for drying screeds so I would suggest force drying is the best option anyway.

It depends on the screed supplier as to the level of prep required. If it is an old fashioned supplier it will have a soft friable skin on it which will need to be removed by either sanding or gentle shotblasting and the residual dust vacuuming. This is essential as the skin is not part of the screed. If it is a more modern supplier it will not have said skin. Then it depends on how old it is as it will be trafficked by site and willl this time of year be dirty. The easiest way to clean this up would be to sand it but in this case you are not looking to "remove" anything other than the site dirt. Again it will need to be vacuumed and then it is ready for priming.

My suggestion if you intend to use a heat mat with levelling compound I would suggest one based on gypsum as this is fully compatible with the screed. The primer will then be a standard acrylic primer. You can then use a gypsum based tile adhesive to stick down the tiles in the normal way.

 

[aflemi]

Ok thanks, Ajax all understood and I remember you posting about Gypsum-compatible adhesive so will get one from that list.
Re drying time, I will check with manufacturer but also, would Dural CI make a difference in waiting time?

 

[aflemi]

Ajax, He tells me now its Gyvlon 40 mm anhydrite screed, which you obviously will know all about. So we can force dry it if required. Assume we would need a moisture measuring device to check when it's ready, any advice on that?
You mention using a gypsum-compatible adhesive, Mapei say that Latexplan can be used over anhydrite screeds if primed http://www.mapei.com/public/GB/products/506_GB.pdf
Is there any reason why that would not work in this situation?
Help much appreciated........

 

[Ajax123]

In perfect conditions with perfect preparation it is perfectly reasonable to use an acrylic primer and cement based adhesive over an anhydrite screed. However I am not convinced that you are dealing with the perfect site here. I will let Mapei stand or fall by their on advice in respect of ther own materials. You have undoubtedly read other posts on adhesive types and the reasons for choosing different types with different screeds. For me it is all about building in robustness in order to protect you and your client in the long term from problems. I can say that I am unaware of any issues where a gypsum adhsive has been used. If you go the cement way you must make sure that the screed is below 75% relative humidity measured with a hair hygrometer. You would need to ensure a high end acrylic primer is used or better still use a water dispersible epoxy primer.

You are right that the material can be force dried. - as it is 40mm I assume he has decided not to go for wet underfloor heating so force drying would involve space heating and dehumidifiers. I say this because generally 40mm is not deep enouh over a standard domestic wet system - needs to be 30mm cover to pipes so 15mm pipe plus 30mm cover is 45mm total minimum depth - I would usually specify 50mm nominal in this situation.

 

[DHTiling]

In perfect conditions with perfect preparation it is perfectly reasonable to use an acrylic primer and cement based adhesive over an anhydrite screed. However I am not convinced that you are dealing with the perfect site here. I will let Mapei stand or fall by their on advice in respect of ther own materials. You have undoubtedly read other posts on adhesive types and the reasons for choosing different types with different screeds. For me it is all about building in robustness in order to protect you and your client in the long term from problems. I can say that I am unaware of any issues where a gypsum adhsive has been used. If you go the cement way you must make sure that the screed is below 75% relative humidity measured with a hair hygrometer. You would need to ensure a high end acrylic primer is used or better still use a water dispersible epoxy primer.

You are right that the material can be force dried. - as it is 40mm I assume he has decided not to go for wet underfloor heating so force drying would involve space heating and dehumidifiers. I say this because generally 40mm is not deep enouh over a standard domestic wet system - needs to be 30mm cover to pipes so 15mm pipe plus 30mm cover is 45mm total minimum depth - I would usually specify 50mm nominal in this situation.

Perfect answer from a perfect member..🙂