Anhydrite Screed and tiling

[St Andrew]

Hi All,

I'm currently looking at putting down a anhydrite screed and then tiling on it and putting down somw timber flooring. the screed is made by Bardons and is called Highflow S; my question is what to put on top of this screed; I've been advised to use a primer and cementacous tile adhesive OR to use a acrylic tile adhesive, any advice would be grratefully received. PS the tiles are ceramic and 300mm square.

cheers,

Andy

[Dave]

Hi Andy..

Is the screed to be heated..?

[St Andrew]

Dave,

I tried to reply the other evening (still getting use to the system, it didn't post) there is a wet UFH system which is sat on a layer of kingspan insulation

cheers,

Andy

[Dave]

Ok..

Well this wet UFH will need to be commissioned before any tiling can commence.. this can start usually as early as 7 days from the pour...

Now this screed will need to be dry enough to tile before you tile it and by using the wet UFH you can force dry the screed..

Turn on the heating at ambient (approx 20 degrees C) after minimum 7 days. Leave at this temperature for 3 days. Then increase the temperature by maximum 5 degrees C per day up to a maximum of 55 degrees C. Leave at this temperature for 7 days then reduce by 5 degrees per day down to its normal operating temperature and leave at this temperature for as long as you like but normally for 3 or 4 days. Then switch off for three days before moisture testing to determine if it is dry. The effect of force drying with the heating must be accompanied by good ventilation i.e. plenty of draught from open windows and doors. Alternatively Dehumidifiers can be used but they need to be man enough for the job as heating the screed will drive ou moisture quite quickly. All temperatures are flow temperatures and not surface temperatures.

The bardon high flow screed is not a low laitance one and thus will need to be removed prior to tiling.. this can be done with either a scarifier or a floor sander.. The laitance is a friable surface that is left on the top as a result of the pour and levelling process from the screeders.. look as it as a creamy residue that dries as a crust on the surface and needs to be removed ..1: to allow the screed to dry quicker and fully and ...2 : to aid in the priming and adhesion of the adhesive.

The floor needs to be around 75% RH before you can tile with a cement based adhesive and 85% RH with a Gypsum based adhesive.. and a method of testing for the content of moisture is the use of a Hair-hygrometer.

[St Andrew]

Cheers Dave,

is there a need to put down a primer before the adhesive? I've read other post's on here and they seem to say there is a need for this. Also I was told yesterday by a supplier (on the phone) that I'd be better off using an anhydrite/gypsum based adhesive, but this is very expensive. thanks for taking the time to reply,

Andy

[Dave]

No probs Andy..

If using a cement based adhesive then yes a primer is required.. IMO the best to use is an epoxy primer.. this if applied properly will stop any reaction between the screed and the adhesive but in all honesty Andy.. You want a risk free installation and Webers Anhydrite-col adhesive isn't really expensive and will give you a risk free bond..

[St Andrew]

Dave,

where would I best off buying the Weber product from, any suggestions?

Cheers again,

Andy

[Dave]

Andy.. The only place i know so far that sells it is CTD.. and Westerhope tile centre.. up in the north east