Thickness of flexible tile adhesive for terracotta tiles on anhydrite screed

[piggy]

Hi - We are nearing the end of a self-build project and have hit yet another brick wall!
We have 100m2 of floor to tile with 25mm thick terracotta tiles over UFH. They are a bit uneven and the makers recommend bedding them on a 15-20mm mortar bed, our tiler wanted 25mm in order to do a good job so we left space for 25-30mm.
We have laid a gypsum screed over the UFH and the advice seems to be to use a flexible tile adhesive 10mm thick (after priming).
We have been supplied with Primer G an acrylic primer supplied by flo screed.
Please can anyone tell us what we can successfully use 25mm thick?
.
Many thanks to one of your threads on preparing gypsum screeds for tiling it was more informative than a whole day spent elsewhere on the internet.
Piggy

[Gazzatile]

I have used Mapei P4 as thickbed adhesive but cannot remember what max dept is may be only 20mm Just looked on website is only 20mm

[diamondtiling]

25mm is on the excessive side for an adhesive bed, you could lift the surface with a flexible self leveller to leave you with 15-20 mm which is more than adequate.

[pjc]

who are the makers ?

[piggy]

Thanks for that- Mapei website wouldn't come up for us last night but found some stuff on the Bal site at about midnight - pourable thickbed up to 25mm. Main thing is to know that there's a solution out there to be found!
Is it ok to use a different make of primer to the adhesive - all the makers obviously recommend their own primer but we've already got primer G courtesy of Floscreed

[piggy]

That sounds like an interesting idea - have to chew it over.

Quite honestly would like to put down a plastic film and fill the void with mortar + additive and bed the tiles on that. That would decouple from anhydrate screed, waterproof and reduce costs - but i'm expecting at least 20 replies to that suggestion telling me i'm barmy.

Thanks for your reply

[piggy]

Do you mean the tiles? if so York Handmade - lovely tiles, don't want to wreck them!

[Dave]

How long has the screed been poured..? and is it a low laitence type..?

[piggy]

Hi Dave

The screed was poured about Nov 1st - We have just delayed tiling when we discovered that the screed needed to be .3-.5% moisture content - bogglingly small - but thank god for the internet!
We are now about to install gshp on a plinth in the utility to help achieve this before going any further as it seems that the best chance of a successful outcome will be achieved by heating the slab first.
Yes the screed is low laitance.
Thanks for your interest

[Dave]

You are right it does need commissioning first.. and the portable gshp will be great..

My concerns are using that depth of adhesive or modified mortar.. are the tiles that bad in calibration to require such a bed depth..?

[Ajax123]

That sounds like an interesting idea - have to chew it over.

Quite honestly would like to put down a plastic film and fill the void with mortar + additive and bed the tiles on that. That would decouple from anhydrate screed, waterproof and reduce costs - but i'm expecting at least 20 replies to that suggestion telling me i'm barmy.

Thanks for your reply

not sure why you can't do that. would not be plastc film as such but Ditra or Dural matting or the like.......

[pjc]

i would have thought morter bed was the worst thing you could have done with terracotta but i have only laid 2000 - 3000 mts in my 29 years of tiling looking at their specs i dont think they no a lot about fixing them imo or have i got it all wrong ?

[piggy]

You are right it does need commissioning first.. and the portable gshp will be great..

My concerns are using that depth of adhesive or modified mortar.. are the tiles that bad in calibration to require such a bed depth..?

Hi
Thanks for replying, Had intended to install gshp on floor once tiled but 'best laid plans'!

The tiles are not so badly calibrated in fact, but that is the gap that we have left to be filled to get the floor level to meet the threshold level to meet building regs.

What 's the problem in using a thick bed depth of adhesive or modified mortar apart from the cost of course?

[piggy]

Only problem with Ditra is that they don't recommend putting more than 10mm thickness of adhesive on top of it. Don't know if Dura and Genesis etc say the same but would expect that they do.

[piggy]

Hi
Thick bed mortar was what the tile makers recommended even on a heated mortar screed.

We have since laid anhydrite screed for what seemed like good reasons at the time thus increasing the gap and causing our current headache,

What do you use for terracotta?

[Dave]

Just a thick bed flexible adhesive to around 10mm deep is easy sufficient IMO..

[pjc]

nothing wrong with thick bed adhesive i used over 200 tons the year before last(not well last year not working) but you would grade the tiles a little as you were laying them so you shouldent need a 25 mm bed

[pjc]

if its a height problem you need to build the floor up then tile it

[pjc]

what area are you piggy ?

[piggy]

ok - I accept that we've left too big a gap and that 10mm may be enough adhesive but that still leaves a 15mm gap to fill.

Given the anhydrite screed, ufh and the 25mm gap what do you think are my best options?

[piggy]

Yeah maybe - what would you recommend?
We are in Pembrokeshire, West Wales.

[pjc]

AAAAAAAjax what do you think not my field what would be cost affective ?

[pjc]

does the whole floor need to be raised or can it bee ramped ?

[Ajax123]

If you are simply looking at the depth in order to void fill a gap and you are looking at a 10mm adhesive bed then the best thing would be to use a levelling compound over the screed to fill most of the void first. Use a gypsum based compound such as NC110 from Uzin or Screedmaster Gypsum from Laybond Products at 15mm. Because they are gypsum based they are compatible with the screed and will not noticably impede the thermal performance of the floor. You could then use a gypsum based tile adhesive to stick the tiles down e.g. Weber Anhydrite.col or GBTA from creative impressions.

[piggy]

does the whole floor need to be raised or can it bee ramped ?

Whole 100M2 unfortunately!

[piggy]

If you are simply looking at the depth in order to void fill a gap and you are looking at a 10mm adhesive bed then the best thing would be to use a levelling compound over the screed to fill most of the void first. Use a gypsum based compound such as NC110 from Uzin or Screedmaster Gypsum from Laybond Products at 15mm. Because they are gypsum based they are compatible with the screed and will not noticably impede the thermal performance of the floor. You could then use a gypsum based tile adhesive to stick the tiles down e.g. Weber Anhydrite.col or GBTA from creative impressions.

Thanks for going into this in depth - Have had a look at gypsum based adhesives but are they OK to use in exterior doorways, kitchen, utility and cloakroom with shower or any place where they may get wet?

[Ajax123]

Thanks for going into this in depth - Have had a look at gypsum based adhesives but are they OK to use in exterior doorways, kitchen, utility and cloakroom with shower or any place where they may get wet?

I get asked this all the time with regard to the screeds. My answer is this. The calcium sulphate screed should not be used in areas where it will become wet or in areas where it will remain permanantly wet. in respect of the screed material itself temporary wetting is not considered a problem. If the screed is dried then covered with a floor covering it is sealed against the ingress of moisture so will not become wet and so does not present a tangible problem. My opinion is that the same advice should be followed with the smoothing compounds and tile adhesives. you would use a standard flexible cement based grout and silicone around the perimeter expansion gap.

If a cement based smoothing compound is used over a gypsum based screed and it gets wet it WILL delaminate from the screed and the flooring will fail. If a gypsum based smoothing compound is used the floor covering itself may delaminate (in the case of tiles they would be reusable if a gypsum base adhesive were used) but the smoothing compound and adhesive would generally remain bonded to the screed.

In areas where there is high risk of moisture ingress e.g. commercial kitchens the I would always recomend the use of an epoxy damp proof membrane over the screed prior to tiling anyway. If this is done then a cement based adhesive would be more appropriate.

I know of hundreds of thousands of square meters of tiles on gypsum adhesives in kitchens, bathrooms, utility rooms etc and there are no serious issues. Won't say there are no issues at all because the human element often comes into play and you find someone has done something daft.