laying reproduction flagstones on a pumped screed

[mj8984]

Hi All, I will start by saying I wish I found this site a long time ago as it is very imformative indeed, basically i have been doing a refurb on a old farmhouse and have to my cost found out that there are pitfalls with laying slabs on a pumped screed floor, The screed was pooring in Aug 2009 and after waiting at least 4 months to ensure the screed was perfectly dry, we fitted some reproduction flagstones throughout the entire ground floor approx 90M2, the flags were 30mm thick and had joints of 25mm wide, the flagstone manuafcturer normally recommended to fit on a sand and cement screed but as we we had underfloor heating he suggested the a flexible tile adhesive suitable for UFH, as the finish on the floor was very good as you get with a pumped screed, we used excel bond rapid set flexible adhesive, as the instructions said to seal wooden floors or worn concrete we DID not seal the floor(Big Mistake) anyway we laid the floor and after another 3 months the heat pump was comissioned and the UFH slowly increased up to temp, after another 3 months i noticed that their was small cracks between the pointing and some slabs , as they were pointed with lime and cement based mortar because the depth was 30mm and up to 25mm wide because we couldnt find any flexible grout that would cover that gap width and depth, initially i thought this is just the grout or slab shrinkage( as the slabs were madde to order and i guess they had not had 6 weeks drying time, after removing the grout i found that the slabs lifted freely from the floor and the adhesive had not stuck to the floor or the slab???? anyway after alot of deliberation I have lifted the slabs and primed the floor with a SBR from Febond, also I have primed the back of the slabs, my question is how many coats of SBR bond should i apply to the floor and slabs and should it be almost tacky when applying the adhesive ?? and as the gap is very wide and deep compared to ordinary tiles would it be ok to add SBR to the pointing mix to give it some water resitance and help the motar adhere to the slabs? i have also read about a matting that can be applied to stop the reaction between the screed and the adhesive, the problem is I will re do each room and i dont want to increase the height by adding a matting even if it is 5 ot 6mm it is a pain as skiting boards and eveything is on now! i know that this started by not priming the floor and gypsum based screed but I am trying to recover the situation, will the floor be ok if it primed with SBR before re applying the adhesive or should i also try and use a gypsum based adhesive? thanks alot for any advice or pointers Martin

 

[faithhealer]

will pm ajax for you, he works for Lafarge. I do think the heat pump should have been commisioned before flags laid but wrong adhesive/prep prob here.

 

[doug boardley]

did you sand off the laitence too? An epoxy primer apparently is the the ideal for prepping an anhydrite screed, applied in two coats at 90dgs to each other.

 

[mj8984]

its seems as though the adhesive that was applied to floor has pulled of the top film of the top of the screed, i have sanded the floor with a sander and it does not change too much apart from the dust i will apply a second coat of primer then thanks alot

 

[mj8984]

thanks, ideally i agree but as the heat pump was being sited on top of the flags it made it a little difficult, the screed had more than 4 months to dry seemed very dry before the flags were laid, it then had a further 3 months before the heat pump was installed and heating to the floor so i guess the adhesive would of had plenty of time to dry before any heat, I know now that the primer seal was the vital step, as it is not my profession i relied on advice from so called specialists and the instructions on the adhesive which did not clearly state to prime the floor unless it was wood or worn, anyway i am trying to deal with it now and hopefully i will be able to rectify it with the correct proceedure now, many thanks

 

[Ajax123]

SOunds like a combination of issues here. Corect prep as follows as far as I am concerned
1. Sand the floor to ensue that there is no loose laitance or other contamination of the screed surface prior to priming. Any disruption to the bond using a cement based adhesive can cause debonding

2. Ensure that the underfloor heating is comissioned and run prior to laying floor coverings - this is non negotiable and MUST be done. Expansion of the screed compared to the adhesive can cause debonding

3. Prime the screed using a good qualitly penetrative primer which wil form a good barrier to the screed from the adhesive. Chemical incompatibility i.e. cement plus gypsum do not stik together in direct contact. The choice of primer will epend on the choice of adhesive.
3a. If using a cement based adhesive in general an epoy primer will gove a more robust barrier. A good quality acrylic primer
will work but only if the screed is fully dried and perfectly prepared. A water dispersible epoxy works best IMO
3b. If using a Gypsum based adhesive then an acrylic or polyurethane primer are fine. SBR will work also as you are only
worried about suction into the screed and chemical incompatibility is not an issue.

Bare in mind that whichevr primer you decide to use it MUSt be able to penetrate the screed surface (i.e. soak in) otherwise it will simply sit on top and form a "film" which is not bonded.

Then stick you tiles down with whichever adhesive you select. FOr my money I would buy Gyspum Based adhesive and Acrylic primer.

Hope this helps

 

[White Room]

Top post again

 

[doug boardley]

I'd have known that if my car had started^^^^

 

[mj8984]

thanks Ajax123 appreciate you advice, the UFH has been running since March but i have turned it off in the rooms i am re doing at the moment, but i may just leave it on very low with the temp as it is at the moment, any ideas where i can get Gypsum based adhesive? i have been surfing the net but they seem very rare and i dont know of any suppliers in my area Stafford. any thoughts on the grout will the lime/white cement based mortar work with a bonding agent such as SBR added? as the joints are quite deep and wide really? thanks again

 

[mj8984]

any ideas on where i can get Gypsum based adhesive from and is it reasonable in price compared to cement based? thanks MArtin

 

[lucius]

Used some last year but the contractor supplied it so cant remember the name but it was definitely German.

 

[jstrats]

I realise this is an old post but you can get adhesive from here:

N&C (Nicholls and Clarke) Building materials manufacturer and supplier