Fixing to Calcium Sulphate Screed & drying times

[Cat]

Right - here goes...:thinking2:

I have a 75mm lafarge gyvron calcium sulphate flow screed floor that was put down over 100 sq m on the ground floor of my self build in December 2008. The kitchen/diner has a flat roof which only became watertight Feb 14th 2009.

When the screed was laid the house was watertight except the kitchen, which ended up with water on top of the screen from december to mid Feb. The official drying time for 75mm screed is 90 days (1mm per day up to 40mm and then 1mm per 2 days for the remainder) but this is from watertight.

The floor has to be levelled by about 7mm before the tile adhesive is laid. Our tiler did attempt to level about 6 wks ago but was not aware of any special requirements for calcium sulphate screed. The PVA was applied in about a 1:5 (I now understand that it should be applied neat to form a barrier between the calcium sulphate and any cement based levelling compound to prevent reaction).

The levelling compound dried and cracked all over the ground floor and had to be taken up. Strangely however, one batch of compound adhered to the floor better than the remaining - however the rep has been out and taken samples and not come back to us. On going back to the screed company I have now been told that the underfloor heating must be in operation before the tiles can be laid to ensure that the screed has settled.

The underfloor heating has now been on a week (since 15th May). As we have a ground source heat pump, water underfloor is at approx 38 - 40 degrees and would have started up gradually due to taking time to warm up - so basically, its not as we have blasted 70 degrees through the screed from start up, so all should be good. The rooms feel warm, not damp, no cold spots, all watertight.

Apologies for the long post but that's the background. My questions are:

1. Without spending loads of money on a special hydrometer testing for calcium sulphate screed floors to check the moisture content (standard meters don't work with calcium sulphate) - I feel that we have been watertight for 3 months plus with the heating on a week and the screed has been down for 5 - 6 months - surely it has to be dry now?

2. We are having 600mm porcelain tiles over the 100 sqm ground floor that has to be levelled by about 7mm. We have a great tiler but he hasn't tiled to calcium sulphate before - any recommendations on using PVA as barrier and then appropriate levelling compound to be used prior to adhesive? Would you trust the pva barrier and then use cement based leveller (which is the main reactant) or go for a calcium sulphate leveller and then apply adhesive to that?

Any help would be greatly appreciated. I'm a new poster to tilers forum but when searching for tiles often ended up here and have found lots of useful information (I've read so many of Gary's posts now that I feel like 'family'!). Kitchen is fitted on 29th June so I need to get something moving soon and want to make sure that it's going to work this time - for definite.

Thanks in advance to all you experts out there - I would appreciate the benefit of your experience,

Cat :54:

 

[DHTiling]

pva???...:yikes:

 

[Cat]

Hi Dave - yes PVA - I've got a quote from the flow screed company saying neat PVA should be used as a barrier. There was no mention to me that special consideration need to be given to calcium sulphate by the people who laid it - not going to name names. When speaking on the phone afterwards they said - "PVA - craft glue - like kids use". My tiler is great but has not experienced calcium sulphate - the screed company say that anyone should have recognised the different coloration and acted accordingly. Saying that - they haven't responded to my help requests. After spending 2 years on my self build - just need to become an expert in the science of levelling and tiling floors and then hopefully one day I'll just have to walk on the damn thing!

Cheers,

Cat 🙂

 

[doug boardley]

does my "o" level chemistry serve me correctly in saying calcium sulphate is a type of gypsum?,just curious as to why you opted for this type of unconventional substrate?

 

[DHTiling]

Why was the floor to level....?..flow screed floors should be like a bowling green...

Was the latitence removed before any prepping was done..?

 

[Cat]

Doug - on recommendation of architect and research, went for flow screed as opposed to conventional. This was because the flow screed enabled better conductitvy for the underfloor and I didn't have to risk a gang of workers trampling over my underfloor pipes which go over 3 floors and over the external garage linked to a ground source heat pump. None of my experts on board sounded any warning bells and I still know more about flow screed that both I and others care to know - but I now understand that flow screed has an alternative compound to standard. Sadly I have had to find that out for myself.

So, to answer your question - not gone for something 'weird' on purpose just 'different' on recommendation. Mind you - having experienced choosing underfloor heating on 3 floors and a ground source heat pump - don't even start me on the rainwater harvesting - Grand Designs operate on another planet - how do they get anything done - I only wanted a watertight roof and a floor!!!!

I know it can work - just need to correct materials and advice. From my experience I can't just rely advice I'm intially given - just need to find out more.

Thanks - Cat

 

[Cat]

Dave - Got a three storey house with a flat roofed kitchen diner with steel frame construction. Steel frame - 26 ft across - 12ft deep - 25K of sliding/folding glass in there. Plan was to have flush levels inside and out - which, annoyingly, I can not now achieve. Levels were left on the conservative side so that when the glass was fitted the chosen tiles could be made flush. There wasn't much laitence on the screed as it was flow screed - really good finish and very level and accurate. However, after putting cement based levelling compound down and then bringing back up the floor surface is definitely clear.

Basically - I know that the flow screed could and should have gone perfectly to the given level which would have been ideal but now I need to make up the level and find materials and adhesive that work with the calcium sulphate lafarge gyvron screed.

Thanks,

Cat

 

[DHTiling]

Ok..Pva will not stop any reaction between the 2 coverings....

once laitence has been removed and moisture levels are down to required level then acrylic primer is applied then when dry it is applied again in the opposite direction to the first coat...

The screed is gypsum based and if you have applied a cement based leveler with out correct prep then you have trouble on your hands..imo.

 

[Ajax123]

Right - here goes...:thinking2:

I have a 75mm lafarge gyvron calcium sulphate flow screed floor that was put down over 100 sq m on the ground floor of my self build in December 2008. The kitchen/diner has a flat roof which only became watertight Feb 14th 2009.

When the screed was laid the house was watertight except the kitchen, which ended up with water on top of the screen from december to mid Feb. The official drying time for 75mm screed is 90 days (1mm per day up to 40mm and then 1mm per 2 days for the remainder) but this is from watertight.

The floor has to be levelled by about 7mm before the tile adhesive is laid. Our tiler did attempt to level about 6 wks ago but was not aware of any special requirements for calcium sulphate screed. The PVA was applied in about a 1:5 (I now understand that it should be applied neat to form a barrier between the calcium sulphate and any cement based levelling compound to prevent reaction).

The levelling compound dried and cracked all over the ground floor and had to be taken up. Strangely however, one batch of compound adhered to the floor better than the remaining - however the rep has been out and taken samples and not come back to us. On going back to the screed company I have now been told that the underfloor heating must be in operation before the tiles can be laid to ensure that the screed has settled.

The underfloor heating has now been on a week (since 15th May). As we have a ground source heat pump, water underfloor is at approx 38 - 40 degrees and would have started up gradually due to taking time to warm up - so basically, its not as we have blasted 70 degrees through the screed from start up, so all should be good. The rooms feel warm, not damp, no cold spots, all watertight.

Apologies for the long post but that's the background. My questions are:

1. Without spending loads of money on a special hydrometer testing for calcium sulphate screed floors to check the moisture content (standard meters don't work with calcium sulphate) - I feel that we have been watertight for 3 months plus with the heating on a week and the screed has been down for 5 - 6 months - surely it has to be dry now?

2. We are having 600mm porcelain tiles over the 100 sqm ground floor that has to be levelled by about 7mm. We have a great tiler but he hasn't tiled to calcium sulphate before - any recommendations on using PVA as barrier and then appropriate levelling compound to be used prior to adhesive? Would you trust the pva barrier and then use cement based leveller (which is the main reactant) or go for a calcium sulphate leveller and then apply adhesive to that?

Any help would be greatly appreciated. I'm a new poster to tilers forum but when searching for tiles often ended up here and have found lots of useful information (I've read so many of Gary's posts now that I feel like 'family'!). Kitchen is fitted on 29th June so I need to get something moving soon and want to make sure that it's going to work this time - for definite.

Thanks in advance to all you experts out there - I would appreciate the benefit of your experience,

Cat :54:

First of all the screed is too deep. It need only be 50mm over underfloor heating but that cannot be changed now so is probably useless info.

The tler should really make himself aware of the requirements for the substrate.

For moisture I often initially use an indicative test which is basically to place a sheet of polythene around 1mx1m on the floor. Leave in place for 3 days and then lift it. If there is no change of colour and no moisture on the polythene then it is dry. If there is some colour change and some moisture then it is nearly dry, if there is moisture on the floor or the underside of the polythene it is still wet. This is an indicator only and does not give an accurate result of course.

The natural drying time is 1mm per day up to 40mm then 2 days per mm thereover so for 75mm it is actually 110days and not 90 days. Who told you 90? - they were wrong. This is based on standard drying conditions of 20oC 60% RH and good ventillation. The last bit is the one people often forget. However if the heating has been running for a while I would expect it to be dry. The correct test is to use a hair hygrometer. These are fairly cheap to buy (around £80 ish last time I looked) so migh be worth investing in one. Electronic resistivity meters do work on this type of screed but cannot be relied upon to give an accurate result. If you get very low results it indicates the screed is dry. Conversely if they show high results the screed is wet. They cannot tell you how dry or wet however if you fail to get a response or get a very low response the screed can be considered dry.

another good indicator is the humidity in the room. If the screed is stil very wet it will feel humid in the room and you will probably get some condensation on the inside of the windows over night.

You should NOT use PVA as a primer for 3 main reasons.
1. PVA (poly vinyl acetate) degrades over time when exposed to alkaline conditions. This will cause the tiles to fail over time.
2. It will not prevent the migration of sulphates from the screed through the PVA which will cause the ettrignite reaction you mentioned earlier and which will cause the tiles to delaminate
3. There will always be some moisture in the screed. This moisture will migrate towards the surface as the heating operates. This will emulsify the PVA and loosen the bond between it and the screed again potentially delaminating the tiles

In terms of options I suggest the following actions

1. Remove any residual leveller that has been placed ad then sand the surface of the screed to remove any remaining PVA.
2. Make sure the screed is dry - an indicative test will suffice at this stage. It can also be carried out with the heating on.

If not dry run the underfloor heating at its maximum operating temperature and get plenty of ventillation over the screed by opening windows and doors. The ventillation is the key.
2. Prime the screed with a good quality acrylic primer such as PE360 from Uzin or Screedmasters Universal from Laybond or Resin Based primer from Creative Impressions. There are plenty of others but here is where you have a bit of a choice to make cos often if you ask the primer supplier to do a moisture test for you they will do provided you commit to using their product. Can't guarantee it but ask them the question.
3. Once primed you need to sort out the 7mm out of level - I am surprised its this far out but it does technically meet BS8204:7 so I guess not a lot to be done about that. To sort out the level use a calcium sulphate based levelling compound. you avoid any risk of ettrignite formation this way. Uzin can supply NC110, Laybond Products Screedmasters Gypsum and Creative Impressions have GBLC01. Al of these can be found on the relevent suppliers web sites. Bal and Dunlop also have some good gear but I can't remember the names off the top of my head.
4. ONce it is flat and at the level you need you have a second choice to make and that is whether to use a cement based tile adhesive or one based on calcium sulphate. There are very few in the UK based on calcium sulphate bt one supplier is creative impressions who supply GBTA. I beleive Laybond also have one but don't know what its called. The benefit of course of this is again the avoidance of ettrignite reaction. If you decide on cement based that's fine bt the primer will be different.
5. Prime the surface of the levelling compound. If you opt for the calcium sulphate addy then use the same primer as you used bfore the levelling compound went down. If you opt for the Cement based leveller go for a water dispersible epoxy. Same suppliers as before should be able to help.
Do NOT use an acrylic primer with cement based adhesive cos it will probably fail.
6. Make sure you allow the primer to dry properly and then tile as normal.

Couple of things to consider when making the choice of levellers and addys. Cement based products expand and contract at different rates to calcium sulphate based materials. this can place a mechanical stress on the interface between the two so I would avoid cement based leveller in this instance.

Secondly if there is going to be a failure and you use cement based addy the cement the failure is likley to be between the screed and the addy so the addy sticks to the back of the tile and makes in un-reusable. With Calcium Sulphate the failure is likely to be at te interface between the tile and the addy so it comes off clean and can be re used.

Thirdly the calcium sulphate addy is more robust to moistrue and can normally be used when the screed is still up to 85% RH (a dry floor for cement based is 75%)

Finally the other alternative would be to consider using a decoupling mat such as Ditra Matting which can be used whilst the screed is still quite wet (up to 2% moisture) These are relatively expensive but worth considering if fast track is needed.

A big read I know but hope it helps

 

[Cat]

Hi Dave - thanks for your reply. The cement based leveller is all now up - it cracked completely off the floor so it is now very clean. Watertight now for 3 months plus and heating on.

What acrylic primer would you use and is there any particular concentration? You mention multiple directions of laying primer - this is a new recommendation to me. Would you go for a calcium sulphate leveller after this?

Have a horrible feeling I just need to cut to the chase and get the hydrometer test done - if they state that moisture levels are correct they underwrite the job.

Any help appreciated,

Thanks,

Cat

 

[Ajax123]

does my "o" level chemistry serve me correctly in saying calcium sulphate is a type of gypsum?,just curious as to why you opted for this type of unconventional substrate?

Gypsum = CaSO4.2H20 - Dihydrous calcium sulphate - this is the resultant molecule in all calcium sulphate based products including plaster, plasterboard and floor screed basedon Hemi Hydrate and Anhydrite

CaS04 = Anhydrous calcium sulphate = anhydrite - can be harvested as a waste product of acid manufacture as flouro anhydrite (this is what Lafarge Gyvlon is made from) or as a a waste product from coal fired power stations - this goes though a process called calcination which converts what starts of s a composite product to a single molecular arrangement.

2CaSO4.H20 = Hemihydrate

Harvested mainly as waste from coal fired power stations but also occasionally mined gypsum. It goes through a process called autoclavin - high pressure with moisture which converts the coposite material to a single molecular structure depending on the temperatures, pressures and source materials forms one of 2 types.

- Beta hemi hydrate which chrystallises to form big gypsum chrystalls - used for making bagged plasters and plasterboard products - a relativley soft form of Gypsum

Alpha Hemihydrate - used for floor screed and calcium sulphate block manufacture - also a major constituent in plaster of paris - forms small tightly packed gypsum chrystals which makes it relatively hard

Something like 20% of the UK screed market, approx 50% of the french screed market, approx 65% of the German and dutch markets are of this type.

 

[Cat]

Ajax - thanks for your detailed reply - yes, a big read but one that might be of a big help. Off to do some printing and discussion with tiler. Sorry about mixing up the hydrometer and hair hygrometer - there's phone conversations for you 🙂

Will continue looking into it - Really appreciate your time - Cat

 

[Ajax123]

Hi Dave - yes PVA - I've got a quote from the flow screed company saying neat PVA should be used as a barrier. There was no mention to me that special consideration need to be given to calcium sulphate by the people who laid it - not going to name names. When speaking on the phone afterwards they said - "PVA - craft glue - like kids use". My tiler is great but has not experienced calcium sulphate - the screed company say that anyone should have recognised the different coloration and acted accordingly. Saying that - they haven't responded to my help requests. After spending 2 years on my self build - just need to become an expert in the science of levelling and tiling floors and then hopefully one day I'll just have to walk on the damn thing!

Cheers,

Cat 🙂

Who said PVA !!!! - sounds like retraining required

 

[DHTiling]

Who said PVA !!!! - sounds like retraining required

loos like these peeps did>>>> lafarge gyvron calcium sulphate company.

 

[Ajax123]

Ok..Pva will not stop any reaction between the 2 coverings....

once laitence has been removed and moisture levels are down to required level then acrylic primer is applied then when dry it is applied again in the opposite direction to the first coat...

The screed is gypsum based and if you have applied a cement based leveler with out correct prep then you have trouble on your hands..imo.

Likely to be skin free as it is Lafarge Gyvlon so does may need laitance removal. You right though if it is "traditional" gyvlon laitance needs to be removed.

 

[Cat]

Ajax - after reading your post with your obvious, indepth knowledge of the science behind the substrate - yes, PVA does sound a bit Heath Robinson. Have to move on and not worry about this advice or I won't sleep at night!

Cat

 

[DHTiling]

But a tiler should always check for laitence, don't you agree Ajax..?:smilewinkgrin:

 

[Cat]

Hi Dave - I have read the floor finishes advice from some of these companies found on google - and it does mention polyurethane or acrylic primer. I can't get any joy from my screed company and my tiler is really trying to find out as much as he can from lafarge etc but as self builders we need to really be happy with our methods ourselves also.

Thanks for your help. Cat.

 

[Ajax123]

loos like these peeps did>>>> lafarge gyvron calcium sulphate company.

Negative good buddy - I know for a fact that this particular organisation would not recomend PVA. (assuming spelling error on Gyvron of course otherwise I in trouble)

Broken Link Removed

I am guessing ready mix supplier - would be interested in knowing whom.

 

[DHTiling]

Negative good buddy - I know for a fact that this particular organisation would not recomend PVA. (assuming spelling error on Gyvron of course otherwise I in trouble)

Broken Link Removed

I am guessing ready mix supplier - would be interested in knowing whom.

Thats what i thought...thats who you work for isn't it..?..thats why i hoped you seen this and corrected who advised pva ffs...


The question now is getting this floor prepped and tiled correctly...

Cat..get your tiler to join our forum and then the info can be given DIRECT to him so he can absorb it at first hand...

 

[Cat]

Hi Ajax - not your company at all so no worries there. Obviously not going into details - guys were great - screed is great - floor is flat - just need to get the 'bloomin tiles stuck to it...

 

[Ajax123]

But a tiler should always check for laitence, don't you agree Ajax..?:smilewinkgrin:

Absolutleytootley Dave.

BS8203 - code of practiv for resillient floor coverings

"the screed should be sound, clean, dry and free from dust and contaminants likely to affect adhesion of the flooring"

ergo Laitance should be removed prior to the application of tiles. If no laitance is present there is no need to remove it. - Screed might need sanding for other reasons of course e.g. removal of mortar or plaster snots, plasterboard addy, expaning foam deposites and such likes.

 

[Ajax123]

did i attach that pic correctly - not sure

 

[Cat]

Hi Dave - yes, agree just need to get the solution now - on my recommendation my tiler has been a member of the forum since I joined a couple of months ago. As a self builder and project managing, I need to be sure that everyone is happy with the plan - if I've managed to get the house up I'll get the floors down! I'll be collaborating with the tiler and I'm sure he will get involved in the thread when I speak to him so that we all know the best route.

Cheers, Cat

 

[DHTiling]

It works if you open it.....nice pic...

 

[DHTiling]

Hi Dave - yes, agree just need to get the solution now - on my recommendation my tiler has been a member of the forum since I joined a couple of months ago. As a self builder and project managing, I need to be sure that everyone is happy with the plan - if I've managed to get the house up I'll get the floors down! I'll be collaborating with the tiler and I'm sure he will get involved in the thread when I speak to him so that we all know the best route.

Cheers, Cat

No probs..look forward to his input.....lets get this floor tiled eh?...:smilewinkgrin:

 

[Ajax123]

Thats what i thought...thats who you work for isn't it..?..thats why i hoped you seen this and corrected who advised pva ffs...

I wish to remain silent on the grounds I might incriminate myself - thought it was supposed to anonymous

but seriously if it helps the industry get things right I am happy to help cos Anhydrite Screeds aint going away (not if I have my way anyway)

The question now is getting this floor prepped and tiled correctly...

I agree

Cat..get your tiler to join our forum and then the info can be given DIRECT to him so he can absorb it at first hand...

good idea

 

[Cat]

Ajax - thanks for the laitance pic - no, my floors didn't have a skin on like that and are all smooth and clear. Really useful though to have a picture - makes all of these terms come to life.

Cheers, Cat 🙂

 

[Ajax123]

Hi Ajax - not your company at all so no worries there. Obviously not going into details - guys were great - screed is great - floor is flat - just need to get the 'bloomin tiles stuck to it...

Don't want to post names and numbers on the forum without their permission but if you want to ring me at work tommorrow I can give you some contact details for the primers and levellers etc if you want.
07545 932723

Where in the country are you

 

[Cat]

Hi Ajax - Thanks for the contact - will give you a call as I really need to get this floor down now. Based in Midlands 15 mins from Bham NEC.

Appreciated. Cat

 

[wetdec]

Using an anhydrite screed is not a problem provided its treated correctly.

When the screed is dried out properly( to 0.5% moisture), remove any latency with a scraper, wire brushes or sanding then hoovering up all dust.

If you have a wet under floor heating system installed increase the temperature in incriments until full heat is reached then work backwards, this procedure helps in the drying process of the screed and aids curing.

When dust free use your adhesive maufacturer recomended sealer to seal the floor, this protects the screed and should be done in opposing directions to ensure coverage. It is important that guidence is saught from the adhesive manufacturer as the bond to screed is their territory.

Screeds move even after drying due to expansion and contraction especially when using wet under floor heating and can cause tiled surfaces to crack from latteral deflection. To combat this cracking an uncoupling membrane such as Ditra (1) or Dura-Ci membrane should be fixed before tiling.

Cut to size and laid using a flexible tile adhesive the membrane is easily tiled over. The membrane acts to protect the screed from moisture from above so should therefore be sealed properly. The channels in the membrane aid air movement over ufh and during adhesive drying.

(1)Please check Ditra for their fixing instructions

There are always questions re gypsum / anhydrite screeds, hope this goes some way to answering some of them.


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