Can it be done? Are my slabs too big! Help!

[g cambrensis]

Want to lay sandstone slabs various sizes 300x300 upto 900x600 over ufh.
Screed was a liquid pour type and these slabs are uncalibrated 25-45mm in thickness.

I know i'm going to need a lot of flex adh to pack up but is it do-able? Last tiler came out and said it wasn't.

I know of an adh called nocros 'thick bed stone and porcelain adhesive'. Would this do the job for fixing? Any help appreciated.

Thanks

 

[jay]

Re: Can it be done? Are my slabs too big!

hi the variation in thickness might challenge you can it be sorted so you start off at 45mm then as you neer end lay 25mm slabs :8:if you can grade it i cant see why it cant be done just remember your finish height

 

[jay]

Re: Can it be done? Are my slabs too big!

forgot to ask whats the sub:8:

 

[heavytrevy]

Re: Can it be done? Are my slabs too big!

You will need to wet bed those big slabs.
Totally doable but u will need somebody specailised in stonework.

Goodluck in finding somebody suitable.

Trev

 

[g cambrensis]

Re: Can it be done? Are my slabs too big!

forgot to ask whats the sub

Sorry, your dealing with a novice! Subfloor?
Liquid screed was poured on to a depth of between 40-50 mm

The 2 guys i've spoken to so far are putting a downer on the whole thing and i need some advice.

Some have told me to use sand-cement rather than nocros flex adh. Whats the opinion on that?

Other question: where do i find people? A lot aren't used to laying large uncalibrated slabs indoors. Should it be that big a deal finding a layer.

Ultimate question! Can i do it myself? (with enough advice from you lot)

 

[heavytrevy]

Re: Can it be done? Are my slabs too big!

yes u will need mortar (sand, cement) for the wet bed.
You will have to create a screed /wet bed and lay ur slabs onto it when wet , tap into place to level and acheive the desired height.


Its a job for a stonemason or large format specialist.
very few tilers can wet bed nowadays .

Trev

 

[Gazzer]

Re: Can it be done? Are my slabs too big!

Thoughts that i am having here are. You say that the thickest slab is about 45mm , so we assume that even if you used an adhesive to lay these with you would have at least 50mm from the sub floor. I assume the UFH is a wet system which wil be about 10mm below the screed, maybe more !. It seems a lot of tile for the heat to come through in my mind. I could be wrong as i have never done such a job as this.

 

[jay]

Re: Can it be done? Are my slabs too big!

as above still wondering on sub concrete or timber can it be graded :8:

 

[g cambrensis]

Re: Can it be done? Are my slabs too big!

yes u will need mortar (sand, cement) for the wet bed.
You will have to create a screed /wet bed and lay ur slabs onto it when wet , tap into place to level and acheive the desired height.

I wanted opinions on this as i was also told that any fixing agent needed to be flexible as the expansion and contraction of ufh would lead to cracks and a lot of heartache.

Took advice from uponor for ufh. Upto 100mm is acceptable but there is a time lag on the warmth.

Jay - at the risk of sounding stupid whats the sub concrete?
Liquid screed was pumped in. Now totally flat.

I had a tiler lined up, and waited for him but he's more or less walked off and i'm left with 80 odd Sq M to lay.

Thanks all

 

[heavytrevy]

Re: Can it be done? Are my slabs too big!

I doubt you will find any fexible adhesive that can be bed up 35mm WITHOUT failing.
Im recomending a wetbed of atleast 50mm reinforced with poultry mesh.

Trev

 

[g cambrensis]

Re: Can it be done? Are my slabs too big!

I doubt you will find any fexible adhesive that can be bed up 35mm WITHOUT failing.
Im recomending a wetbed of atleast 50mm reinforced with poultry mesh.

Trev

finished floor is planned to be 50mm from current slab so 5mm bed for largest of the stone slabs 25mm bed for thinnest slabs. At least thats the plan!

Thanks

 

[heavytrevy]

ur gunna need 10 - 15 mm bed for the 900x600

Trev

 

[Gazzer]

I think a call to Mapei technical dept would get a quicker and better answer.
0121-5086970

 

[Ajax123]

Might be worth a call to Creative Impressions - they do a gypsum based adhesive for these types of screeds but I am not sure what the maximum depth it is able to go to. I know they say minimum 3mm but the data sheet I have does not give a maximum depth. I am guessing you might be able to mix it dry and use it much deeper. I know some of the gypsum levelling compounds can go up to 75mm deep but understand that tile adhesive might be different. Other than thay this one has got me. PM me if you want some contact details.

 

[Dan]

I'll be honest and admit this is over my head, so I'll stay out of this one. What I would say is, even I wouldn't attempt the above without being shown on a few jobs beforehand, and I can tile.

Good luck either way. Perhaps post the job in the "I'm looking for a wall or floor tiler" forum and see what sort of a response you get?

 

[DHTiling]

You need to go heavytrevy's route....use a semi coarse sand cement mix and modifyer....Only difference i would do, is slurry fix into the mortar bed..

But we also need to know before you do any tiling, is the flowable screed gypsum based..?

 

[Ajax123]

Thanks Dave - that was a bit of an assumption on my part. I am guessing as it is underfloor heating and the depth is 45mm ish it will be gypsum based., If it were cement based it should be minimum 65mm deep.

My concern with sand cement bed even if it is modified on top is the shallow depth it looks like going - using sharp sand and cement will mean a sand grading of up to 6mm grains. This will not allow enough cement matrix cover to the sand and when the heating is run it will crack.


Will the weight of the slabs prevent them from moving if this happens. I have seen these types of slabs laid over gypsum but the depth of mortar I have seen has been upwards of 30mm.

Other issue is of course chemical separation of the cement from the gypsum which will be best acheived either with a water dispersible epoxy or uncoupling membrane.

The gyspum adhesive can go to minimum 3mm but I don't yet know the maximum depth - will be finding out tomorrow though. If it can be used it avoids both these issues.

Underfloor heating needs to be comissioned before the slabs are laid as well.

 

[DHTiling]

If you use the sand cement method then it is just like using a thick bed adhesive,,this type of mix can be bought already prepared...A builder i do work for uses it quite a bit...for laying slab floors indoors onto wet ufh.....

Rob z uses mud fixing a lot as well and as heavy said they use mesh inlay...Methods that are not used much in the uk...

 

[Ajax123]

So a lot like what they do in France with quarries and the like - standard wet bed method.

 

[DHTiling]

It is yes...but personally i think it can cause efflorescence if grouted to early....it's just an option Ajax....

Another is to lay the thick peices first with a thick bed adhesive, then latex slc to build up for the thinner stuff, but would cost a fortune....

 

[Ajax123]

Sounds like a reasonable solution to me Dave...

Just need to make sure the risks of delamination are managed cos adding the level of moisture in a wet bed mortar to the surface of the screed (assuming it is gypsum) will make it difficult to maintain a bond between the cement and the screed. Would need epoxy type primer to prevent sulphate attack on the cement interface (don't think acrylic would work with that level of moisture). If you go down the route of wet bed it would be a good idea to use a water reducing plasticiser to minimise the amount of mixing water needed.

You are right about the efflouresence both short term and long term with the action of the underfloor heating. Don't think the gypsum would cause efflouresence but it might take a long time to dry sufficiently to allow the slabs to be sealed. You can use the gypsum as a grout as well which would allow the system to "breath" so moisture could egress through the grout joints.

Would you normally seal sandstone slabs - would this prevent efflouresence in the long term?

(When I say you Dave- i mean it figuratively - I know it is not one of your jobs)

 

[DHTiling]

It's the salts coming through the joints that concerns me with wet bed fixing... even if it's a gypsum base and you wetbed with portland then it this that is the the possible cause...

Anyway, lets see what flowable screed it is and then take it from there...

 

[Perry]

or Use PTB and backerboard under the thinner ones

 

[g cambrensis]

Many thanks for all the replies.

Liquid screed was gypsum based i think and as such we used an acrylic sealer over the top.
Ufh has been commissioned and screed had been dried out fully prior to sealing.

Have spoken to Mapei and Nocros technical and they say that their flex adh can be bedded up to 25mm which is about the limit with some of the slabs.

Not to sure about the grouting yet but that will have to be flexible as well i'd guess

 

[DHTiling]

I don't know of a mapei that will go 25mm...P4 goes to 20mm...?

Also you say you have primed....Have you removed the laitence first..?

 

[g cambrensis]

I don't know of a mapei that will go 25mm...P4 goes to 20mm...?

Also you say you have primed....Have you removed the laitence first..?

sorry, your right 20mm the mapei goes to. Any idea of any that can deal with a deeper bed. the Nocros brand goes to 25mm

Washed floors down, dried them out, and hoovered the lot before sealing.
Another tiler called in today. See what he comes back with price wise. Any ideas on what would be reasonable on 80 sqm?

Thanks

 

[DHTiling]

Have you removed the laitence first????? it won't wash off...it needs to be scraped or scabbled off..

 

[g cambrensis]

Have you removed the laitence first????? it won't wash off...it needs to be scraped or scabbled off..

How do you scabble the floor? Sorry Complete novice on your hands!

 

[Gazzer]

How do you scabble the floor? Sorry Complete novice on your hands!

Various ways from a good stiff floor scraper and loads of effort to hiring a scabbling machine. The main thing is that the top (soft ) layer shoould be removed as this is the laitance which will cause the problems later on.
When scabbled or scraped use a hoover to remove the left over laitance and prime floor as nescessary

 

[Ajax123]

YOU DO NOT SCABBLE A FLOOR SCREED!!!!!!

Scabbling is used for removing laitance from a concrete floor in order to expose the coars aggregate within the surface matrix. This provides a suitable mechanical key when bonding levelling screeds or structural toppings. It is far too aggressive to use this method on a gypsum scred or any other screed for that matter. If you scabble it you run the risk of turning it into crazy paving or even to gravel. additionally if you scabble it you are likely to remove between 5 and 15mm of the screed. As these types of screeds require a minimum 25mm cover to underfloor heating pipes this could also lead to movement cracking during the operation of the heating as there is not enough screed left. You also run the risk of damaging the underfloor heating pipes.

DO NOT SCABBLE IT!!!!

Firstly you need to determine if there is a skin on the surface of the screed which is not a physical part of the screed. This will be apparent if the screed is the old fashioned anhydrite type. It is not present in either the new generation low laitance anhydite screeds or in alpha hemi hydrates. If you let me know which screed supplier was used I will be able to tell you which it is although if there is a skin it will be immediately apparent. To remove this the best method is to use a rotary floor scarrifier with a 60 grit carberundum sanding disk. It depends how long the skin has been left in place as to how difficult this will be as it hardens with age. If this proves difficult a copper disk can be used although you need to take a bit of care as the copper disk is quite aggressive. If it is old fashioned anhydrite and the skin is not removed the tiles WILL fail.

If it is the new generation low laitance anhydrite this is designed so as no skin forms on the surface. This means that you only need to sand the surface in order to rough it up and form a key for the primer and adhesive, rather like sandiong woodwork before painting. This will also have the benfit of removing any construction contamination such as plaster and mortar debris. Again a rotary floor scarrifier with a 60 grit paper or a carborundum block sander is suitable for this application. Make certain that you vacuum the dust up.

Scraping can be adequate in some curcumstances but the manufacturers of the screed usually recomend sanding as the best method.

DO NOT SCABBLE THIS FLOOR!!!

I do not rate most acrylic primers on these screeds although as with all things some are better than others. The key is to ensure that the primer forms a chemical barrier between the cement based adhesive and the screed. If this barrier is insufficient the cement will react with the sulphates in the screed which migrate through the primer during the normal aquadynamic operation of the floor. It is even more important with heated screeds. I have unfortunately seen many instances where tiles have failed as a result of inadequate priming. I would suggest several coats personally if you must use this type of primer. A much more robust solution would be to use a water dispersible epoxy primer which will act as a barrier to the aforementioned sulphates and is not affected by moisture and does not soften when heated.

Even better still would be to use a gypsum adhesive so you have like for like materials. I am awaiting a call from someone to let me know if it can be used at 25mm deep. I have seen no failures to date where this type of adhesive has been used.

Alternatively use a decoupling mat which wil also serve to divorce the tile adhesive form the screed. I have also seen very few failures where decoupling mats have been used.

Just in case you did not hear me YOU DO NOT SCABBLE FLOOR SCREEDS!!!!