Limestone tiling over a dry sand and cement screed with UFH

[emmettcraik]

Hi Guys, Im new to this forum. I am going to do a job on a new build in a couple of days and would like a little advice. I will be laying limestone tiles over a 'dry' screed of sand and cement with UFH (air to water heating system). The builder has said he had left the heating on for two weeks for me. I am wondering is it standard practice to use uncoupling matting in this situation or am I being to cautious? How long should the screed have been down for? Really don't want any problems.If not using matting would any particular primer be best. Thanks guys!

 

[White Room]

Yes, a decoupling membrane as ditra would be required...

 

[Chalker]

Welcome to the tiling forum :welcome:
Make sure you moisture test before laying tiles. Air source runs at low temp, don't rely on this to dry the screed.

 

[Glynn]

The sand and cement screed should not be forced dry as this may cause cracking. Check the screed to make sure it is sound, remove the thin film of laitance, this is a residue sometimes left after the screed has dried. If you can get hold of Tilemaster products; Primeplus at three parts water to one part Primeplus. Their Ultimate white is for green screeds. Use this with their Anti-Fracture matting and Grout 3000 mixed with Flexmaster additive. All the information is on their web site.

 

[Ajax123]

Uncoupling mat is required. Provided the screed is hard and the heating is commissioned you can lay the matting straight away. If the screed is still damp tape the joints to stop moisture coming up through the joins and staining the tiles.

 

[emmettcraik]

Thanks guys for the advice, much appreciated!

 

[Brian the Tile]

Sorry but i cant agree that a uncoupling mat has to be used ,not at all ,only if it is gòing to make you feel better or the customer doesn't mind paying extra for it ,if the screed is done with the right mix by a professional deppending on the area to be tiled and has expansion joints are in the screed ,a minimum of three weeks to cure and dry out primed and a good flexible adhesive has always worked for me with no problems to report

 

[Andy Allen]

imo its worth using a uncoupling mat with a stone tile and ufh..

 

[Ajax123]

Sorry but i cant agree that a uncoupling mat has to be used ,not at all ,only if it is gòing to make you feel better or the customer doesn't mind paying extra for it ,if the screed is done with the right mix by a professional deppending on the area to be tiled and has expansion joints are in the screed ,a minimum of three weeks to cure and dry out primed and a good flexible adhesive has always worked for me with no problems to report

Bs 5385 part 3 which is the standard for tiling and also the British stone federation specifically say that natural stone tiles should always be uncoupled from heated screeds. Whether you agree or disagree is not really the debate. If it fails and you don't uncouple you would find yourself liable as the courts and all the experts will quote the British standards. If you have got away without then that is fantastic but at the same time is probably very lucky.

 

[Brian the Tile]

ALAN, never had a problem ,not Diy with little tiling experience, not lucky at all as you call it ,re read my post please it says with no problems to report, been fixing tiles forty odd years you do not need mats

 

[Time's Ran Out]

The original post was asking for advice as to the best/correct method of fixing Limestone on a heated screed.
The best / correct method has to be to British Standard as quoted by Alan.
Although experience is a great asset even 40 years ago sand/cement screeds were uncoupled by a membrane be it dpm or waterproof paper over wood.

 

[Brian the Tile]

So if ive been doing it wrong all theese years without a problem, im willing to learn,tell me why it has to be used,dont ģo all technical on me i want it in laymans terms,hope its good while you must take in the forty odd years or has it all been wasted and i dont know what im doing,if i dont reply its cos my tablet keeps dying

 

[Brian the Tile]

John i do know about sand and cement on wood floors lol i done plenty of them believe me

 

[Brian the Tile]

As arny said i will be back lol

 

[WetSaw]

It may not have been wrong 40 years ago but times change, along with materials and methods.

 

[AliGage]

Oh my days!!!

 

[Ajax123]

ALAN, never had a problem ,not Diy with little tiling experience, not lucky at all as you call it ,re read my post please it says with no problems to report, been fixing tiles forty odd years you do not need mats

Please don't mis understand me. No offence intended. I am not disputing your experience, longevity or the fact that you have not had an issue. What I am simply saying is that the British standard, which is the bench mark for the minimum level of best practice and the document that would generally be referred to in the event of a dispute or failure, states quite categorically that natural stone should be uncoupled.

Im not wishing to pick a fight or criticise or anything and I really don't need to re read your post. I read it properly first time round. If you get away without it then that is fine and a matter for you. If someone were to ask my advice not as a tiling expert but as a materials technician and a substrate "expert" the i would always advise uncoupling natural stone from UFH and so would the vast majority of tiling and flooring "experts" including as I said the British stone federation. I know of many installations where stone tiles have failed on heated screeds. There are a whole variety of reasons for these failures of course but in the vast majority of cases that I know of where the tiling contractor has not followed the standards he has ended up with the short straw of putting it all right.

 

[Ajax123]

So if ive been doing it wrong all theese years without a problem, im willing to learn,tell me why it has to be used,dont ģo all technical on me i want it in laymans terms,hope its good while you must take in the forty odd years or has it all been wasted and i dont know what im doing,if i dont reply its cos my tablet keeps dying

I am a techy so I can't help but go all technical. However in as laymanny terms as I can underfloor heating causes thermal deformation of the screed I.e. Expansion and contraction under thermal cycling. Natural stone is generally relative weak in tension. The act of sticking it to a screed which is effectively moving places internal stresses on the stone. As the stone expands and contracts to a different level than the screed there are differential deformation stresses that transfer through the system and it can fracture at the weak points e.g. The veins in travertine and marble. These differentials can rarely be accommodated by the flexible adhesives as these are not flexible enough... They are only flexible to a point.

Also so especially where sand and cement screed is used this undergoes shrinkage due to cement hydration and carbonation. Modern cements are significantly different to cements from 40 years ago. One of the main differentpces is the level of compressive strength they acheive. Problem is when you increase compressive strength you I crease "hardness" and when this incRease the screed can become brittle.

also sand cement screeds from 40 years ago would have been laid very differently from modern screeds in the respect that most modern screeds are poorly compacted leading to low flexural strength which in turn can lead to both plastic and long term shrinkage cracking.

There re are a whole raft of differences between what happened then and what happens now.

I dont think anyone is one is saying you have been doing it wrong. It is a simple fact though that you have not followed the advice of the British standard and as such that puts you at risk in the event of a failure. Your experience means that you are happy to accept that risk into your working practice. Nowt wrong with that and as long as nothing goes wrong you are fine.

 

[Ajax123]

I must be recovering.... It's a long time since big long posts came out....

 

[Brian the Tile]

Why is it i spend ages typing and it tells me im not logged in when i am ,anyway spokke to adhesive tech man , no need for uncoupling mats end of from me but im only a lucky guy i dont know what im doing

 

[Andy Allen]

How long have you been tiling brain? :sofahide:

 

[Brian the Tile]

Only just finished my two week course alun,but i got my city and guilds in that time so im set up for lifebefore that i was a bus driver for sixty eight years and also worked in a Chinese laundry part time to make my wage up

 

[stusgsx]

Brian do you do ironing too?

 

[Just Rizzle]

ive been trying to teach my old dog a new trick he still cant get it

 

[Brian the Tile]

Brian do you do ironing too?

Only on a Wednesday my half day