Sand cement Floor Screeds

[john raymond]

Guy's I'm sure this has been asked before. How long do you need to leave a new screed floor (Sand cement) before tiling can take place.
The screed will be going down within the next month. The builder has told the householder I will be alright to tile after about 4 days of it being laid.

Regards,

John (Hampshire)

 

[prceramics]

Re: Floor Screeds

Think its one day per mm depth

 

[Diamond Pool Finishers]

Re: Floor Screeds

minimum 3 weeks , we leave them

 

[Gazzer]

Re: Floor Screeds

Guy's I'm sure this has been asked before. How long do you need to leave a new screed floor (Sand cement) before tiling can take place.
The screed will be going down within the next month. The builder has told the householder I will be alright to tile after about 4 days of it being laid.

Regards,

John (Hampshire)

You could tile onto it if you took the right approach, ie Green screed adhesive or even Ditra matting approach but its not one that I would be happy with.
1mm per day is drying time.

 

[prceramics]

Re: Floor Screeds

The ditra approach I am not sure of I know you can but I feel if the tiles won't take to a new screed how will ditra as you are using the same Addy

 

[Gazzer]

Re: Floor Screeds

Schlüter-DITRA - Schlüter-Systems

 

[Ajax123]

Re: Floor Screeds

Depends on the screed type.tere ar additives which can be included in the mix which make it fast drying but if these have not been used it should be cured for 7days under polythene and then left for 1 day per mm depth. (BS8204:1:2001 is very misleadingin this respect in that it says leave for AT LEAST three weeks to dry thoroughly.... A 1:4 sand cement screed will take considerably longer than 3 weeks tomdry thoroughly)

If there is underfloor heating this must be comissioned and run prior to tiling and this must not be done for at least 28days following installation.

 

[Diamond Pool Finishers]

Re: Floor Screeds

alan just out of interest ,if you have an outdoor screed, that is exposed to the rain ? how will this dry out, on outdoor pools we tend to leave it three weeks then carry on mosaic tiling when its dryish obviously we use mainly water soluble adhesive and can only tile if the surface of the screed is not ringing wet , and in the main we tend to cover them in more nowdays

 

[Stef]

Re: Floor Screeds

John, I would listen to Ajax on this one as he is the "Man" when it comes to floor screeds.

Tilers Forum resident expert on all things screeding.

 

[Ajax123]

Re: Floor Screeds

alan just out of interest ,if you have an outdoor screed, that is exposed to the rain ? how will this dry out, on outdoor pools we tend to leave it three weeks then carry on mosaic tiling when its dryish obviously we use mainly water soluble adhesive and can only tile if the surface of the screed is not ringing wet , and in the main we tend to cover them in more nowdays

Standard screeds are not designed to be used out of doors and should only be placed in a weatherproof building envelope. That said both sand cement and calcium sulphate screeds are regularly placed in less than suitable environments. The main benefit of using cement screed out of doors is simply that the cement will hydrate whilst there is water present. If the screed is still green when it rains on it there will generally be surface damage which can range from a light sapping to a very dusty surface.

In tms of drying the screeds as you know should be dried to below 75% rh but this is unlikely to be possible with out door screeds.

So why do tile stick. Well in reality some times they don't but the bond between cement and cement I.e. adhesive to screed is generally very robust and therfore the tiles stick despite their being abused. Asside from delamination due to moisture the next most common issues are tile staining (particularly in stone) and efflorecence.

I guess the other sideways benefit of screed being out doors is that it is unlikely to dry too rapidly if it is damp cool weather. If you lay it in the sun it will crack.

 

[Ajax123]

Re: Floor Screeds

John, I would listen to Ajax on this one as he is the "Man" when it comes to floor screeds.

Tilers Forum resident expert on all things screeding.

Thank you Robson for your glowing vote of confidence .... Much appreciated

 

[john raymond]

Re: Floor Screeds

Guy's I'm sure this has been asked before. How long do you need to leave a new screed floor (Sand cement) before tiling can take place.
The screed will be going down within the next month. The builder has told the householder I will be alright to tile after about 4 days of it being laid.

Regards,

John (Hampshire)

Thanks for all your advice chaps, much appriciated.

John

 

[Stef]

Re: Floor Screeds

Thank you Robson for your glowing vote of confidence .... Much appreciated

You gave me sound advice on an anhydrite screed to pass on to a builder & every time i see someone posting about screeds then the advice you give is always the best you will get on this site.

 

[Diamond Pool Finishers]

Re: Floor Screeds

Asside from delamination due to moisture , yes but in a swimming pool environment you can have cement based grout/adhesive then screed the swimming pool water will migrate through all of these layers, and its not just water it's got chemicals in, and it is warm/cold its amazing it all stays together

 

[Ajax123]

Re: Floor Screeds

Asside from delamination due to moisture , yes but in a swimming pool environment you can have cement based grout/adhesive then screed the swimming pool water will migrate through all of these layers, and its not just water it's got chemicals in, and it is warm/cold its amazing it all stays together

Badly worded, should probably have said aside from the occasional delamination due to moisture. Moisture delamination is relatively rare on sand cement screeds with cement adhesives.

 

[widler]

Re: Floor Screeds

alan just out of interest ,if you have an outdoor screed, that is exposed to the rain ? how will this dry out, on outdoor pools we tend to leave it three weeks then carry on mosaic tiling when its dryish obviously we use mainly water soluble adhesive and can only tile if the surface of the screed is not ringing wet , and in the main we tend to cover them in more nowdays

i supose its the same with rendering,i rendered a supermarket front years ago and tiled it within days,big buggers as well,still drive past it,and still looks the same,well apart for grout being a bit darker

 

[Tile Marble Granite]

Re: Floor Screeds

The way i do it (US standards) I can lay tiles on it the very next day. It dries over night

 

[Ajax123]

Re: Floor Screeds

The way i do it (US standards) I can lay tiles on it the very next day. It dries over night

No Detroit .... It sets over night.... It does not dry overnight .... There is a big difference.

 

[Fliselege]

Re: Floor Screeds

I used 2 new products last week, will take photos of it Monday and post back, one was a hey'di self levelling "turbo", you can tile on it after 4 hrs, the other was a PCI ready mixed normal "?" screed, tile on it after 5 hrs, I walked on it after an hour and it was solid as a rock. Christ knows what additives they put in it, viagra maybe. It was supplied by the builder, now we've worked with it once, we'll never work with it again.

 

[Ajax123]

Re: Floor Screeds

There are loads of screeds availale tha will set very quickly. Most have hydraulic binders which tie the residual water up in crystals of one sort or another. They have a place in the market Most deffinitely it realistically they are relatively pricey and your timing needs to be good. I have seen a few issues with some of them due to cracking. Not yet convinced of their long term stability against long term shrinkage but they satisfy a demanding some respects. Really though most projects are not tiled within 5 weeks let alone 5 hours so the advantages are limited ...... That is my opinion anyway.......awaits barrage of contrariness

 

[Ajax123]

Re: Floor Screeds

By the way these screeds are not "dry" it is simply that the moisture is locked in.

 

[Diamond Pool Finishers]

Re: Floor Screeds

very interesting what are the BS STANDARDS ON EXPANSION JOINT BAY SIZE?,in screeding ( just to enlighten some peeps ) you would have thought that they would match tile bay size? i love talking screeds/render and potential faults

 

[Fliselege]

Re: Floor Screeds

You're probably right in most cases but we work mostly in peoples houses with only 1 bathroom so it's screed floor 1 day, spend 2 days tiling the walls then start on the floor quick as poss, and if the grout lines on floor have to match the walls we do the floor first. The product we use the most is hey'di B20, hey'di is a brand name and B20 is the mix, loads of manafacturers produce it but hey'di is the best, saying that not impressed with their self levelling gear. It's become the norm for me now, but when I first started here I thought it was a lazy cheat, I'd only ever done screeding in U.K with a labourer on a mixer pump, shovel in hand with a pile of sand on one side of him and a pallet of blue circle cement on the other!

 

[Ajax123]

Re: Floor Screeds

very interesting what are the BS STANDARDS ON EXPANSION JOINT BAY SIZE?,in screeding ( just to enlighten some peeps ) you would have thought that they would match tile bay size? i love talking screeds/render and potential faults

Bs8204:1 which covers sand cement screeds does not actually give any specific guidance on bay sizes which is always a bit of a bone of contention amongst screeders and tilers alike.

The generally accepted rules are though that Traditional unmodified sand cement screeds should be split into bays of no larger than 6m in length with an aspect ratio of no more than 2:1 so the max bay would be 6mx6,. This fits in with the acceptable limits for concrete as well. Some will say the max bay length should be no more than 40times the depth.

When it comes to heated screeds I usuall offer the advice given in the NHBC handbook chapter 8 which says sand cement bays should be o more than 15m2 or room size whichever is the smaller.

The standards do go on today that sand cement screeds tend to crack uncontolledly.......

Calcium sulphate screeds are covered in a different part of the standard BS 8207:7:2003 which says that these can be laid to bays much bigger 1000m2 unheated and 300m2 but it does not give advice on aspect ratios so we say unheated 8:1 and heated 6:1.

In both screed toes there are guidelines on additional jointing e.g. Door thresholds and independent heating zones as well as areas of high thermal gain......

The tiling standards then conflict by saying 40m2 in all cases which is about ok for cement but is not even close to correct with gypsum. Trouble is all the standards are written by differentcomittees comprising different "experts". Trouble is the comittee members often have vested interests at some level so it is really difficult to remain objective when looking at the requirements of the standards. The other issue I find is that some of the standard tolerances allow interpretation and you can drive a bus through some of the tolerances.

My view is that BS are for guidance only and should be over ridden by manufacturers requirements. Bit of a nightmare really........

 

[Diamond Pool Finishers]

Re: Floor Screeds

god that is a minefield of as you say conflicting committees ,with that old chestnut vested interests , they should be based on scientific facts,but as you say alan BS standards are only recommendations ,that i presume would not even have any weight in a court of law ? should we not all be adopting european standards anyway !!? a bit like imperial/metric systems that will be dragging about for years to come ?