tiing on green screed

[huckin]

hi lads is there a adesive that can be used on a greed screed thats suitable for stone

 

[wakes]

When you say geen screed how long as it been down??

 

[White Room]

What stone will you be fixing....

 

[Peter]

Would be better just Ditra'ing over it.

 

[wakes]

i disagree there wingn. if its a green screed and not fully dry then covering with ditra
the screed may never dry as screed needs air to breath and dry so will break up over time.

 

[White Room]

Ditra will allow moisture out due to fluting on the under side of the matt.....

 

[wakes]

imo you never tile on a screed until it is fully dry. ditra is just hiding the problem that may occur.
The fluting as you say may allow a small amount of air flow but with the condensation that will
accur under the matting the screed will never dry properly.

try this on a new screed after 2 weeks of being laid, put a cup, plant pot, bucket, anything you like
and i garentee if the screed is not fully dry there will be a moisture mark on the screed.
So by covering over with ditra you are just masking a problem waiting to happen.

 

[Peter]

Schluter may go back to the drawing board then, they're obviously wrong.

 

[wakes]

are you a rep for DITRA??? screed will not dry properly whilst coverd.
if you let a screed dry properly uncoverd and aired why would you want to use ditra?
or maybe to rush a job when you aint got much work.

I would say NEVER tile on a screed until its fully dry. then you dont need to cover things up
and you know youve done a proper job.

 

[Ajax123]

i disagree there wingn. if its a green screed and not fully dry then covering with ditra
the screed may never dry as screed needs air to breath and dry so will break up over time.

Thats not correct. Sand cement screed dries via two mechanisms and requires moisture to undergo hydration of the cement. Te first mechanism of course is evaporation which should occur at a controlled relatively slow rate. The second is the hydration reaction itself which chemically binds the water into new molecules. Covering with ditra is a great way to trap the residual moisture so that the screed can gain strength and properly harden. If you want it to dry the first thing you should does cure it under a polythene sheet for at least seven days. Sand cement screed do not generally break down as a resultof exposure to moisture unless there is chemical contamination e.g. Chlorides or sulphates. Breaking up ver time would generally suggest the opposite of moisture retention I.e. over rapid drying of the screed leading to poor and incomplete hydration of the cement. Alternatively it might suggest a lack of compaction which is probably the most common cause of failure in sand cement screed.

Im not a big fan of green screed tiling simply because the screed can have a tendenc to shrink and crack causing a potential failure of adhesion or cracking tiles.

Anhydrite screed on the other had does not need curing as it is self curing. It works on a different principal. However exposure to moisture is still n likely to cause a break down although there will be a loss of mechanical robustness if it is saturated. If it is exposed to ish levels of moisture or long term exposure over several years then some gypsum swelling can occur but it still remains hard. It does not like to be damp primarily because when itis damp it is almost impossible to get a cement based compound to stick to it.