I knew anhydrite screeds had their issues but having followed this and other threads then there are a few questions i must ask you Ajax, as i've never known anyone so knowledgeable about this stuff!
Right, As a tiler i have turned up at a job to measure, and i find its an anhydrite screed. I guess i need to ask:
What make of screed?
Yes
How long has it been down?
this wil give a guide only to drying as screeds do nt dry unless in a drying environment. It does nt matter how old t is if it has not been in such an environment.
What thickness is it?
This is nt really really relevent other than to assist with the above. You are more interested in asking is it dry and how do you know its dry, ie has it been tested
Does it have underfloor heating?
very important as no screed should be covered untill after the ufh has been comissioned and run.
Now comes the tricky parts.
Would you advise i go to the manufacturer and gather ideal drying times and moisture content before laying?
Once ou know it is calcium sulphate whether synthetic anhydrite, thermal anhydrite or alpha hemihydrate you can estimate drying based on ideal conditions cos they all declare the same drying rates ie 1mm per day up to 40mm and 0.5mm per day for anything over this depth so 50mm takes 60 days in ideal conditions.
If a floor had no underfloor heating, so can't be forced dried, i take it the customer has to wait until the screed is deemed dry enough (from moisture test), this could take over a year, from what i read.
All calcium sulphate screeds can be force dried. If no ufh is present then space heaters, dehumidifiers or specialist techniques can be employed. I have never heard of a screed taking over twelve months to dry although i too have read stories. I usually take these with a pich of salt as i think a screed would have to be kept in pretty poor conditions to not dry sooner than this and i cannot see any builder waiting that long anyway. The longest i was involved in that i remember was the shirley school in the midlands where i was called to site to advise on drying a screed which had about three inches of standing water on it. It took about 7months to get it dry. Usually when we see "long" drying time complaints it is because moisture testing has been carried out incorrectly or the site is just not suitable for drying. bear in mind that whilst not a cure all the use of ditra matting or gypsum based adhesives can be extremely helpfull in sorting out moisture issues as they can both be used before the screed is fully dry.
And if the screed has underfloor heating i take it it is recommended that the heating is fully commissioned (as above), before laying any tiles?
Yes, without exception with all screeds.
Many customers have the floor screeded whether it be sand and cement or poured, and want the tiling done asap, way before the plumber gets to commission, what are the screed manufacturers opinions on this?
this is no excuse. The plumber should know, the builder should know, the tiler should know and the screeder should know that the underfloor heating is essential and not doing so is prone to disaster. If the boiler cannot be connected hire a temporary boiler for a few quid a week.
Sorry for asking so many questions, but these screeds are appearing everywhere and i have a good reputation that i want to uphold.
