[?] Floor screed - drying out time and sealing

We've recently had a screeded floor (around 50mm thick) laid in a new conservatory.
We were told by the builder who laid it that it would take 3 weeks to dry out, and someone else in the trade said that we should leave it for at least six weeks.
I'd appreciate hearing the views of the experts in this NG about its likely drying out time (at this time of year in London) and what we should be doing to speed it along.
Also, when it is finally dry enough, what sealant(s) should we use to prepare its surface ready for Flotex floor covering?
Many thanks,
- Dave
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David C.Chapman - ( snipped-for-privacy@minda.co.uk)
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David Chapman wrote:

Conventionally, concrete sets in 24 hours. You can walk on it after 48 hours; you can build on it after 7 days and by 28 days it's reached well over 90% of its final strength and is considered "fully cured".
However, you don't want to cover over an internal screed until it has fully dried out, which is not the same as curing.
If the screed was a very dry mix (likely), then the original builder was probably right and the screed will likely be substantially dried out after three weeks and you can happily lay tiles on it. The other trade person is being more cautious and suggesting that if you give it six weeks you are less likely to get damp problems. This may be because you've mentioned that you're carpeting it, rather than tiling.
Only you can judge whether it appears to be dried out or not. Take a hair drier to a particular spot and heat it up for a few minutes. If the screed gets noticeably lighter in shade, then it's not fully dried out.
You can speed up the drying using a dehumidifier.
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My advice first off is ... water it, use a watering can and water it well for after first 48 Hrs .. and then at least 2 consecutive days ... semi dry has so little water when it is laid that a good soaking will help it cure.
The seal; surface with a watered on surface hardener/anti-dust treatments (Feb and Marley both make them.)
I used a professional sealer, but that would be difficult to get in less that 25Lit containers.
Then leave at least 3 weeks to fully dry.
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Are you planning to use underfloor heating? If so, running the heating at a low temperature will help speed up the drying process, but it is still VERY slow. Either use a dehumidifier or leave the windows open. Circulate the air with a fan. I believe that the main reason for wanting the screed to be as dry as possible is to avoid condensation forming at low temperatures in the glue that sticks down the floor finish. If there is condensation for a long time, then the glue gets mouldy, smells nasty and eventually comes unstuck.
Ideally, the dew point of the air spaces within the screed should be below the lowest temperature the screed will reach. In a conservatory, if you leave the heating off for a few days in the winter the floor temperature will quickly drop to something like 5 deg C.
Flooring installers will often place a hygrometer on the screed for a few days to check the humidity before laying impervious finishes. It can take 24 hours for the air space in the hygrometer to reach equilibrium with the screed. They generally have a target of about 75% RH at 20 deg C and it can take several months for this to be reached with a thick screed.
Have a look at BS8201 "Code of practice for flooring of timber, timber products and wood based panel products" and BS8203 "Code of practice for installation of resilient floor coverings" for details of recommended humidity limits and measurement methods.
When we built (well, the builder did most of the work) a conservatory with hot water underfloor heating I arranged for a 2m long horizontal open-ended length of plastic conduit to be embedded in the screed and for one end to be accessible from the underfloor space of the adjacent room. Inserting a temperature-humidity probe into this showed that the surface humidity measurements significantly under-reported the humidity in the bulk of the screed.
If you have underfloor heating, the humidity will end up redistributed with least moisture around the heating elements or pipes and the most moisture at the coldest location which is the glue line of the surface finish.
The conclusion - wait as long as you possibly can before finishing the floor.
John
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Very many thanks to those who took the time to post such useful information in response to my queries.
Your helpful comments and constructive suggestions are really appreciated.
Thanks again - Dave
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David C.Chapman - ( snipped-for-privacy@minda.co.uk)
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wrote:

Are you planning to use underfloor heating?
I hope not as 50mm screed would crack .. even if fibrous reinforced 75mm is minimum.
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No, we are not, but thanks for your concern. I did realise at the outset that if we wanted to have underfloor heating then the floor composition and thickness would have to be quite a bit different.
All the best - Dave
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On Sunday, September 19, 2010 10:07:09 AM UTC+1, David Chapman wrote:

-
David,
Screed floor dry out at a rate of 1mm per day in ideal conditions. Suggest you keep all windows and doors closed at all times as in the early stages s creed acts like a sponge and draws moisture back in from the atmosphere. Yo u can help speed up the drying out process by getting a de-humidifier and p ut your underfloor heating on low if you have it. I have had a screed floor laid and have allowed it 6 weeks to dry out prope rly. Be sure to dry it out as floor tiles will only start to lift up if you don' t.
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On 29/09/2014 18:29, snipped-for-privacy@googlemail.com wrote:

Was the screed more than 1.471 metres thick? Or the conditions not optimal? Otherwise it might be dry.
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Rod

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On 29/09/14 18:29, snipped-for-privacy@googlemail.com wrote:

I would not force dry it during the first week - give the cement a chance to set as it is a water based reaction.
After that a dehumidifier could be used - I did, and it's amazing how much water it pulls out.
Without forced drying, 6 weeks sounds reasonable this time of year. With forced drying, could be down to 3 weeks including the first week.
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As the original post was 4 years ago I expect it's dry by now! ;)
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TOJ.

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The Other John wrote

Bollocks, beat me to it.
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On 29/09/2014 20:12, Jabba wrote:

Beat you both to it!
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On 29/09/14 20:37, polygonum wrote:

Wibble...
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