We've recently had a screeded floor (around 50mm thick) laid in a new
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?
David C.Chapman - ( firstname.lastname@example.org)
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.
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.
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
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
The conclusion - wait as long as you possibly can before finishing the
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
David C.Chapman - ( email@example.com)
On Sunday, September 19, 2010 10:07:09 AM UTC+1, David Chapman wrote:
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
Be sure to dry it out as floor tiles will only start to lift up if you don'
On 29/09/14 18:29, firstname.lastname@example.org 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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