I've been looking through old messages on this group and the message I seem
to be getting is that screed requires moisture to dry so presumabely my
builder using a de humidifier to speed the normal 3 week drying may not have
been successful!? The tiles were laid on adhesive today, 5 days after the
screed was laid. As this cannot really be undone now will it make any
difference if we just go ahead and seal and grout the slate tomorrow or
would it be better to wait(if wait how long?)?
The builder should not have been using a dehumidifier to rush the
curing process - this is a sign of rushing to completion to get paid,
and he shouldn't have done it.
Since the tiles are now laid it is too late to avoid any cracking and
other damage which may occur.
I doubt whether it will make any difference now whether you seal and
grout or not because if the screed does crack and crack the tiles then
it is likely to do so randomly across some and not along a join.
Given that situation, waiting to grout is not going to make a whole
lot of difference in terms of reducing the risk of cracking.
You have to seal when you grout anyway to prevent the grout from
picking up dirt and marks so I see no reason not to go ahead at this
What I would do, if you still owe the builder a final payment is to
withhold an amount equal to the cost of the tiles plus relaying them,
for say, a month.
If the screed does crack, it is likely to do so in that time.
Perhaps it would also teach the guy not to cut corners next time.
To email, substitute .nospam with .gl
I think this is a bit alarmist Andy. The drying out process of concrete floors
is very slow. With other floor finishes it would have been necessary to wait
until the moisture content in the floor had reduced to sufficiently low levels
in order to prevent damage such as swelling timber or lifting vinyl etc. and
generally the rule of thumb for this is to allow 1 month for each 1" thickness
of the concrete slab + screed. So a 4" slab with 2" screed would take 6 months
to properly dry out.
Hydration problems can occur when screeds are laid on a dry slab, or in hot
weather, neither of which apply in this case as I understand it. If the water
in the mix evaporates too quickly or is sucked out of the wet mix before the
cement is cured the screed will become crumbly, but this has to happen in the
first 24 hours to have any serious effect. Laying impervious slate tiles over a
damp screed will delay drying out and so this will in fact aid curing. And
using the dehumidifier for a short time will have made very little difference -
it may have made the surface of the screed appear dry, but that's about it -
there will still be plenty of moisture underneath.
There are two things that do concern me over this. Will the tile adhesive be
effective when laid on a damp surface, and will the sealer be affected by the
dampness in any way? I can't answer these - you need advice from the
That's a bit unfair! Using a dehumidifier to aid drying out, especially in
winter, is a normal and harmless procedure if not all that effective. I think
this simply amounts to a genuine attempt to finish the job on time, and he
probably didn't allow for the hire cost in his estimate.
On Wed, 25 Feb 2004 03:48:26 -0000, "Peter Taylor"
Well OK. I would defer to your professional knowledge on this,
Peter. However, I have seen a slate floor that was laid in the
conditions described and several slates had cracked. The homeowner
had the slates lifted and there were various cracks in the screed
underneath. However, when the screed was removed as well, these
cracks were only in the screed - they weren't in the underlying
I suppose it's possible that the screed was mixed incorrectly, and
it was done a couple of weeks after the oversite, however I do know
that artificial drying had been used and the slate laid only a few
days after the screed.
Maybe this is a coincidence or some other mechanism was in play,
however the result was pretty expensive to fix.
I don't think it's unfair at all. The manufacturers clearly state
the usage conditions for the products. Bending the rules to
accelerate a job may work, but it's taking a risk. Sometimes that
will work, sometimes the result will be a disaster. Slate is an
expensive flooring material, relatively speaking, so cutting corners
can be a false economy.
I think we agree it would have been preferable to allow the floor to dry out for
longer before laying the slate. But for how long? And how dry? As I said, new
floors take ages to dry out properly, one of the main reasons being the
damp-proof membrane underneath. Timber and vinyl flooring, even carpet, would
be susceptible to damage if laid on a damp floor but I can't see how slate would
be affected by it, any more than ceramic tiles. If there is a particular issue
about these slate tiles then it should be stated in the manufacturer's
If you go back to before DPM's were widely used, a concrete floor would be laid
over the bare earth, maybe with a 12-25mm thick screed, and finished with quarry
tiles to hold back the damp. Often the tiles were actually bedded in the wet
screed, or were stuck and grouted in one operation using a cement slurry.
Either way the concrete would never have dried out properly.
I think your case of cracking was much more likely to be due to a poor bond
between the screed and the concrete, rather than the drying process. This would
happen if the concrete was not cleaned or damped down, or if the screed was too
thin, too lean a mix or too dry. However, causes of flooring problems are
notoriously difficult to pinpoint and even the experts often disagree!
Accepted. But clients often put great pressure on builders to complete,
especially when they can see the end in sight. If the builder followed the
manufacturer's recommendations, or if there are no recommendations, or if the
client forced him to ignore the recommendations against his own advice then he
is in the clear and, unless there is a retention clause in the contract with the
builder, there is no moral or legal reason to withhold payment from him. If he
clearly went against the recommendations of his own accord then I agree he is
On Wed, 25 Feb 2004 10:29:34 -0000, "Peter Taylor"
The manufacturers say 3 weeks.
It isn't forgiving of stress across the surface and will crack. I'm
not sure that complaining to God here is going to help though :-)
One could use a flexible adhesive as would be used on a wooden base,
but probably that won't have been done here since the assumption is of
a solid floor.
Sure, and were stable for donkey's years.
Well exactly. I wasn't trying to suggest a causal link, more of a
correlation. The trouble is that if one were to try to go back to
the suppliers/contractors on this basis, it would be an uphill
To email, substitute .nospam with .gl
YES THATS WHAT IT SAYS ON THE TILE ADHESIVE INSTRUCTIONS BUT NOT WHAT
YES ITS JUST NORMAL ADHESIVE
WE WOULDN'T HAVE TILED + LAID THE ADHESIVE AGAINST THE ADHESIVE INSTRUCTIONS
IF STONELL HAD NOT TOLD US IT WOULD BE OK
YES WE ASKED STONELL IF THE TILES COULD BE LAID ON SCREED ONLY AFTER 5 DAYS
BECAUSE WE ARE HAVING THE KITCHEN DELIVERED SOON AND WE COULDNT CHANGE THE
DATE. STONELL SAID LAYING SLATE TILES ON 5 DAY OLD SCREED WOULD BE OK IF YOU
DON'T SEAL IT
OK. Well basically you are in a situation of bending the
manufacturer's and other's recommendation to meet a kitchen fitting
date. I had a similar situation and postponed the kitchen fit.
It meant that the project slipped by a month but there were no
problems with the floor.
As I said in another post, you will almost certainly get away with it,
so there is no point in worrying at this stage.
I found Stonell good to deal with, but whether it would stretch as far
as replacing the slates if they crack, I wouldn't like to wager.
By pushing the completion dates you are gambling (probably at low
Just one other small point, and it doesn't matter here at all.
Generally in Usenet posting, using capitals means that you are
shouting. I know that you've used it to differentiate so no
problem; however, in a different context it could create the wrong
impression. Just a note for reference - no big deal here.
To email, substitute .nospam with .gl
Thanks Andy, you have really been very helpful in making everything a lot
clearer to me. The builder tested the screed today for evaporation by using
a rubber mat and there was none on it, so it is assumed the screed is dry
and the grouting is going ahead tomorrow so it's fingers crossed really!
Sorry about the caps I'll remember that in the future.
Thanks for everyones replies, it seems like the main problem is that the use
of the de humidifier to try and accelerate the normal 3 week drying time for
screed may not have been at all successful because screed actually needs
moisture to set? So basically the setting of screed cannot be accelerated?
So my tiles are probably going to crack??? :( Stonell definitely said that
the slate could be laid on screed after just a few days as long as you don't
seal them. That sounds like they think screed dries by letting off moisture
and if you don't seal the moisture by sealing it there won't be a problem?
But surely even if that was the case the screed may expand and contract if
letting of moisture and therefore cracking the adhesive or the tiles? And
according to replies here the screed actually requires moisture to set
anyway so Stonell are wrong on thinking not sealing would have any benefit?
So can we hold Stonell responsible if the tiles crack?
Thanks again for everyones help
On Wed, 25 Feb 2004 10:35:35 -0000, "Martin"
You can try, However, I wouldn't worry about it. The chances are
that nothing deleterious will happen. If nothing does happen then
you will have worried unnecessarily. If it does, then you will have
to deal with it at that point.........
I think you may be confusing "drying" and "curing" Martin. Cement needs
moisture to cure (gain its full strength). If this is not present it will be
weak and crumbly, will not have bonded properly to the concrete slab, and can
shrink and crack as Andy found. But after 5 days with adequate moisture the
cement will be quite close to its full strength, certainly enough for a screed,
and all initial shrinkage will have ceased.
However, once the cement is cured properly you need the screed to dry out as
quickly as possible so that the dampness doesn't affect any floor finish. There
is no "normal" drying period - it depends on the screed thickness, its moisture
content and the environmental drying conditions. The thickness and moisture
content of the concrete slab under the screed has to be taken into account too.
If there is a new slab underneath then I think, even if the dehumidifier was on
constantly, it is very unlikely to have dried out the screed sufficiently to
cause a setting or shrinkage problem, certainly at this time of year.
Not the setting (curing), unless you add special chemicals. The drying can be
accelerated a lot, but it is unwise to do this until the cement has cured
No, I don't think they will "probably" crack. There is a slight risk, and
Stonell are saying that dampness in the screed will not affect the slate
(although according to your first post it may affect the adhesive). Andy is
worried the builder's attempt to accelerate the drying out too early might have
affected the curing and possibly cause shrinkage cracking. The two points are
not related. I'm not sure why Stonell are saying don't seal the tiles - maybe
locking the moisture in could affect the sealer.
Everything depends upon the moisture content of the screed. Nobody here can
tell you what that is, or whether the dehumidifier made any real difference.
You need to rely on your builder's advice and experience as he is the only one
on the spot, but you also need to make sure he knows you will hold him
responsible for any defects caused as a result of trying to do the work too
quickly. This is why Andy suggested holding some money back.
Perhaps it would be wise to delay the sealing of the tiles for now, as Stonell
recommended. I suggest you wait at least another 2 weeks more to see if any
cracking, crumbling or other happens. My guess is it will be fine and you can
go ahead with grouting.
Thanks Peter for your amazingly thorough response! I can't believe how
helpful people are on here! The cement under the screed was laid about a
month ago above a layer of insulation above another layer of concrete. The
screed was added 5 days ago with the dehumidifier on for about 3 days but
not in the night time. I am just hoping all the estimates that are given for
the drying times are conservative, and the dehumidifier may have helped a
bit. The screed was tested today to see if it was dry by seeing if water
evaporated onto a plastic mat over the day which it didn't so the screed is
assumed to be dry and the slate will be grouted tomorrow and stainstopped as
normal so I am just going to have to hope for the best on this.
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