Screed not drying at all?

[houseonahill]

We had 70mm of floor screed (sand cement with accelerator) laid on 9 March. It was tested on 5 April and found - via a carbide test - to be too damp to lay laminate flooring. We had a number of locations tested and they were all roughly the same. One, for example was 4.6%. (Laminate flooring company stipulates 2.5% or lower).

We just had the floor tested again (four weeks to the day since the last test and nearly 8 weeks since screed laid) and the results have barely changed! The same location mentioned above read at 4.5%.

I am baffled. We've had the heating on morning and night, ventilated regularly and there are no signs of leaks. We are living in the house (mostly upstairs!) so I know it is being kept warm and there is movement of air.

The screed is laid on top of a brand new flooring system (the concrete slab itself had a month to dry out before anything put on top).

Floor is as follows:

TOP
70mm Screed
CH pipes here and there
Polythene slip layer
100mm insulation boards
DPM
100mm concrete
DPM
hardcore
Clay / ground
BOTTOM

And yes the DPM layers are overlapped and come up the sides of the floor. All was checked by building control during and after.

I've been advised to try a dehumidifier but I just cannot believe that in four weeks there has been basically no progress. Has anyone else experienced this? Got any advice?

 

[Wayners]

Fan is better than heat.

Get the summer fans out and open windows. Open doors if wind is blowing and get the air through

 

[Deleted member 174758]

I suspect that there isn't sufficient ventilation/airflow to remove the moisture, so to add to what has already been written, if you absolutely need to you can seal that moisture in by applying 2 or 3 coats of a liquid epoxy DPM, but it isn't cheap stuff (expect to spend £200 plus for a large room)

 

[houseonahill]

Thanks @JobAndKnock but I'd really rather not seal the moisture in. I want it to evaporate, I'm just perturbed that it doesn't seem to be happening after such a long time! We have introduced air flow but not quite to the level of large fans running constantly (not cheap at the moment!) but I suppose we could try that.

Is no-one surprised by this though? It seems crazy to me that there would be no discernible drying after 4 weeks (8 weeks total).

 

[Deleted member 174758]

I've been forced into the "seal it in" approach a couple of times on commercial jobs where we were up against a fixed deadline. It isn't a nice option, but it does work. The last one was where the concrete had been exposed to rain due to a missing section of a glazed roof over a stairwell for the whole winter (it is a public building, so very big stairs). After more than 4 months in a weathertight building it was still showing 4% plus on our Tramex in multiple places so £400 for materials and two man days labour worked out a lot cheaper than ripping out a £20k floor 6 months down the line. 5 winters on and it's still good, so I think we are past the warranty stage!

 

[houseonahill]

@JobAndKnock I hear you - though in our case it's a 1950s residential property, not particularly big (but detached) and no missing roofs / ceilings / cracks in walls / leaks etc. We are renovating it. I just can't imagine why the screed is not drying.

 

[Notch7]

We had 70mm of floor screed (sand cement with accelerator) laid on 9 March. It was tested on 5 April and found - via a carbide test - to be too damp to lay laminate flooring

Screed Accelerator can mean more than one thing may reduce time before it’s hard enough to stand on, not be dry.

it depends on the product and method of mixing.

if a product was used that reduces drying time, the screed must be mixed with a forced action mixer or screed pump, if was mixed with a drum mixer, that won’t mix the additive into the cement sufficiently.


Typically 70mm screed would take:

1mm per day for first 40mm
0.5mm per day for rest ie 30 x 2

so your screed would take 100 days to dry

 

[JohnD]

heating will not help because warm air rises. I bet if you take the temp of the floor it will still be cold.

ventilation is needed.

you can put a battery of fans on the floor, all pointing downward, or open all the doors and have a real air current blowing through.

 

[houseonahill]

@Notch7 The company that laid the screed told us to wait 3 weeks and it would be ready for floor to be laid. I thought that was optimistic so I had it tested after 4 weeks. Still clearly damp. Waited another 4 weeks. No change. You say 100 days but if it's not actually drying now what makes you think it will be dry another 50 days from today?

Not quite sure some people replying here are getting that the problem is not screed taking a while to dry but that it is barely drying, if at all!

 

[Wayners]

Just borrow or buy some cheap fans. Any will do nothing special.
Cheap enough to buy and seen some in charity shops and DIY.

Get the air moving.
I tell my customers to do this and they are always shocked how well it works. Yet we have all seen rain dry up quickly.

 

[dazlight]

Get a moisture suppressant on the floor. ardex MVS , fball F78 , uzin PE404 , Bostik rapid Dpm
all of them let moisture through at a slow rate. We use them under laminate and LVT all the time.