Plastic sheet moisture test for thin 'screed'

[Julestools]

Hi all, 

I have levelled and flattened my old ground floor screed, ready to take a DPM and a floating floor of PIR and t&g chipboard , mainly using sand and cement with 'RonaScreed fast dry prompt' added, for (hopefully) a more rapid drying. Thickness is from 35-5mm. We're now at day 14 and having taped squares of polythene to it in a couple of places for 48 hours we find no condensation, moisture or darkening of the screed below. 

The plastic sheet test, from what I can gather, is only a rough indicator of dryness and more accurate tests should be used before laying final floor coverings like wood laminate or tiles, but would I be right in thinking that when laying the DPM, PIR etc over the screed, moisture content (and more accurate testing) would be less critical, and therefore the plastic sheet method will do in this situation? 

I'd be very grateful for any guidance on this. Many thanks. 

 

 

 

[Nickfromwales]

On 18/01/2026 at 10:07, Julestools said:

Hi all, 

I have levelled and flattened my old ground floor screed, ready to take a DPM and a floating floor of PIR and t&g chipboard , mainly using sand and cement with 'RonaScreed fast dry prompt' added, for (hopefully) a more rapid drying. Thickness is from 35-5mm. We're now at day 14 and having taped squares of polythene to it in a couple of places for 48 hours we find no condensation, moisture or darkening of the screed below. 

The plastic sheet test, from what I can gather, is only a rough indicator of dryness and more accurate tests should be used before laying final floor coverings like wood laminate or tiles, but would I be right in thinking that when laying the DPM, PIR etc over the screed, moisture content (and more accurate testing) would be less critical, and therefore the plastic sheet method will do in this situation? 

I'd be very grateful for any guidance on this. Many thanks. 

 

 

 

Sorry, we seem to have missed this.
 

It’s the original slab you’ve levelled? 

[saveasteading]

There won't be any free water in this by now unless it's getting wet from rain or other source.

Even if there was some dampness it won't get through the dpm and insulation. 

[Julestools]

Hi Nick, yes I've levelled the original screeded slab. Since I posted the above, I decided the RonaScreed FastDry stuff had done it's job and went ahead with the floating floor, so there's no going back now! 

 

The original slab seemed bone dry, though my gut feeling was to not trust the old thin DPM below. I used a mix as dry as I could get away with for levelling the worst low areas and SBR to bond it. Technically, adding the RonaScreed, it should have been dry enough in 4 days to take 'moisture sensitive floor coverings' so all should be ok. I was just curious about what others thought of the old 'plastic sheet taped to the floor' test. 

Thanks for the reply👍

 

[saveasteading]

6 minutes ago, Julestools said:

plastic sheet taped to the floor' test.

Works well except in very damp environment when there is no cause of evaporation.

[Julestools]

6 minutes ago, saveasteading said:

There won't be any free water in this by now unless it's getting wet from rain or other source.

Even if there was some dampness it won't get through the dpm and insulation. 

Hi, and thanks! I had read something about the risk of trapped moisture (between the old DPM below the slab and my new DPM above) potentially causing issues with mould, but decided I was over thinking it and as my leveling stuff passed the old polythene sheet taped to the floor test, I've gone ahead and done the floating floor.. almost finished it today, a productive was to spend a rainy Sunday😊

[Julestools]

1 minute ago, saveasteading said:

Works well except in very damp environment when there is no cause of evaporation.

Ah ok, that's good. We had plenty ventilation, fans going and woodburner chugging away👍

[Nickfromwales]

With DPM and PIR over it, any issues will

stay subterranean for sure. 
 

Full steam ahead!