Liquid DPM bonding on floor slab

28 Aug 2020
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United Kingdom
1933 semi detached 2 bedroom house.

I am planning to pour self levelling screed over the old slab, in what will be the kitchen, which will only be 15mm at deepest, and 4mm at most shallow. I've had a devil of a job getting a liquid DPM to stick before pouring the screed. I have removed about 3-4 mm off the top surface, swept it, hoovered and mopped it. I've used Drybase liquid DPM on various test patches. I've tried to apply it on
  • clean but dry patch-barely adheres at all
  • clean and damp patch-barely adheres at all
  • with tacky SBR patch-only moderately better than above
  • with dried acrylic primer-best adherence but still easy to peel away
Hpw hard should it be to peel off? I'm no expert but it feels a bit too easy. I really have prepared the surface well, nil laitance, nil dust. I don't trust the the 1930s DPM under the top slab, of which I have repaired a few cracks.

I have used a moisture content meter, pin type, and have a consistent reading of 1.2 - 1.4 % across the floor. I have no idea what is or isn't an acceptable moisture content. Is this a good figure? Am actually needing to paint a liquid DPM at all with this kind of reading? I'd rather do so in truth.

Thanks, really appreciate any advice
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the pin type moisture meters are designed to measure moisture in wood.
if you want an accurate reading the meter will cost upwards of £200.
the easiest way to check for moisture would be to tape a square of polythene down and see if it condensates inside.
tanking may be a better option than the dpm.
Hi, thanks for the reply. The meter I have has a concrete mode.

I've been cracking on with this floor for 4 months now, a real pain. I've done a polythene test, nil moisture. I've also tried KA tanking slurry, which did not bond properly approx 40% of the floor, despite doing it absolutely by the book, also using KA SBR bonding as well as meticulous prep. It all had to come up again!
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In the hands of an experienced investigator pin m/m's are an excellent tool - even used by any common sense diy'er they will show a helpful pattern of damp in timber, plaster or render.
Using pin m/m's on a concrete or s&c screed floor needs experience - more important is what is the composition of the slab or the in-situ screed and if there is an actual DPM below the floor.

Ask the mods to move your thread to Flooring forum and see: dazlight Fri 29 aug 2020 for a specific response to a difficult surface.
And crazydaze - both are pro floormen with lots of knowledge.
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Thanks Ted, will do.

I've ordered some Sikabond Rapid DPM, apparently good on difficult surfaces

Mods, would you mind moving to flooring please?
What are you putting the liquid Dpm on? Old concrete ?
You would be best using a crack repair on any cracking then a latex like ardex NA , then the liquid Dpm , we only use ardex DPM1c on concrete older then 1965.
Thanks mods for the move.

Hi dazlight. Yep. 1933 concrete. Plan was to put down a liquid DPM, use the bags of Mapei self leveling screed I've already brought, then a layer of anti fracture membrane, then tiles.
Have I just wasted more cash on 5 lt of Sikabond Rapid DPM?! o_O

I've used a concrete repair epoxy for the cracks which seems to have a good job,

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Dazlight, would you recommend I use ardex DPM1c? I've cancelled the sikabond rapid dpm order as they don't have it in stock anyway.
It has been challenging to get anything to stick on the slab, is the DPM1c good on challenging surfaces in your experience?
Also, the surface is pretty rough after being sketch chiselled to get rid of the first few mm to give a new face that something might bond to.
Any other suggestions gratefully received(y)
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No mate. Best to grind off any old adhesive or pre screed the floor with ardex NA so then it’s nice and smooth.
righto. so a create a smooth surface with ardex NA, then a DPM? Followed by a self leveling screed?
NA, DPM1c , NA is what we do most jobs.
Sometimes grind clean , 3 coats uzin PE404 Dpm then prime and NC150 screed
I've got 10 bags of Mapei self levelling screed dazlight, I'm thinking Na, DPM1c and the mapei as final coat.
Whats the best primer to use on the DPM1c to get a screed to bond? really appreciate your advice mate, cheers
Nice one. I've seen some recommend bunging down 'sand' on epoxy DPM so a screed will bond to it, I'll be well pleased that I don't have to do that as well! You're a gent dazlight, cheers mate(y)

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