Builder wants to use liquid DPM....



We have a concrete floor with 70mm insulation board over which the builder will be pouring 70-80mm screed.

Although he says in normal circumstances we would need to wait 1 week for each mm to allow the screed to dry, we can get round this by using a liquid DPM - he says in that case the tiles and engineered wood for the separate rooms can be laid immediately (although in practice he won't lay these until 2 weeks later).

If it is that easy, why does not everyone use a liquid DPM?

I will be very grateful for advice as to the pros and cons of doing this. After all, if you seal in the moisture, where will it go?
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It normally takes 30 days per inch (2.5 cm) of new concrete to dry out. 1 week per 1mm would be extremely long, so guessing he means 1 week per cm (too short a period, it is roughly 1 week per 3/4 cm).

Your 70-80mm screed would take around 1.5 - 2 weeks to dry, so can't see why he wants to apply a liquid dpm now and install any flooring 2 weeks later anyway?

Using a liquid dpm is only advisable when the concrete/screed has a moist content of less then 4.5% - any higher and the liquid dpm will fail.
Thanks WoodYouLike,

Sorry, my mistake, I did mean to write 10mm per week.

Based on 1 week per .75cm, I work that out as 9 weeks (not 1.5-2 weeks?) i.e. (7/.75).

Even the builder would admit the only reason he wants to use liquid DPM is it enables him to finish the job quicker!

If the moisture content needs to be less than 4.5%, then that means he still needs to leave the floor to dry before applying the liquid DPM - might it reach this content level within 2 weeks? It's a 48m2 floor space in total.

Are there any downsides to using liquid DPM?
My mistake indeed: calculated on 7-8mm instead of 70-80mm. (time to get new glasses!)

Yours will take roughly 3 months indeed. You or your builder needs to check the moist level after say 1.5 months to see if a liquid DPM is feasible at that point in time. If still higher than 4 - 4.5% he/you should not risk applying a liquid DPM and installing a floor, that's asking for troubles (and fairly quickly too!)

What you can do to reduce the waiting time - but only with a bit - is to ventilate the room/rooms as much as possible for the excess moist to escape. DO NOT force dry the floor
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Thanks again

I spoke to my builder, and he assures me there are different types of liquid DPM and he will be using one that is appropriate, which I take to mean one suitable for whatever the moisture level is likely to be in 2 weeks.

Also, I asked about the moisture - he says it will ultimately leak through, but at such a slow pace it will not be noticeable or cause any problems.

Is he right?
I mean - IMHO nothing will reduce the normal drying time of 3 months suddenly back to 2 weeks!

Thanks, but I think the builder is not saying it will fully dry in 2 weeks - he is saying it will be ready to accept the particular liquid DPM he will be using (which pre-supposes a certain level of drying - maybe requiring a specific maximum mositure level).

I know nothing about DPMs, but will ask him which one he will be using so I can check with the manufacturer.
I understood what he meant, but IMHO nothing can be applied to a concrete floor that normally takes 3 months to dry after only two weeks of drying time to install any floor without causing problems later.

Concrete dries at a certain pace so it can perform at its best over the longest time possible, a liquid DPM can help you speed up things but not already after two weeks! Perhaps in the last two weeks of the normal drying time.
You run the risk of having a failed concrete floor and a failed floor-covering when you try to force nature ;)

The builder accepts the floor will remain moist (for some time!). he is saying it will not cause any problems if we laid tiles and engineered wood on top as the moisture will escape slowly and the whole thing will dry over a long time.

He also says DPM is not strictly needed for the tiles, and using it would be pre-cautionary.

I have no idea whether he is correct, but I hope so!
He's asking for problems! Wood will absorb the moist - even wood-engineered - and will expand more than it would with the normal seasonal fluctuations in air humidity.
But as long as he's willing to give you a guarantee to sort it all out on his own costs when things will go pear-shaped ;) (They will and rather quickly too).
The builder wants to apply a DPM to the top of the slab before it is dry. . .

There will be a DPM under the slab and up the edges already. Ask yourself where the gallons and gallons of water in the slab are going to go - apart from potentially lifting the DPM it will come through any defects in the DPM and cause problems. Let the slab dry first.
Get the builder to check the relative humidity of the slab with a hygrometer box. Some liquid DPM's can be used up to 98% RH.
Thanks Petest67, but are you saying what he is doing is OK (provided he uses a liquid DPM suitable for a high RH, which pre-supposes there are such products available)?
Even if thatks the case - and I do hpe your builder knows how to actually measrue for moist - the water on the concrete has to go somewhere.

Remember what ijsw said: trapped between horizontal barriers the water could end up in yiur walls.

What's yoiur hurry anyway? Wouldn't you rather take the time needed and end up with a proper result, or depend on your builder's short-cuts and hope for the best?

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