Is a Liquid DPM needed on a New Build concrete subfloor?

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Never float a solid.

And even if you have a working DPM under the slab, a surface DPM is also required when bonding wood floors.

Like I said, I don't recommend gluing a hardwood floor, but people do do it, and do it successfully, so it can't be ruled out as a method. I recommend laying plywood or battens and nailing to that.

A surface dpm is not required when bonding wood floors, what for?

Either the screed is dry and has a dpm, or it doesn't and isn't suitable for gluing to.
 
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A surface dpm is not required when bonding wood floors, what for?

Either the screed is dry and has a dpm, or it doesn't and isn't suitable for gluing to.


what does the RH have to be below when installing wood flooring direct to a concrete base?



I presume you are not a professional but you do DIY ?
 
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Using f.ball dpm if the RH% is below 92 then its one coat of F76 and if above 92 them 2 coats of F75
 
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what does the RH have to be below when installing wood flooring direct to a concrete base?

BS 8201:

For floors which are to be directly stuck down with full-spread adhesive without a vapour check membrane, this figure should not be more than 65% RH but for materials with a width-thickness ratio of 4:1 or less, 75% RH may be considered acceptable.


Whilst rare, I've seen instances of people relying on sealing rather than drying, enough that I am not entirely comfortable with it, dry the screed, then if it's dry there is no moisture issue to seal away.

Though of course if you float or fix it to battens/plywood you can lay a physical DPM, which can be lapped and taped.

And as said, whilst I discourage gluing solid floors, some people do it and do it successfully.
 
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Problem is mate sometimes a slab of concrete can take over a year to dry out. So using a dpm with suppress the moisture not trap it. So the moisture will still release but at a controlled rate which won't upset the floor covering above.
 
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Problem is mate sometimes a slab of concrete can take over a year to dry out. So using a dpm with suppress the moisture not trap it. So the moisture will still release but at a controlled rate which won't upset the floor covering above.

I would mostly agree with you, except it does (rarely) fail, and the reasons are never clear (in that there is no obvious "fault" with the DPM or the way it has been applied).

Again though, rare, I only stress it because when it does occur, no one ever takes any blame, just a lot of finger pointing.
 
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what does the RH have to be below when installing wood flooring direct to a concrete base?

BS 8201:

For floors which are to be directly stuck down with full-spread adhesive without a vapour check membrane, this figure should not be more than 65% RH but for materials with a width-thickness ratio of 4:1 or less, 75% RH may be considered acceptable.


Whilst rare, I've seen instances of people relying on sealing rather than drying, enough that I am not entirely comfortable with it, dry the screed, then if it's dry there is no moisture issue to seal away.

Though of course if you float or fix it to battens/plywood you can lay a physical DPM, which can be lapped and taped.

And as said, whilst I discourage gluing solid floors, some people do it and do it successfully.

you shouldn't nail down any board over 150mm personally i'd say 130mm as they have too much movement and should be bonded direct to subfloor mate also narrow boards under 110mm wide should be bonded too.
 
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