new concrete floor

[kent]

I am planning to have the ground floor of a small terraced house (about 100 years old) dug out and replaced by a concrete slab (with dpm and insulation). One builder has suggested that instead of a screed finish he could put a 25mm layer of insulation on top of the concrete and then put a floating chipboard floor on top of this. He reckons this could be done immediately after the concrete has been laid, but I am concerned because I cannot see how the concrete would be able to dry out. Can anyone give me any advice on this? Many thanks.

[kent]

I am planning to have the ground floor of a small terraced house (about 100 years old) dug out and replaced by a concrete slab (with dpm and insulation). One builder has suggested that instead of a screed finish he could put a 25mm layer of insulation on top of the concrete and then put a floating chipboard floor on top of this. He reckons this could be done immediately after the concrete has been laid, but I am concerned because I cannot see how the concrete would be able to dry out. Can anyone give me any advice on this? Many thanks.

[Tim Watts]

Personally, I'd ask him to quote for putting insulation *under* the slab (which is possible if done the prescibed way).

(there is another way which is celotex type insulation between the concrete and the screed, but the overall depth is much larger and the screed should be reinforced).

This gives you three advantages:

1) You are not limited to chip on top - eg you can tile direct 2) Top level insulation has some risk of compression under localised heavy loads (washing machine) or heavily-walked areas (doorways). 3) You can consider adding underfloor heating to the slab.

[Roger Mills]

It doesn't answer your question, but have you considered taking advantage of the situation and installing wet under floor heating. Seems like an ideal opportunity to me. You'll need a concrete slab, then rigid foam insulation with the pipes clipped to the top, and then screed to finish off.

We did this when we converted an integral garage to a kitchen, and needed to raise the floor level (the slab was already there in our case). It's by far the most comfortable room in the house - temperature distribution wise - the rest of the house having conventional radiators.

[kent]

My original post: "I am planning to have the ground floor of a small terraced house (about 100 years old) dug out and replaced by a concrete slab (with dpm and insulation). One builder has suggested that instead of a screed finish he could put a 25mm layer of insulation on top of the concrete and then put a floating chipboard floor on top of this. He reckons this could be done immediately after the concrete has been laid, but I am concerned because I cannot see how the concrete would be able to dry out. Can anyone give me any advice on this? Many thanks."

Thanks for all your advice guys, but what about my question about the concrete drying out? If the concrete has dpm & insulation underneath and then more insulation and chipboard is placed immediately on top of this, surely the concrete won't be able to dry out. Any thoughts on this? Thanks.

[The Natural Philosopher]

It will. Eventually. But it doesn't matter if it doesn't, does it? Remember concrete MOSTLY sets by hydration. And is quite happy fully submerged.

All YOU care about is the damp not getting out of it to your precious organics and places where moulds can propagate.

[Tim Watts]

I'd give it a week per inch thickness. It will not be totally dry after that, but it will have gone a long way down the road to dryness...

[Steve Firth]

Your concrete won't completely dry out anyway. It can take decades for the water in a slab to evaporate. It shouldn't be a case of pour, harden, slap on insulation though. Allow some time for the surface to dry. For a slab sealed at the bottom drying time to 10% moisture is about 6 weeks. Getting to 2-4% moisture will take about a year.

You can do a rough test to see if moisture is a problem by covering the slab with polythene and seeing if moisture collects below the sheet.

[kent]

This is very interesting, but is leaving me a bit confused! Another builder who quoted for this job said the concrete would have to be left 2 or 3 months before putting down a floating wooden floor.

[Tim Watts]

Sounds reasonable for a floating wood floor.

[John Rumm]

who quoted for this job said the concrete would have to be left 2 or 3 months before putting down a floating wooden floor.

It depends on which aspect you are looking at. The concrete itself will achieve adequate strength without actually getting completely dry (as others mentioned, it sets by chemical reaction).

Once it is "dry enough" you can cover it with flooring chipboard or something else that will not be upset by the slight increase in humidity from being close to a bit of still drying concrete.

A posh hard wood floor however that really needs to be kept at a relatively constant humidity, could risk deterioration if laid too soon.

[kent]

ilder who quoted for this job said the concrete would have to be left 2 or =

3 months before putting down a floating wooden floor.

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Thanks all for some interesting and useful comments. Just going back to my original question, do we really need to have a new fl= oor at all? Could we just cover the quarry tiled floor with a dpm and then= lay carpet or wood on top of this? I read elsewhere that this would cause= problems because the tiles need to breathe and this would therefore cause = a build up of moisture under the dpm. At the moment the house does not act= ually have a damp problem, we just want a more modern and cosier floor. An= y comments gratefully received. Thanks.