Drying out floor slab

d2ww

Here's my situation, The 150mm slab for my extension got poured 2 months ago, just before christmas, but the roof is only on for the last week. Now the builder wants to put the foil backed rigid insulation down so he can do the ufh pipes and pour the 50mm easi screed.
My problem is that the slab was a swimming pool for those 2 months, with all the rotten weather we got. I know from other threads that people say a month for every inch, but do clients really say to their builders "Oh that's a lovely slab, see you in 6 months time"!!!!
While the surface of the slab has dried out, I have no idea how much moisture is in the body of the slab.
So do I say to him, go ahead or wait til it's dried out, and how dry is dry enough?
A perplexed d2ww

dunie001

Well the worst possible scenario you could have would be to have a body of water trapped in the floor with no means of escape.

As well as that, it's your house not the builder's so you should do whatever you think would be right...after all it'll be you living in the house once the builder is gone. The builder doesn't care whether or not the floor is dried out enough as long as he can get on with his work.

I'd advise you to seek the help of an engineer, probably a structural engineer would be the best person to get advice from. The builder probably would ignore your advice but he can hardly ignore the advice of an engineer...

Personally though, I'd tell the builder to bugger off and go somewhere else until the floor is fully dried...but considering the rain, maybe it'll never dry!:p

RKQ

The one month per inch refers to drying out the finished floor slab to receive a timber floor.

You seem to be discussing the drying out of a sub floor slab?

If the builder is putting down a radon barrier / dpm over the sub-fllor slab, installing floor insulation and pouring a screed then the moisture in the screed is important.

If it was my extension, I'd have the windows installed, 1st fix electrics and have the walls plastered before pouring a screed - especially an "easy screed" which can be basically pumped in. This gives your Builder plenty to get on with.

If the screed is installed later, most of the wet trades will have finished, the structure will also have been weather tight for afew weeks / months.

Good luck!

d2ww

Hi RKQ dunie001,
Yes I suppose you would call it a sub floor slab. What the builder did is lay foundations, then put down a dpm, then pour this sub floor slab. My worry is that with the dpm below and foil backed PIR above, there's nowhere for any moisture to go. But, then again does the presence of this moisture matter? My concern is that it drawn up into PIR by the heat above and effectively "shortcircuits" the insulation.
I am awaiting a response from my engineer about this, but the poor fella has enough difficulty figuring out his ar*e from his elbow, never mind all this technical stuff :cool:

RKQ

If the damp-proof-membrane is below the sub-floor slab, as stated then I would agree with your concerns. The better the sub floor dries out the less the chance of the screed moving and cracking!

Its a good time of year for drying out, so get the building weather tight and leave laying the screed as long as possible, definately after plastering.

You could also put a 1000 guage dpm/slip sheet over the sub-floor slab, laid the insulation horizontally and vertically at external wall perimeter. This 2nd dpm will allow the screed to dry out without any possibility of rising moisture.

Poor Uncle Tom

RKQ wrote: »

You could also put a 1000 guage dpm/slip sheet over the sub-floor slab, laid the insulation horizontally and vertically at external wall perimeter. This 2nd dpm will allow the screed to dry out without any possibility of rising moisture.

This could also provide a sandwiched layer of stagnant moisture incorporated in the make up of your floor....potentially very unhealthy.

As stated above, best get the house weather tight, get on with other works and allow the sub floor dry out.

RKQ

RKQ wrote: »

Its a good time of year for drying out, so get the building weather tight and leave laying the screed as long as possible, definately after plastering.

You could also put a 1000 guage dpm/slip sheet over the sub-floor slab, laid the insulation horizontally and vertically at external wall perimeter.

You have to read all the advice Uncle Tom to understand it:D

2 month old slab, roof just on, so lets guess 6 weeks - 2 months before plastering is complete.... In April a 4 month old sub-floor should be perfectly dry and ready to receive the UFH and screed.
The likelyhood of trapped moisture is minimal... to be honest I can't see any alternative.

Maybe you can you suggest one Uncle Tom?

Poor Uncle Tom

RKQ wrote: »

Maybe you can you suggest one Uncle Tom?

I wouldn't put in the DPC, if the existing course is correctly installed it will be sufficient. We need to remember that the plastering will add a lot to the moisture in the slab and the insulation will not provide a barrier for the slab drying out. So the slab and screed will dry out as normal.

I would do as you suggest and leave the screed pouring until near the end of the job, I just would not put in the extra DPC course.:)

d2ww

Thanks for all the replies, anyway fact is that I blinked first and told him to get on with it. We've been 5 months in a rental house already, and weren't willing to delay things for weeks and weeks based on a hunch that this might pose a problem. Things like how dry is dry enough, and how you measure it accurately just added to the uncertainty.