Still Damp! - How Long to Dry Out?

Hi all
Tricky one this. Four years ago I bought a 1970s dump and discovered that a damp patch (believed to be a leaking patio door) was actually a fractured water pipe. A copper 15mm elbow had split on its internal "corner?" on a buried hot water feed to a sink and was showing as a damp-to-wet area of floor. On further investigation, water had obviously saturated the sub-base and travelled along all heating channels rising in places to rot skirtings etc. I lifted all the plastic? tiles that had been bitumened over the ground floor and a film of water was found under most of them. As I say, that was four years ago and since then, all the upper surface of the ground floor slab appears to have dried out (sorry, forgot, solid concrete ground floor).
But, even now, I have discovered that if something impermiable is placed over the limit edge of the floor dpm where it rises to floor level (eg at all internal and external door thresholds), a damp film forms on its underside. This is true of a rubber backed doormat at the front door and a length of skirting which was being stored close to the rear patio door.
So the question is - how long should it take for the entire sub base to dry? Is it likely that this damp is coming from "outside" rather than "inside" the dpm IYSWIM? (ie it is general ground water rather than a leak)
This is causing concern for two reasons:
1. I am considering converting heating system to sealed and am concerned about possible leaks. 2. SWMBO was after wood floor in the dining room (with patio door) and I'm not sure whether I should consider this if there is a source of damp (however minor) beneath it.
Is it possible to locate minor leaks without actually digging up all the floor channels?
Thanks to anyone who's got this far.
Phil
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TheScullster wrote:

I started a job a few weeks ago in a living room ext to take up the screed with the intention of putting battens and a t&g floor down but soon realised that all was not as it should be. The cowboy who built the extention was to tight to put a dpc in so are you sure that you don't have the same problem. The thing I find odd though though is whilst the builder was to tight to by a membrane he through enough bricks to build a wall into the concrete base and then went out and bought thermalite blocks. I say concrete base but used the term concrete lightly.
Kevin
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"Kev" wrote
snip...........

Thanks Kev
No the top edge of the dpc is visible and the damp is in the original house rather than an extension. Could the damp be coming from below the dpc and up the "outside" of it? I know the floor dpc should lapped into the wall dpc to prevent this, but I suspect it is damaged in parts.
Phil
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TheScullster wrote:

I'm not too clear on the details of what youre describing but below the dpc will always be damp. If youre getting damp up at floor level from below dpc, your dpc is not sealing somewhere. You can use a liquid sealant on a concretef loor slab to remedy this.
NT
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Thanks NT
The damp is not appearing as visible in the surface of the floor slab at all. It is presumably evaporating or tracking up from below (not sure which side of the dpm) and arriving on the underside of objects placed over cracks in the floor where the edge of the floor membrane stops.
Maybe I just need to fill the cracks with water-proof sealant?
Phil
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TheScullster wrote:

Thre may be a bit of a draught bringing in damp air. I'd be inclined to cap odd the void with some sort of mastic, rather than filling it up.
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If I recall correctly there is a problem with deterioration of copper pipes buried in concrete (Isn't this why copper gas pipes are clad in plastic?) If that is the case do you have any more buried water pipes that may be leaking?
Tony
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TMC wrote:

IUC the risk of corrosion is higher, but still very low. Lots of gas pipes are buried in crete up and down the country, and resulting leaks are not a significant prolbem.
NT
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"TMC" wrote

Thanks Tony
Yes, all the ground floor piping is buried copper. I was hoping someone might have a clever method to locate leaks and confirm if there are any. All I can think of is to tie up the ball valve in the heating header tank for a short while and monitor the water level closely. Not sure that this will highlight leaks which are just big enough to cause pressure loss in heating system though. As noted in the original post, when I took over the place, the floor and sub-base must have been sodden.
In my last place, built in 1988, all buried copper pipes were plastic coated. In the present earlier build property, they seem to have been clad in some hession sort of stuff and laid in sand filled channels before concreting over (both gas and water).
Phil
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Could you substitute air for water, and do a drop test, or some such?
Thomas Prufer
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on 23/06/2006, TheScullster supposed :

Connecting a high resistance meter to a suspect pipe, one probe to pipe the other to a damp area might provide some usable clues - the lower the resistance the nearer you might be to a possible leak. I would not rely too heavily on the outcome, but along with other clues it might be of use.
If it uses a header tank, could you perhaps shut the ball valve supply off and see if the level in the tank drops?
--

Regards,
Harry (M1BYT) (L)
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