New oversite concrete slab leak

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Just need to ask advice on this. Had a new extension to the rear of our house. It's a 60s build with suspended timber floors. For the ease of running existing services to the new area, we are continuing with the suspended timbers. A new sub base was laid down in the extension, butted up against what appears to be the old slab running under the house. The sub soil is heavy clay.

The extension wasn't covered during the recent heavy rain so it ended up filling like a bath. Fortunately there's now some cover and we've been able to get the water out however it's been noted that when empty, the water was rising up where old meets new and that the water is clear and cold.
The sub base is probably 100mm below the outside ground level and with a high water table during heavy rain, it appears to be finding a level inside. There appears to be a point along that line where there is a hole filling with water and at that point there is no concrete at all (on the old slab side)

Anyone with any ideas on what can be done to rectify before I put a floor down? I don't want a spring under the house every time it rains.

For reference, the wall to the right of shot is the old house wall which now becomes a sleeper wall to take the floor joists. Top of shot you can just make out the new dpc and blockwork.

 
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So it's got ventilation from new air bricks, has a DPC all round and the DPM under there isn't obviously doing much good as it's just getting under where it's not.

As I don't have much more than 150 to where bottom of the floor joists are, I'm wondering whether it needs some kind of fill at that point, of some water resistant medium to plug the hole.
 
the DPM under there isn't obviously doing much good
You've got a new slab with DPM, butting up to to a strip of concrete with no DPM, so you can't really wonder why water is getting up.

I don't know why its been built like this but the only way now is to put a DPM on top of th earea where there is none, and a DPC on the wall.

Your big problem now is that oversite slabs should not have a DPM under them, as now that's going to hold all water like a swimming pool.
 
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You've got a new slab with DPM, butting up to to a strip of concrete with no DPM, so you can't really wonder why water is getting up.

I don't know why its been built like this but the only way now is to put a DPM on top of th earea where there is none, and a DPC on the wall.

Your big problem now is that oversite slabs should not have a DPM under them, as now that's going to hold all water like a swimming pool.

I thought that they needed a DPM laying under and any material on top was to keep them in place? Shouls it have been laid with a permeable membrane then rather than to prevent any water getting in?

If they're designed to be wet on top, what prevents the smell of soily water coming through the house each time it rains? I've got 50 years of muck under there from previous work or bits of old rotten timber. I'm hoping to get most of it cleaned out to stop the smell at least.
 
Anyone with something constructive on what should be done before builder finishes on site?
 
I thought that they needed a DPM laying under and any material on top was to keep them in place? Shouls it have been laid with a permeable membrane then rather than to prevent any water getting in?

If they're designed to be wet on top, what prevents the smell of soily water coming through the house each time it rains? I've got 50 years of muck under there from previous work or bits of old rotten timber. I'm hoping to get most of it cleaned out to stop the smell at least.
Through-ventilation prevents the humidity getting high enough to be an issue. It does not matter if the sub-floor is damp, and ther should not be any smelly smells (soily water does not smell), if there is a smell then that's a different problem. Damp prevention to the floor is dealt with via DPC's on the sleeper walls.

Has someone designed this for you or are you making it up as you go?

I would suggest you leave it and let it ventilate, as its supposed to be. Possibly drill several holes through the slab and membrane in case you get ponding and stagnant water.
 
the water was rising up where old meets new and that the water is clear and cold.

Have you considered the possibility of a leaking water pipe?

Is there a water meter?
 
No it's being built from full building reg drawings. Definitely no leaking water from mains or rainwater gulleys. had it tested with dyes and it only occurs during heavy rain.

It's an earthy silty smell rather than a drain smell.
 
I dont understand what the picture is showing. It seems that in the top part of the picture around where the concrete block is, is where the original external brickwork finished. What Im not understanding is why the dpm doesnt finish tight up to that and the new concrete run across to it.

The think is, concrete oversite is used to cap off the soil preventing any weed growth or rodents getting into the void. It doesnt therefore need dpm as it isnt part of the structure of the building.
 
Isn't the underfloor void below a suspended timber floor likely to fill with water if the top of the oversite concrete is below external ground level :?::!:
 
Isn't the underfloor void below a suspended timber floor likely to fill with water if the top of the oversite concrete is below external ground level :?::!:
That's why the oversite should be higher than the external ground levels, and if not laid to falls to a drainage system. In my area the water table is often near ground level at certain times of the year, so any oversites below ground level are flooded.
 
That's why the oversite should be higher than the external ground levels
Yes exactly, it was a rhetorical question, top of oversite concrete to be at or about 25mm above adjacent ground level.
 
we have pretty much the same although 20s house, the new extension slab area is totally dry but the other areas are sopping because they didn't have plastic in the 20s. Also the new DPC tends to stop water evaporating so the other areas become even more wet. With the air bricks reinstated and some modern dpc inserted below the existing wall plates on the sleeper walls, it should be fine.
 

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