damp/water ingress under floor

6 Apr 2007
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United Kingdom
Hi folks, looking for some advice please

I'm currently refurbishing a 1970's dormer bungalow and have been working top down - the house is watertight etc. But i do still need to put up guttering at the rear.

After clearing out the rubble from underfloor and drying everything out I'm finding damp in the concrete slab under the floor. De[pending on how heavy the rain is there can be a trace of surface water. It seems to come from the vent built into the brickwork on the inner wall.

Hard to tell where it comes from but there is no sign of damp/water above the dpc on the wall and only appears after rain.
My thought is rainwater is seeping in through the brickwork below the dpc.

Should I worry? and if i should what do i need to do?

The house is built on a concrete slab which is solid and in good condition.

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You do not say if the vent under the floor goes out through the wall as well - which it should.
It would be better if there was no free water on the concrete slab as its just adding to the overall "dampness of the house, and especially the under floor joists.
As far as I know a cavity wall has the cavity filled to within 6" of the DPC, the slab would be laid below the DPC. This would mean that if the slab is 12" below the DPC, that water penetrating the wall could rise in the cavity until it leaches out and if the slab is lower then then the 6", then the water would pool on the slab. If the vent is sitting directly on the slab it would not help. Could just be some mortar above the vent in the cavity is allowing water from the outer leaf to run down the inner leaf and exit via the vent.
What to do? One start would be to get hold of a wire coat hanger and turn it into a straight bit of wire. If you put a bend in it you should be able to thread it through a hole in the vent and rotate it to see if the cavity is empty to below the slab 's surface. If it is empty , I would drill a hole through it at a steep angle so you have put a bleed hole so any water accumulating in the cavity will trickle outwards.

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