Damp patch on concrete floor - not sure what has happened.

[roofmog]

Not sure if this is the correct forum or if it should be in the" floor" section.

I have been investigating the cause of a small damp patch that has appeared on the junction of one of my outside walls and the neighbour's party wall. The plaster had started to look a bit damp in the bottom corner and the carpet on top of the concrete floor was cold and smelly. I started by knocking off the damp plaster which disintegrated very easily. In fact it had pretty much turned back into sand. I was somewhat surprised to discover that the plaster appeared to go about 15-20cm below the surface of the floor. It is almost as if the walls were plastered first and then someone added the concrete slab. However, further along the wall it seems normal, the plaster stopping just before the concrete (see second picture). One of the bricks that I have exposed on the outside wall appears to be broken so I think that may be where the damp was getting in.

Any suggestions as to what I need to do to remedy this. There is theoretically a chemical damp proof course in the brickwork, and I presume that there is a DPC under the concrete slab as I haven't had any other damp issues in the past 20 years.

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• roofmog
• 1 Mar 2015
• 1

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• roofmog
• 1 Mar 2015
• 1

 

[JohnD]

water pipe in or under floor.

Have you got a water meter? Does the bubble ever stop moving?

 

[roofmog]

I've finally managed to excavate all the damp sand render. For some reason in this corner the render went about 6 inches below the level of the concrete floor. There is no obvious source of the damp. I think it is basically caused by the lack of a damp proof course where the concrete floor stops. If I dry the trench out with a heat gun then the brickwork at the bottom is damp to the touch within a couple of days. I'm not quite sure what to do to fix it. My current plan is to fill the bottom of the trench with a couple of inches of concrete, then seal it with a couple of layers of liquid damp proofing and then more concrete up to the level of the existing floor. Will this work, or is there a better solution.

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• roofmog
• 7 Jun 2015
• 1

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[JohnD]

have you or your neighbour got a sink, drain, radiator, tap or downpipe near this wall?

show us a pic of the outside please

 

[roofmog]

The mains water comes in at the other side of the properties so I'm fairly certain that a pipe isn't to blame. However the neighbouring property has an un guttered sloping porch roof which is probably concentrating the rainfall into this area. The biggest mystery to me is why the sand based render went about 4 inches below the concrete floor in this corner of the room, when everywhere else the floor goes right up to the brickwork and the render ends above it. I'll try and post a picture later. Thanks.

 

[JohnD]

Offer to buy him a gutter.

 

[roofmog]

He's actually gone to Australia for a couple of years, and the property is currently between tenants. I might just put one up anyway. By the time he gets back he'll have probably forgotten that there wasn't one on it to start with.

Any suggestions as to how I should fill the hole in the floor?