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Historically we've had a leak from our shower in our bathroom which has leaked into the neighbour below's bathroom and kitchen (this has gone though the bathroom wall to the kitchen which is adjacent). This caused staining on the ceiling and dividing wall of the neighbours kitchen / bathroom.

Was fixed in Feb this year and the neighbour reported no further problems. We decided to wait a long time for everything to dry out - the building is an old 120 year old tenement flat with a large amount of deadening material (soil, rubble, stone, sand) between the floors so a lot of water could be absorbed and need time to dry out.

About mid July this year we had a leak from our kitchen sink into our neighbour kitchen. This found a path through the interfloor material quite quickly so we are not sure how much water was absorbed. I'm told it leaked through the ceiling although I suspect water would have been absorbed everywhere behind the ceiling plaster, including going down the wall against the brickwork. Anyway, leak fixed.

Now neighbour is telling me that they are getting further marks / stains on their walls and wants me to take down my shower and pull up my kitchen floor to investigate. They have been away for three weeks so I am not sure how much of this has suddenly appeared and how much of this happened while they were on holiday.

To make matters worse, I am away from home at the moment and have let my flat out. I won't be back for about 7 weeks without making an unplanned flight home.

I think this might be residual marking as the wall dries out / water comes to the surface - plaster on these walls is about 30-50mm thick, but I'm no expert. I'm not able to actually see any of this other than some photos so it's difficult to tell if this is actually a further leak.

We've contacted a firm who say they can do a thermal image of the wall to show the temperature differences along with actual damp meter readings which will indicate how damp the wall is (cost ~£100).

We could buy one of these Damp Test Meters, or Non-contact Moisture Meters from Amazon.

Anyone have any suggestions? Are stains appearing 2 months after the initial escape likely to be the wall drying out?

Any suggestions on ways to find the leak if there is one?

(sorry for the long explanation and perhaps incorrect keywords - there's no "damp" keyword)

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We had a leak going into our ceiling. Luckily, as a wood turner, I already own a moisture meter. This can test the moisture content of wood (but other similar density substances too) merely by pressing a button. That meter worked very well for my problem. Of course, if you do buy one, I don't expect you will find many other uses for it than catching an occasional leak, but it can still be cheaper to buy one than to pay someone to make a simple reading for you. Just store the meter after you are done with it with the battery removed.

It will take several weeks or even months for a ceiling to dry out. This depends on how enclosed the area is, and whether there is still water getting in. Readings made over time should be decreasing. If not, there is still a problem. The moisture content of even dry wood will be around 8%. Check other walls in your house, because this number will vary by where you live, and what time of year it is.

There are several styles of moisture meters. One relies on pins that must be inserted into the material, testing electrical conductivity between the points. As such, it will leave small holes. Better are the styles that do not leave a hole, but they are more expensive. Note that the cheaper models use a probe. This probe will have pins to stick in the wood.

By the way, while the ceiling is still damp, stains WILL continue to spread by capillary action. The water will continue to wick the stain further away. 2 months later is a long time for new stains to appear, however, old stains can bleed through new paint. You may wish to use a product like Kilz to seal in those stains. But wait until it is surely dry before you paint. This is where the meter will help you out.

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  • I'm going to accept this answer as it's the most informative and I believe more technically correct. Long story short, it turns out there was still a leak (now fixed), but I have to agree with the comments on capillary action - the stain appears to spread as the moisture soaks out from the bulk of dampening material between floors. Like woodchips says, it's difficult to know how long this can go on for and in my particular case it was continuing to spread as the water was not stopped. However, even once we found the leak and fixed it, the stains still continued to spread as the ceiling dried.
    – Jamie McNaught
    Oct 11, 2012 at 14:44
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I'm afraid that no, in general stains do not spread for weeks after the stop of a leak. If a stain is spreading it is because there's additional moisture coming from some source.

Are there pipes running behind that wall? It is possible it's not a leak from your apartment, but from a leak behind the wall itself. In this case the stain would spread out from the source of the leak. If the stain is spreading down from the ceiling it's probably coming from your apartment though.

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  • Nope, there's no pipes behind the wall. It looks like it's a leak from an outflow and that's why we're seeing so little water / marking. It's unfortunate that we've managed to get two leaks at exactly the same time making it very difficult to ascertain if it's residual or otherwise.
    – Jamie McNaught
    Sep 28, 2012 at 9:00

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