Where's the leak?

I have a detached 1930 house, conventional construction (rendered concrete block). There is an intermittent leak in the upstairs bathroom I haven't found a cause for.

The problem is dampness/water on the cork tiled floor of the bathroom, across the corner (one outside wall, one inside wall) where the toilet is. It seems to build up slowly, and has dried up since I've stopped using the toilet (legs not crossed, I have another downstairs!), but I can't see any leaks from the toilet itself. There's an overhead tank for the toilet - I've put tissue paper round all the joints, including those to the waste pipe and flushed a few times with no dampness. I can't see any cracks inside or outside of the pan, and the water level in the pan seems constant when its not being used. I've checked under the bath, which is dry, and also the pipework to the basin is exposed and seems OK. The loft over the bathroom looks dry and there's no staining. Outside the roof seems OK and the guttering is clear. There is no unexplained water running through the water supply system when everything is turned off. It's not direct human action - I haven't spilt the water and nor have guests.

So what's the cause? All I can think of is that there might be a crack in the (cast iron) waste pipe as it goes through the outside wall to the soil stack. Is this likely? The waste pipe looks OK from both the bathroom until it goes through the wall and from the outside, but there's a few layers of paint on it, and of course I can't see what's happening inside the wall. Have I missed something and there's some other cause? Would taking up the cork floor tiles in the bathroom help? How do I ask a plumber to come and find a leak which has now dried up and which I can't attribute to anything without looking stupid?

If it is the waste pipe, is replacing it a job for a plumber or a builder?

Thanks for reading this far!


Faellie

Do you leave the bathroom well ventilated with either a fan or a window open?

Bathroom is smallish (3m by 2m apprx), and the window 90cm by 60cm apprx) is usually open during the day (it's sheltered from the prevailing winds), shut at night. There has been some condensation on the ceiling (inadequate insulation, in process of being dealt with), but the damp/water on the floor doesn't seem to be a part of that - everything else in the bathroom (including walls and the porcelain) is dry except for the damp bit of the floor.

wait til its wet and call a builder or plumber.

Have you had the blocks tested in case they are Mundic ?

Tony

Has the water been coming and going or did you notice it, stop using the loo, and then it went?

Is the water just against the wall or could it have come from the pan and pooled in the corner?

Is it a new pan?

I was mystified in my last house after I put the bathroom in to find water on the floor. After blaming my son, I eventually found a tiny crack under the rim of the loo.

Whilst the weather is dry, flush the loo a few times and see what happens. Leave the tissue on the floor and see where the water is coming from - the wall or the pan. My feeling would be that your diagnosis of a leaking pipe in the wall would be correct.

Thinking about it, before flushing the loo, use a bucket to send water down the pipe - this eradicates a leak between the cistern and pan.


G

Thanks everyone for your help and interest.

Big G
The cork tiles on the floor make it a bit difficult to tell what happened (I won't want them in a bathroom again). Mostly I hadn't been using that loo much, and I noticed the problem after I'd been using it a lot for about a week. I stopped using the loo and the problem seems to have gone away - there's no damp there now, as far as I can tell.

The damp/water was in an arc across from one wall to another with the loo in the middle. The bit of floor between the loo and the corner of the two walls seemed pretty dry, as did the rest of the floor (the bathmat stayed dry). Judging just from the pattern of the damp it could have been either coming from under the skirting boards on one or both walls, or coming from the pan and going in both directions. Perhaps I could use a spirit level and see whether there's a slope on the floor which might help to tell which? And yes, I could probably crawl round the pan again (its an old one) looking for cracks.

Otherwise, I think you and Agile are probably right: start using it again, and have a plumber/builder ready to come round while the evidence is fresh! Although I didn't want to add to any problems a persistent slow leak might have already caused to the joists.

Agile - yes, tested for mundic and clear - I'm on the Roseland, and apparently the blocks were made with china clay waste rather than tin mining waste!

Thanks again