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I have just noticed that I have a wet patch around my external tap on the outside wall.
The wall seems to dry throughout the day.
I have turned off the water to the tap, but the following morning, the outside wall is again damp.
I have looked at the internal wall and there is no sign of water.

I do not know where to look/what to do next. What could be causing this wetness, and how can I fix it?

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2 Answers

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It sounds like condensation is probably forming on the cold metal pipe and wicking into the surrounding concrete.

Has it been particularly humid of late in your area?

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Unfortunately not. Its the hottest October on record with no signs of rain. – Steve Oct 3 '11 at 13:12
Rain & Humidity are different things. Humidity is the measure of how much of the air's current moisture holding capacity has been met. 50% humidity at 100F is a lot more humid than 50% humidity at 50F because as the air temp increases, the amount of water that can be stored by the air increases and it's a non linear increase. So - if it's really warm and humid in the day and cools dramatically at night, the metal will cool faster than the concrete, and that will cause condensation on the metal, even w/o rain. – The Evil Greebo Oct 3 '11 at 13:15
What region are you in? I can look at the historical weather data. – The Evil Greebo Oct 3 '11 at 13:15
Thanks for the explanation. I'm in Dorset, England – Steve Oct 3 '11 at 13:22
Yeah you're seeing BIG shifts in temperature from day to night - 21.3C at 1pm vs. 14.9C at 1AM, and 67% humidity at 1pm vs. 91% at 1 AM. The warmer air during the day is absorbing large amounts of moisture which, at night, as the air cools, it cannot hold onto, thus condensation. Is the faucet itself wet/damp when you check the wall? – The Evil Greebo Oct 3 '11 at 13:31
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up vote 3 down vote

My first guess is that the threads where the valve is screwed onto the pipe have started to leak. Even with the water turned off, there will almost certainly be water in the line that would leak. With the water turned off, you can try unscrewing the valve from the wall (as long as the plumber didn't solder the threads, it should come off). If the valve is relatively new and the threads are in good condition, then you can clean the threads, apply plenty of pipe dope to the threads, and screw the valve back on.

Note that if you don't see any mineral buildup around the pipe or threads, then the problem may be inside the wall and require removing drywall to locate the problem. If that's the case, I wouldn't delay in opening things up, since a small sign of water outside could be concealing lots of water inside.

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God point - did you drain the line after you shut off the water? – The Evil Greebo Oct 3 '11 at 13:39
Thanks. No I did not drain anything, just took turned the water off. Will attempt to unscrew tonight. Many thanks. – Steve Oct 3 '11 at 13:50
Attempted to unscrew tap from wall, but its well and truly stuck. Guess its "call a plumber time". Thanks for your help. – Steve Oct 4 '11 at 11:55
@Steve: sounds like you had the same installer that I did, soldering the thing on. Hopefully this will be a small surface leak and not anything more serious inside your wall. – BMitch♦ Oct 4 '11 at 12:11

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