We had a slight discolouration in a small area of the bathroom ceiling
appear a few months ago. It has I think been correctly traced to wind
blown rain making its way under roof tiles, which hopefully is now
fixed, but I have been keeping an eye on the area with a damp meter and
an IR thermometer, just to ensure it is drying out.

Initially the meter was showing full scale in contact with the stain
and nothing on the rest of the ceiling. Now it shows it is just
slightly damper than the rest, except when the bath or shower have been
used, but it soon drops back to its usual low value. My guess is that
the tiny extra bit of moisture where it was slightly damp, makes the
area cooler and therefore moisture in the air more readily condenses
there. Checking it with the IR thermometer, the stained patch is a
couple of degrees cooler, than the rest of the ceiling.

Apart from the subject line making me think this was some kind of adult
newsgroup I'd wandered into by mistake. I do agree with your diagnosis here,
We had this in a box room on the side of the house where the coldest winds
come from and the ceiling curves to avoid hitting the roof, so the gap is
very small and of course gaps between tiles and no lining added to the
problem. We did do a little blocking up in the loft and drying out and its
gone away for over 20 years now. Took quite a while to dry and we alwso had
mould issues to start with.
Brian

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"Harry Bloomfield" wrote in message
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We had a slight discolouration in a small area of the bathroom ceiling
appear a few months ago. It has I think been correctly traced to wind
blown rain making its way under roof tiles, which hopefully is now fixed,
but I have been keeping an eye on the area with a damp meter and an IR
thermometer, just to ensure it is drying out.

Initially the meter was showing full scale in contact with the stain and
nothing on the rest of the ceiling. Now it shows it is just slightly
damper than the rest, except when the bath or shower have been used, but
it soon drops back to its usual low value. My guess is that the tiny extra
bit of moisture where it was slightly damp, makes the area cooler and
therefore moisture in the air more readily condenses there. Checking it
with the IR thermometer, the stained patch is a couple of degrees cooler,
than the rest of the ceiling.

On 16/05/2018 11:23, Harry Bloomfield wrote:
We had a slight discolouration in a small area of the bathroom ceiling
appear a few months ago. It has I think been correctly traced to wind
blown rain making its way under roof tiles, which hopefully is now
fixed, but I have been keeping an eye on the area with a damp meter and
an IR thermometer, just to ensure it is drying out.

Initially the meter was showing full scale in contact with the stain and
nothing on the rest of the ceiling. Now it shows it is just slightly
damper than the rest, except when the bath or shower have been used, but
it soon drops back to its usual low value. My guess is that the tiny
extra bit of moisture where it was slightly damp, makes the area cooler
and therefore moisture in the air more readily condenses there. Checking
it with the IR thermometer, the stained patch is a couple of degrees
cooler, than the rest of the ceiling.

We had some damp from a small crack in a leaded. I have a thermal
imaging camera, and that clearly showed the damp patch as cooler,
presumably due to water evaporation. This is lath and plaster.

The gully was fixed, the damp dried out and there is no longer a
temperature difference. It's not been painted or touched in any way.

So I wouldn't be so sure of your diagnosis.

Cheers
--
Clive

Oops. ...leaded valley.

Cheers
--
Clive

Clive Arthur used his keyboard to write :
The gully was fixed, the damp dried out and there is no longer a temperature
difference. It's not been painted or touched in any way.

So I wouldn't be so sure of your diagnosis.

There are no pipes at all in the area, so it had to be rain water
ingress and the best guess, because it had never happened before and
the tiles are fine - that the wind (from the east) was driving rain in
the edge gaps of the tiles. That was flaunched, since when there hasn't
been much wet weather.

There is no mould at all, just the very slightest of damp stains, close
to where the ceiling meets the wall.

on 16/05/2018, Clive Arthur supposed :
Oops. ...leaded valley.

Nope, nearest leaded valley must be a quarter mile away lol