While shaving this morning I was dismayed to find a foot size brownish
stain on the newly-painted bathroom ceiling. My worry increased when I
located it to a damp patch on the water tank 'platform' above the
ceiling plasterboard, just on the edge. How on earth, I wondered,
could a thick plastic tank spring a leak? But then I discovered that
the cause is probably much less serious, although still surprising.

Covering the top and most of the sides of the tank is a black plastic
cladding thingie, with thick fibre glass insulation inside it. (This
in turn sits on top of a plastic lid, that fits loosely over the lip
of the square tank.) On the underneath of the cladding's top surface
there was quite a lot of what appeared to be condensation. And I could
just see a trace of water on the outside of the tank itself, below the
rim, at the point which would be above the stain.

I know nothing about these things, so would appreciate a few words
from anyone who does please. Is that really likely to be the cause?
IOW, condensation building up sufficiently to leak out?

Or is it more likely to be this other possibility. At that same point,
the overflow pipe leaves the tank, and it was close to that where I
saw the water on the tank outer surface. The tank water level was an
inch or so below the outlet, but (some 10-20 mins time after using any
taps or loos) there was still a drip-drip coming in through the valve.


--
Terry, West Sussex, UK