On Fri, 8 Jan 2010 22:03:20 -0000, "Cash"
wrote:

It's interstitial condensation - and I've got the very same problem

Thanks for the reassurance that it is "just" warm, moist air from the
house condensing on the unusually cold roof. Hopefully when this
unusual weather goes away, so will the problem.

my case, it's caused mostly by the steam from the bathroom

We do have a fan in the bathroom but it's a 4" connected to flexible
ducting, so it's not ideal. Sadly the window lintel got in the way and
prevented us fitting anything bigger or better.