wet gyproc inside roof - photos now..

Hey folks,
was in the loft to sort some stuff and looks like I have a leak.

It's a 25 yr old semi with usual modern tiled roof.
Weird setup though - from the inside I can see it has joists as normal, but on the outside of the joists is a layer of gyproc board (ie outside of house side of joists, so from in the loft you can see the joist then the gyproc above it). Then a layer of felt then the tiles.

The gyproc half way down one side is damp on the inside (large area spanning maybe 3 metres length) with some mould. When it rained yestereday I could see the drips coming off the edge of the gyproc which sits over the edge of the outside wall in the soffit. So guess the felt is leaking, soaking the gyproc but running down it, which saves my bedroom getting wet!

There's no tiles missing, no chimney or vents or anything through the roof anywhere on that side of the roof. Not sure where it could be leaking. (Fairly sure its not condensation- there are brick vents in the side wall and vents all away along the soffit..)

Should I cut away the wet gyproc from the inside to see where its leaking from? My worry is that if I do that and don't find the point of entry and then it rains the water won't run down the gyproc anymore (cos it won't be there) and will come in the bedroom... or I might damage the felt cutting away the gyproc?

Is it generally easy to spot where water is coming through the felt?

Will probably try and get a roofer before xmas, but thought it might be worth investigating a bit from the inside...
Any advice folks??
Cheers, Andy.

rain should not get onto the felt... there is probably a roof tile issue

thanks, tiles look fine outside as i say but something must be up with them. They're large size black, flat tiles. 25 yr old house shouldn't have this should it? Worth asking the neighbours maybe...
photo of area - spans about 4 joists:

closer, you can see the water on the joist at the worst bit:

http://www.flickr.com/photos/45767064@N06/4201221586/

Thing that also concerns me is the back of the house has a smaller bit that looks a bit damp too:


Any thoughts?

my thoughts are a total lack of ventilation in the loft space. this combined with deep insulation of the upstairs ceiling is the problem

its condensation! slate vents high and low should solve it.

thanks folks! happy not having to redo roof!
It is cold at the moment so condensation makes sense. Also have expansion tank for heating sitting in loft so that would add to it i giuess.

The soffit has vents all the way along though.

Should I just leave it seeing as its probably not damp all year round or would you recommend putting vents in?

cheers, Andy.

you should make sure the soffit is clear of insulation and i would install vents higher up the roof to create an airflow.

thanks for that alistair.. will do.
yes, soffit area is a bit blocked with insulation.

should i remnove the damp gyproc- isn't doing anything useful is it?
could the gyproc be adding the trapping moisture?

it wont be gyproc andy more like celotex or fibreboard which is quite commonly used this side of the border due to our more extreme weather conditions.

the sagging of the sheets is due to the condensation in your attic and wont get any worse once you've sorted out the vents, they wont be adding to the moisture either.