damp problem solved (?)

I've had a damp problem in my 1920's detached house, and have slowly been trying to eliminate things. Does my latest theory sound reasonable?

West-facing gable wall had bad damp problem in upstairs bedroom when we moved in three years ago. Rerouted shower vent so that it vented outside, rather than into loft space.

Had bedroom thermally boarded (to prevent condensation). Small amount of damp still at ceiling - wall join in bedroom.

Lobbed an extra foot's worth of lagging in loft, plus fitted airbrick into gable end. Also, re-waterproofed chimney and confirmed flashings were ok.

Final thing that was nagging at me - wet verge looked a bit ropey - undercloak appeared to be delaminating, with some cracking of the bedding mortar. So i decided to dry-verge it.

When I took the roof tiles off, there was (historic) water-staining of the sarking, all along the wall (I'd have expected it to have been concentrated at the chimney end, if it was due to a problem there).

My guess is that the undercloak (which was supalux, rather than a cement-like material) has been "wicking" the rain onto the internal face of the gable wall, and causing the damp. The fact that the cloak was angled towards rather than away from the house makes that seem plausible.

DOes that seem reasonable? Is supalux a poor choice for undercloak (as it is much more porous than cement sheet?

What type of wall construction is it, and what sort of roof tiles, and how is the eaves constructed - does it have the undercloak right across the wall?

What were the signs of the previous dampness -black mould or brown staining

Undercloak actually causing or allowing water penetration is not common

Thanks Woody,

Brick wall (double skin with 1 inch cavity, I think).

Double roman concrete pantiles.

Supalux undercloak spanning cavity, nailed.

Cloak overlapped sarking.

When we moved in, the wall was mouldy, actually wet at times (vendors had stapled the wallpaper up, as I presume the paste wouldn't hold in the conditions).

And don't even start me on the surveyor / damp surveyor

Never simple......

good news - despite Lake Baikal falling on the house last week, I can see no obvious ingress of water into the loft, but

there's still one brick inside the loft at the gable that's (efflorescing?). It's not flush with the rest of the gable wall (sticks into the loft about a quarter inch), so could form a condensation point.

When I get chance, I'll be fitting further airbricks in the gables.

Chopping out the offending brick and replacing it with an airbrick - good move or not?