Rain getting in gable wall/roof

danrv

Hi
I’ve noticed some water damage along a wall/ceiling join in a front bedroom built in wardrobe. It’s around 3m in from the corner and it’s an external side wall which is exposed to the elements. 
Had a look in the loft and it doesn’t seem wet but just trying to source the leak. 
The mortar in the top half of the wall has worn away and I can poke a screwdriver right through in a few places. Not sure if this is the problem as it’s a cavity wall.
Also had a look along the tile edge and there’s gaps where the soffit board joins with the asbestos roofing sheets and also between the 
mortar and asbestos. I could probably fill these.
The other side of the apex was done a couple of years ago and the roofer said this side was ok. Tiles are held with clips.
Any help appreciated.

danrv

After a bit of research, I now know that it’s called the roof verge.
Repointing or patching up the mortar is all I can do DIY but probably a waste of time as the end row of tiles really need to be lifted and rebedded.

Quote from great-home.co.uk: 

‘When the mortar on a verge crumbles or disappears then there is a risk of leaks from water being blown into the roof space. The roof verge will need to be rebedded with mortar. This is generally a straightforward thing to do.

Update:

Have redone the join between the asbestos sheet and soffit board with some Soudal exterior sealant. Also patched up some cracked mortar as that was over where the water damage was.