Help - Newbee - Roofing - Bonding Gutter!!

[cotswoldbuilders]

Have you sorted the leak yet, ?
Although most here agree, lead work needs addressing around stack, bit of pointing.
Only other obvious signs, couple tiles cocked up a bit, need some tingles.
After re-reading the post (carefully this time) you actually own both roofs, and the leak is from the older roof, but not in attic so its the first floor directly below the gutter, r/h of join.
You state, the water mark, which would be outside (as we cant see inside) and you are not really bothered about it, which suggests to me, that you have water dripping outside and not inside.
On that part of the roof there are some wide gaps on the joint, but my money is still on the guttering ie stop ends fitted to both sections.

 

[^woody^]

.... it most likely that the water is finding its way in at the chimney and running down the felt

Get the lead re-done properly around the chimney - .... and all shall be well.

Even if the lead is letting water in to run down any felt, then surely it should be exiting into the gutter?

So if this was the case, then there is still a problem with the felt at the eaves which needs sorting out

 

[datarebal]

there could be a punture in the felt anywhere between the chimney and gutter, the water could be running down a radter even...

The gaps between the slates at the join are unlikely to let in and could easily be sorted by simply cutting down some doubles (double width slates)

Chimney or extractor have my money.

 

[catlad]

The O.P mentioned seeing felt in the gutter so it seems there are no felt support trays so it may be ponding at the facia were both roofs meet and felts meet. water coming down from poor lead work on chimney.

 

[BethesdaSlate]

So if this was the case, then there is still a problem with the felt at the eaves which needs sorting out

Yes, my mistake, this would need looking at also. My guess is that either the felt is punctured behind the fascia or the felt ends at the join of the old roof and water is tracking in there.