Eh?

Sarking felt is a very beneficial development.
Realise a lost slate means you are getting water ingress from that
point to the ridge, not just that "small hole".
The problem with sarking felt is that it pools & rots at the bottom,
so eaves protectors are a good addition along with replacing the bit
of felt at the bottom.

Slate roof.
Slate fixings do fail eventually, if you lose a slate, the sarking
felt catches the rain and directs it into the gutter.

Marley single or double camber tiles.
The camber provides ventilation & wind driven rain ingress, the rain
runs down the sarking felt accordingly. They are really not a
brilliant roof tile yet were commonly used on a 22-degree roof with
sarking felt & close boarding (Scottish style).

Interlocking tiles.
Great in that water ingress is much reduced, good for very low angles
without close boarding.


Whoever did the chimney removal should have cut a piece of sarking
felt & overlapped correctly.

Did they vent the chimney stack if part of the wall is outside facing?
You do not want to vented into the loft & house, because it will
transfer moist air into the loft. You want an airbrick vent at the
top, at the bottom likewise or an internal vent. It does not need to
be big, but realise water will penetrate single brick and the chimney
can get running in water. Any joists tha abutt it will get rot in
their ends eventually, it can cause blown plaster issues.


Need Kipper At Sea to give a full answer, think he has retired.