Will I need a new Roof?

Time for new felt, probabley new slates as well, depending on their condition.

Having my roof done here, sarkin is 60 years old, tiles are obsolete, so got to have new tiles, some rafters have rotted away on the wall plate, right mess.

 

You can't replace the felt , without removing the slates anyway. A couple of photo's from the outside would be of some use too. We could tell if the rafters were sagging and see more of the condition of the slates.

 

Probabley never felted under the slates orginially, might had problem with rain getting under slates, tack some felt on the underside to get water to run down to eaves, it's the same here, hence some of the rafters have rotted away on the wall plate, I also have rot in the joists around chimmey due to flashing failing.

You have a few loose slates come away by single chimmey, a bit of sagging on front, rear viewa few loose slates on left side & a metal plate for a repair.

Seen worse though.

 

Originally, there would have been no felt. The slates are that close together , they should be able to provide all the waterproofing the roof needed. Modern slated roofs , would have half the amount of slates you have on that roof.
The roof appears to generally be in very good condition though (as Kiab says , there appear to be a few slipped slates near the chimney on the left of (what I assume to be) the front view of the house, and not much sagging at all.