Rotten felt under roof tiles

Need meter of felt breathable and drip trays .

Run any water getting under the tiles into the gutter & directs wind blown rain into the gutter, it should be replaced sooner rather than later to prevent dampness occurring.

Again sounds like you could do with have the bottom run of felt and batton re done along with eve trays or 5u felt as a water escape

Eve/drip trays bridge the gap between your roof tiles/slates into the gutter to prevent the fascia rotting which can lead to problems. This is due to the membrane even if it has one failing over time it’s always best to solve the problem rather than not too as will save you money in the long term

Replace first meter of felt before any structural damage is caused to rafters connected to barge board and facia capping/ so replace first meter of felt make sure there's no damage install permanent breathable membrane along with Eps eave surrport system

If it’s just the edge of felt which is common I come across it all the time installing soffit fascia and gutters If you was to have an eave trays installed correctly that will fix the problem

Tapir damaged felt before any damage done,don’t wait for it to rot anymore it’s just going to get worse Kind regards paul

The bottom course of felt goes always first because this is where the waters stays the longest and the biggest volume. It doesn't indicate about general state of the felt on the roof it can be anywhere between ok to completely gone. So that need to be assessed separately. If generally felt is ok just the bottom course, fastest way which dramatically will improve performance is to slide plastic damp proof membrane UNDER the existing felt or it you have slates a GRPplastic tray to create continuous drip to the gutter