When we were doing some other work to a pitched roof to a 2 story house in Bournemouth, Dorset, we noticed that the felt had sagged a lot over time under the bottom course of tiles. The watermark on the felt suggested if had been collecting water for a long time. This can cause damp to get into the property and we suspected this was causing some mould found on the wall in one of the bedrooms.
We decided to remedy this we would install felt support trays along the front and back of the roof. these would prop the felt up and allow any water that gets under the tiles to run off the felt and into the gutter.