The under felt has failed after the stone tiles were sealed to the gulley and water getting under the tiles at the ridge has been unable to get out. The weight of the water gathering at the bottom course has caused the felt to sag & tear. To solve the problem we have installed drip trays & a new course of under felt which allows the water to be delivered into the gulley without gathering under the bottom course of tiles. We have then put the rest of the tiles (including some new ones) back correctly and re-bedded/pointed the ridge line. We undertook this job over a year ago & the home owner has not had any problems since.