The felt on my 3 year old pitched roof has failed in several places adjacent to the leadwork of a valley gutter and the recent heavy rains have allowed water through to the ceilings beneath. The felt that the builder installed looks to me the sort of quality that would be more suited to a rabbit hutch with large areas of it now very brittle. I will have to refelt and I am looking for some advice on the correct construction method to create a watertight seal about the valley. The present felt is lapped over the top of the lead to the valley ( by about 10cm ) and then a mortar fillet between the top of the felt and the underside of the roof tiles. To my untutored eye it appears to be easier to create a lasting seal by carrying the felt underneath the lead ( which will mean stripping the lead and then reinstalling after the felt has been replaced ) as the present system seems to rely entirely on the cement fillet not cracking at the junction with the leadwork. Should the fillet crack water is free to seep under the mortar and then on under the felt. Any roofers care to comment ?