We have a roof valley where the original roof meets an extension roof. The "obvious" part of our problem is slowly worsening leaks in the valley during heavy rain. An outside repair or rebuild of the valley would fix this on the face of it. However, the secondary part of the problem relates to the felt underneath. Under the valley the old roofing felt follows the "old" roof line (now underneath the extension roof) down to what used to be the eaves at the top of the outside wall. With the extension built, that outside wall is now an inside wall (in between the house and the extension). So if any water makes it in through the valley, the old roofing felt guides the water down inside the loft where it either escapes through tears onto the ceiling below or makes it all the way to the top of the wall where it can pool. When it comes to getting this repaired properly, it seems to me that the felt in the valley should be redone completely to make sure the felt under the original roof and under the extension roof also meet to form a "safe" runoff valley down the line of the valley. Is this something that would be expected where an old and extension roof meet at a valley? If so, I assume it would be reasonable to suggest to a roofing contractor that a "good" repair would require a proper re-do of the valley felting as well as the tiling above? Is there anything else about how felting is done (or supposed to be done) under a valley that might be relevant? Right now the felting seems to be the weak link - it's as though any rain that makes it through is instantly inside the loft on the old felt rather than getting channeled away.