One of the most common areas neglected by most Roof Restorers etc is the valley irons. Valley irons are the metal flashing that is under the cut tiles in the gully that runs up the roof. If the house is over 15 years old then it most likely has been bed and pointed rather than left open.
The reason that we now don’t bed and point valleys (Victoria and Tasmania are the only states that ever did) is that the mortar resting on the metal surface accelerates the rusting process because it retains moisture on the metal and takes much longer to dry out than the metal that is exposed. If any rusting is visible where it is exposed, it’s highly likely it’s much worse where you can’t see it.
On a roof that’s 30 years or more in age it’s likely that there is significant rusting of the valley irons. It is much cheaper to replace the valleys at the same time as rebedding and pointing or during a restoration as you are only paying for the valley replacement not extra for the bedding and pointing you would have to damage if the replacement was done later. You would usually be given a better price if done in conjunction with other work.
If the valley isn’t bed and pointed it’s essential that the cut tiles are secured by nails or clips or even better Stormseal should be installed under the valley cuts. Stormseal is a bitumen impregnated foam rubber that comes in various shapes and forms. In valleys we use it in 2m long lengths 50mm X 50mm square, it’s vermin proof (they don’t eat bitumen) adheres to the tile and metal and keeps pests out as well as aiding to repel water.