The roof valley is a very important part of the roof that determines to a large extent the integrity of the roof. Depending on where it is positioned and the angle, the valley can carry large volumes of water when it rains. Any slight damage could lead to serious leakages.
This calls for accurate installation and so it is best to use the services of a skilled and experienced roofer. Also, roofs should be inspected twice annually. With this, the roof valleys will be maintained in good condition and the integrity of the entire roof preserved.
See image of replacement valley install