The valley gutter works at the point where water from two parts of the roof arrives. Measuring its length is only the beginning of the preparation. You need to identify the layout of the slopes, the line of their intersection and how the covering is finished. The area from which the water flows also matters. Not every pair of slopes forms the same angle and the same detail. A well-described valley allows the flashing to be selected without accidentally narrowing the channel during installation. On the plan, mark both slopes and the line of their intersection. Add the flow directions and any obstacles that may change the path of the water. A dormer, chimney or roof break affects the local layout. The length of the valley alone does not show how much of the slopes the flashing has to serve.