When it comes to roofing, valleys are the straight lines formed when two different planes of the roof meet. Valleys are usually found where gable dormer roofs meet the main roof. The opposing surfaces of the roof planes is where the water runs off and is channeled towards the edge of the roof. This vital area really handles a lot of the water run-off. So, if the valley’s flashing is not properly installed, the water will leak into the home, which can cause damage to its internal structure.