Valley flashings are used where two different roof slopes come together to form alow trough or valley. Valleys should be sloped so water runs off one end. Valleys that are dead fl at along their length are likely to be sources of leakage. Valleys obstructed by chimneys or parapet walls, for example, are also likely to leak. Valley fl ashings may be visible ( open valley ) or may be completely covered by the roofi ng material ( a closed valley ). Valley fl ashings are typically a shedding system rather than a watertight membrane.