NHIE exam practice question · Roofing
A valley is where two roof planes meet, so it carries the runoff from both of them down a single line. That concentration is the whole story: the volume and the velocity of water in a valley exceed anything on the open field of the roof, and any deficiency in the detail there is tested by every rainfall. The same geometry that concentrates water also collects debris — leaves and needles wash down both planes and lodge in the valley — and the debris holds moisture against the covering, dams water so it backs up under the courses, and in cold climates seeds ice dams. Valleys are also the hardest detail to execute, whether woven, closed-cut or open with metal, so the combination of concentrated flow and demanding workmanship makes them the leading location for roof leaks. Inspectors examine them closely and check the ceilings directly below.
A valley carries the combined runoff from two roof planes, so both flow volume and velocity are concentrated there, and the same geometry collects debris that dams water back under the covering.
Valleys convey runoff from two converging roof planes. Concentrated flow, debris accumulation and demanding flashing details make them the most common leak location, and ice damming is a further risk in cold climates.
Two Roofs, One Channel. Everything that falls on both slopes goes down that single line, debris included.
In open metal valleys check that fasteners are outside the water channel and that the metal is not corroded where debris has been sitting.
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