What is the purpose of a valley in a roof and how should flashing be installed there?

• A. To seal the gap around penetrations
• B. To provide decorative trim
• C. To channel water where two roof planes meet; flashing should be installed overlapped with proper fasteners
• D. To insulate the attic

Answer: (C)

A valley is the channel where two roof planes meet, and its job is to collect and carry rainwater down to the eaves. Flashing in the valley creates a waterproof path at that intersection, so water doesn’t seep behind the shingles and into the roof deck. The proper way to install it is to lay a continuous piece of metal valley flashing in the valley, fastening it along both sides with corrosion‑resistant fasteners. The joints should be overlapped in the direction of water flow and sealed, so water moves over the flashing rather than finding a seam to leak through. The underlayment beneath should extend into the valley, and the shingles are installed in a way that the flashing remains the watertight channel. In short, the valley channels water, and correctly overlapped flashing ensures that water stays on the surface and off the roof deck.