whats a roof valley and why does it always leak

Most of a roof does very little work. A tile or a sheet in the middle of a plane sheds a small area of water and is protected by everything around it. Valleys and flashings are the exceptions: they concentrate water from large areas into narrow channels and seal the awkward junctions, which is why they carry a disproportionate share of failures.

How valleys fail

• Corrosion, particularly in older galvanised valleys and in coastal areas.
• Debris. Leaves collect in valleys, hold moisture against the metal, and eventually dam water high enough to go sideways under the tiles.
• Tiles cut too close, leaving too little valley width for storm flow.
• Incompatible metals in contact, which drives galvanic corrosion and can eat through a valley in years rather than decades.

How flashings fail

• Sealant used instead of a properly formed and fixed flashing, which lasts a few years rather than decades.
• Insufficient upstand where a roof meets a wall, so wind-driven water gets over the top.
• Movement between two elements, particularly where a newer extension meets an older structure.
• Penetrations added later for flues, vents, aerials or solar, which is a common source of leaks in otherwise sound roofs.