Why is my roof leaking in the valley?

A roof valley is one of the most vulnerable areas of a home's exterior because it is tasked with collecting, holding, and channeling massive volumes of water runoff from multiple intersecting roof planes. Leaks in the valley commonly occur due to the accumulation of organic debris like wet leaves, pine needles, and twigs, which clog the channel and cause water to back up underneath the shingles or flashing. Over time, physical wear and tear, rust holes in metal valley liners, cracked or unsealed seams, and improper initial installation can also compromise the waterproof barrier. Furthermore, heavy winter freeze-thaw cycles can create ice dams in the valley, forcing standing water to push upward past the underlayment and seep directly into the attic or ceiling joists, highlighting the critical importance of keeping valleys clean and well-maintained.