Trapped water activates aging processes.

Water's central role in stone decay is well established. Water activates the processes* that age building stone — of any kind. When stones dry, aging stops. Even delicate sandstone can last indefinitely if kept dry.

*Aging processes: Water activates the processes that age building stone of any kind, including slate. Water permits acids to attack. It gives effect to freeze-thaw cycling (which causes material fatigue). It converts calcite to gypsum (which swells and undermines the slate's substrate). And, together with chemically unbound oxygen, it oxidizes "pyrites".