Our chimney is exposed two stories interior, one story on the south exterior, two stories on the north. Material is gneiss, struck joints, bluestone rain cap, constructed 2006.
When new it experienced water wicking during heavy rains, especially when wind-driven, sufficient to destroy ceiling and wall drywall, and threaten dry rot. Treatment of the exterior in 2009 with silicone masonry sealer (to refusal to accept additional solution) significantly mitigated the problem, but it has begun to recur.
The flashing is NOT the problem.