There are 3 ways water can penetrate a chimney and cause damage, the obvious one is straight down an uncapped flue. The flashing that provides a seal between the roof and the chimney may become damaged and allow water to come in the house. Water may also penetrate the masonry either at the concrete crown or be absorbed through the brick or block.

A Sweep Above The Rest can remedy the situation by reflashing, waterproofing the masonry or capping the flues.