Chimneys must extend at least 2 feet higher than any portion of the roof or any structure within 10 feet, but must be not less than 3 feet above the point where the chimney passes through the roof.

Masonry chimneys in modern construction are normally lined with terra cotta liners. Terra cotta liners deteriorate over time.  If the flue needs cleaning, it may be appropriate to have the flue re-evaluated after cleaning.