Chimneys develop defects which if not promptly repaired cause trouble.
Most masonry requires replacement of worn or weathered material or
repointing of mortar joints, while flues become clogged and flashings
fail.
It is advisable to test a chimney every few years for tightness by the
smoke test just described; to examine the inside of the flues by lowering
a lantern or flashlight on a strong cord down from the top of the chimney
or by holding a hand mirror at the proper angle at a stovepipe hole; to
inspect the masonry for loose units, which are most likely to occur at
the top (fig. 3) where the action of the flue gases, especially when soft
coal is burned, disintegrates the mortar; to test mortar joints from the
outside by prodding with a knife or similar tool to determine if the
mortar is loose clear through the joint so as to leave a hole; and to
notice if the chimney is damp because of leaky flashings, absorption of
moisture from the ground, condensation, or excessive rain entering the
flues.