Solid masonry is the most satisfactory and safest material to use
for chimneys and fireplaces. If a chimney fire occurs, the safety of
the building may be dependent on the soundness of the flue walls
(fig. 2). Cracked and leaky flues not only are inefficient, destroying
the draft as well as permitting smoke and gases to pass into adjacent
rooms, but are a dangerous fire hazard. The chimney as known today
was developed about 600 years ago. Experience has shown that the
satisfactory performance of a chimney flue is determined by its size,
direction, shape, height, tightness, and smoothness.