All chimneys should be built from the ground up. None of the weight
should be carried by any part of the building except the foundation.
Proper foundations should be provided at least 12 inches wider all round
than the chimney. If the chimney is an exterior one, and there is no
basement or cellar, its foundation should be started well below the frost
line. Otherwise the base of the chimney should be at the same level as
the bottom of the foundation of the building.
No chimney should rest upon or be carried by wooden floors, beams, or
brackets, nor should it be hung from wooden rafters. Wood construction
shrinks, and beams supporting heavy loads always deflect in time. Sagging
of the beams injures the walls and ceilings of the house and is apt
to crack the chimney and render it dangerous. Chimneys usually extend
several feet above the roof, exposing considerable surface to the wind,
and unless the support is stable they are likely to sway during a gale
with the possibility of the joints at the roof-line opening. Openings in
a flue at this point are especially dangerous, for sparks from the flue
may come into contact with the woodwork of the roof. This swaying may
also cause leaks in the roof.
[Illustration: Fig. 7.--Chimney and roof connection. Sheet metal A should
have shingles K over it at least 4 inches. Apron B bent as at E with base
flashings C, D, and H and cap flashings P and G, lapping over the base
flashings provide watertight construction. When the chimney contains two
flues the joints should be separated as shown.]
The brickwork around all fireplaces and flues should be laid with cement
mortar, as it is more resistant than lime mortar to the action of heat
and flue gases. It is well to use cement mortar for the entire chimney
construction. All mortar used for chimney construction, except for laying
firebrick, should be proportioned as follows: Two bags of Portland
cement, not less than 188 pounds, and one bag of dry hydrated lime, 50
pounds, thoroughly mixed dry, and to this mixture should be added three
times its volume of clean sand with sufficient water to produce proper
consistency. When dry hydrated lime is not available, 1 cubic foot of
completely slaked lime putty may be substituted for the dry hydrate.