=Supporting the Chimney=
Stable foundations, preferably of concrete, should be provided, at least
6 inches wider all around than the chimney and 8 inches thick for
one-story and 12 inches thick for two-story houses. When there is no
basement or cellar (fig. 7, _A_), start the foundation of an exterior
chimney well below the frost line; otherwise, extend the base to the same
level as the bottom of the foundation of the building. Foundations for
tall, heavy chimneys require special consideration.
Where the wall of the house is of solid masonry 12 inches or more thick,
the chimney may be offset and carried on corbels or masonry brackets
instead of being carried down to the ground. The offset should not extend
more than 8 inches from the face of the wall, each course projecting not
more than 1 inch, and should not be less than 12 inches high. Often the
corbeling is started at the second- or third-floor level so that the
chimney is only one or two stories high.
[Illustration: Figure 8.--For structural safety the amount of offset must
be limited so that the center line, XY, of the upper flue will not fall
beyond the center of the wall of the lower flue. _A_, Offsetting of the
left wall of an unlined flue is started two brick courses higher than
on the right wall so that the area of the sloping section will not be
reduced after plastering; _B_, a lined flue showing the method of cutting
the tile.]
Chimneys in frame buildings should be built from the ground up or should
rest on the foundation or basement walls if of solid masonry 12 inches or
more thick.
A chimney resting on or carried by wooden floors, beams, or brackets or
hung from wooden rafters (fig. 7, _B_) is a fire hazard. Wood framing
shrinks, and beams supporting heavy loads deflect in time. Sagging beams
injure the walls and ceilings of the house and are apt to crack the
chimney, rendering it dangerous.
=Flue Linings=
Although, to save expense, chimneys are built without flue lining, those
with linings are more efficient. When the flue is not lined, the mortar
and bricks directly exposed to the action of fuel gases disintegrate.
This disintegration and that occurring from changes in temperature
frequently cause cracks in the masonry, thereby reducing the draft. An
unlined chimney is best if not plastered except at the sloped section
(fig. 8, _A_). However, the vertical and horizontal joints should be
filled with mortar and struck smooth and flush with the wall. Offsets or
bends in flues (fig. 8) should not be greater than 30° with the vertical.
This slope can be obtained by offsetting or corbeling each brick course
only 1 inch.
Flue lining must withstand rapid fluctuations in temperature and be
resistant to the action of ordinary flue gases. The shapes used as flue
lining should be of fire-clay, with shells not less than five-eighths
of an inch thick, and should be vitrified. As a safeguard against
over-burning and brittleness, the lining should be tested by submersion
in water at room temperature for 24 hours, during which a quantity of
water weighing more than 3 percent of the dry weight of the lining should
not be absorbed. Place each length of flue lining in position, setting
it in cement mortar with the joint struck smooth on the inside, and then
lay the brick around it. If the lining is slipped down after several
courses of brick have been laid, the joints cannot be filled and leakage
is almost sure to result. Fill any spaces between the lining and the
brickwork completely with mortar, especially if the round type of flue is
used.
The lower section of flue lining, unless resting on solid masonry at the
bottom of the flue, should be supported on at least three sides by brick
courses projecting to the inside surface of the lining. When laying brick
and lining, it is advisable to draw up a tight-fitting bag of straw as
the work progresses so as to catch material that might fall and block the
flue.
Where offsets or bends are necessary in lined flues, tight joints can
be made by mitering or cutting equally the ends of abutting sections
(fig. 8, _B_). This can be done if a cement sack of damp sand is stuffed
firmly into the lining and a sharp chisel is tapped with a light hammer
along the line where the cut is desired. If the cutting is done after the
lining is built into the chimney, the lining may be broken and fall out
of place. The hole for the thimble can be cut the same way when a special
thimble section is not used.
The linings commonly used are rectangular or round. Rectangular linings
are better adapted to brick construction than round linings, but the
latter are considered more efficient. The sizes commonly used are
indicated in table 2.
=Wall Thickness=
Walls of chimneys not more than 30 feet high when lined should be 4
inches thick if of brick and reinforced concrete, 8 inches if of hollow
building units, and 12 inches if of stone. Linings may be omitted in
chimneys having walls of reinforced concrete at least 6 inches thick
or of unreinforced concrete or brick at least 8 inches thick, although
lining is desirable in the case of brick construction. Also the outside
wall of a chimney exposed to the weather is best made at least 8 inches
thick.
In chimneys containing three or more flues, building codes generally
require that each group of two flues be separated from the other
single flue or group of two flues by brick divisions or withes not
less than 3¾ inches wide (fig. 9) . Where two flues are grouped
without divisions, joints in the linings of adjacent flues are safer
if staggered at least 7 inches, and particular care should be taken to
have all joints filled with mortar. Individual flues are advisable for
fireplaces and heating furnaces or boilers.