Inside Cross-sectional area Wall
diameter --------------------- thickness
(inches) Inside Outside
----------- ---------- -------- ----------
Square Square
inches feet Inches
6 28.3 0.29 5/8
8 50.3 .49 3/4
10 78.5 .75 7/8
12 113.0 1.07 1
15 176.7 1.62 1-1/8
18 254.4 2.29 1-1/4
20 314.1 2.82 1-3/8
22 380.1 3.48 1-5/8
24 452.3 4.05 1-5/8
27 572.5 5.20 2
[B] All rectangular flue lining is 2 feet long.
[C] Round flue lining, 6 to 24 inches in diameter, is 2 feet long; that
27 to 36 inches in diameter is 2½ or 3 feet long.
[Illustration: Figure 9.--Cross section of chimney showing the proper
arrangement for three flues. The division wall should be well bonded with
the side walls by staggering the joints of successive courses. Note the
studs are kept 2 inches away from the brickwork for reasons explained on
page 14.]
When two or more flues are used in unlined chimneys, they must be
separated by well-bonded withes 8 niches thick. An attractive and
effective method of separating unlined flues in colonial times is shown
in figure 10.
Chimneys extending above the roof are exposed to the wind and may sway
enough during a gale to open up the mortar joints at the roof line.
Openings in a flue at this point are especially dangerous because sparks
from the flue may come in contact with the woodwork of the roof. It is
therefore good practice to make the upper walls 8 inches thick (fig. 11)
by starting to offset the bricks just below the intersection with the
roof.
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. A good mortar to use in setting flue linings and all
chimney masonry, except firebrick, consists of 1 part portland cement,
1 part hydrated lime, and 6 parts clean sand, measured by volume.
Slacked-lime putty may be used in place of hydrated lime; firebrick is
best laid in fire-clay.
[Illustration: Figure 10.--This Williamsburg chimney shows the pains
taken to make the chimney attractive. The three flues are arranged as a T
with well-bonded withes between them. Often four flues were used in the
form of a cross.]
_Openings Into the Chimney_
No range, stove, fireplace, or ventilating register should be connected
with the flue being used for the heating apparatus because this is a
frequent cause of unsatisfactory operation. Fires may occur from sparks
passing into one flue opening and out through another where there are two
connections to the same flue. If an abandoned fireplace chimney is to be
used for a range or stove, close the fireplace flue tight about a foot
below the smoke pipe hole.
[Illustration: Figure 11.--Greater resistance to the weather is provided
by building the exposed upper section of a chimney with 8-inch walls.
Also the mortar joint, in which the counter-flashing is embedded, is not
so likely to fail as it is when the wall is only 4 inches thick.]
Gas-fired house heaters and built-in unit heaters, if not connected to a
masonry chimney, may be connected to flues of corrosion-resistant sheet
metal not lighter than 20-gage, properly insulated with asbestos or other
fireproofing material that will comply with the recommendations of the
Underwriter's Laboratories, Inc. Such flues should extend through the
roof.
A soot pocket[2] is desirable for each flue. Deep pockets permit the
accumulation of soot, which may take fire; therefore start them from a
point preferably not more than 8 inches below the center line of the
smoke pipe intake and fill the lower part of the chimney with solid
masonry instead of extending the pocket to the base of the chimney as is
often done. Clean-out doors are necessary at the bottom of deep pockets
and, if used, must fit snugly and be kept tightly closed so that air
cannot get in. Clean-outs should serve only one flue, for if two or
more flues are connected with the same clean-out, air drawn from one to
another affects the draft in all of them. Sometimes a door is placed just
below the smoke pipe, but one is not really necessary since the pipe, if
taken down each year for cleaning, allows removal of soot from shallow
pockets through the pipe hole.
[2] See soot pockets and clean-out doors in figures 12 and 35, pp. 13 and
38, respectively.
Close pipe holes, when temporarily not in use, with tight-fitting metal
flue stops; but, if a pipe hole is to be abandoned, fill it with bricks
laid in good mortar. This stopping can be readily removed. The practice
of closing a pipe hole with papered tin is dangerous, for if there is
another stove connected with the flue, the metal may become hot enough to
scorch the unprotected wallpaper or even set it afire.
Proper care in setting and looking after pipe at its connection with the
chimney will greatly lessen the number of fires chargeable to defective
construction. Fit the pipe so that no opening will be left around it,
and keep it from projecting into the flue. The connection can be made
airtight with a closely fitting collar and boiler putty, good cement
mortar, or stiff clay.
Smoke pipes should enter the chimney horizontally, and the hole through
the chimney wall to the flue should be lined with fire-clay, or metal
thimbles should be securely and tightly built in the masonry. Thimbles
or flue rings can be had of 6-, 7-, 8-, 10-, and 12-inch diameters and
6-, 9-, and 12-inch lengths. If the walls are furred (fig. 12), the space
between the thimbles and the wood furring should be covered with metal
lath and plaster.
[Illustration: Figure 12.--_A_, Connection to chimney where furring is
used. The brick are built out around the thimble as a protection against
its cracking. This is a fire hazard that is frequently overlooked. _B_,
Connection when plaster is applied directly to the masonry. Note that the
pipe extends too far into the flue. It should be as shown in _A_.]
When a smoke pipe is less than 18 inches from woodwork, the woodwork
requires protection against charring. A metal casing or asbestos board 2
inches from the upper half of the pipe is sometimes employed to protect
woodwork directly above it. A pipe, even so protected, should never be
closer than 9 inches to any woodwork or other combustible material.
Commercial fireproof pipe coverings can be purchased.
If a pipe must be carried through a wood partition, protection for the
woodwork can be provided by cutting an opening in the partition and
inserting a galvanized-iron double-wall ventilating shield at least 12
inches larger than the pipe (fig. 13) or by using at least 4 inches of
brickwork or other incombustible material. Smoke pipes should never pass
through floors, closets, or concealed spaces or enter a chimney in a
garret.
Gases formed by burning the sulfur contained in coal are the main cause
of corrosion of metal smoke pipes. Little corrosion occurs during the
heating season, when the pipe is kept hot and dry.
The life of metal pipes can be prolonged if each summer when they are not
in use they are taken down, cleaned, wrapped in paper, and stored in a
dry place. This is especially true of pipe to heaters in damp cellars.
[Illustration: Figure 13.--_A_, Elevation of protection around a
stovepipe passing through a frame partition; _B_, sectional view.]
[Illustration: Figure 14.--Method of insulating wood floor joists and
baseboard at a chimney with 4-inch walls. A single header is used as it
is less than 4 feet long.]
_Insulation_
No wood should be in contact with a chimney. Leave a space of 2 inches
between the outside face of a chimney and all wooden beams or joists
except when 8 inches of masonry is used outside flue lining, in which
case the framing may be within one-half inch of the chimney masonry.
The space between the floor framing and the chimney may be filled with
porous, nonmetallic, incombustible material, such as loose cinders.
Brickwork, mortar, and concrete are not suitable. Place the filling
before the floor is laid, as it not only forms a fire stop but prevents
accumulation of shavings or other combustible material. Subflooring may
be laid within one-half inch of the masonry. Baseboards, when fastened to
plaster that is directly in contact with the wall of a chimney, can be
protected by a layer of fireproof material, such as asbestos, at least
one-eighth of an inch thick between the woodwork and the plaster (fig.
14).
Wooden studding, furring, or lathing should not be placed against a
chimney but set back, as indicated in figure 9; or the plaster may be
applied directly to the masonry or to metal lath laid over the masonry.
The former is the better method, as settlement will not crack the
plaster. It is recommended that a coat of cement plaster be applied
directly upon the outside surfaces of masonry chimneys that are to be
incased by a wooden partition or other combustible construction. Metal
lath, lapped 6 inches on the masonry, at the intersection of chimneys
with partitions prevents corner cracks. (See plan in fig. 34.)
_Chimney and Roof Connection_
Where the chimney passes through the roof, provide a 2-inch clearance
between the wood framing and masonry for fire protection and for
expansion due to temperature changes, settlement, or slight movement of
the chimney during heavy winds.
[Illustration: Figure 15.--Method of flashing. Sheet metal, _h_, over
the cricket, extends under the shingles _k_, at least 4 inches and is
counter-flashed at _l_ in joint. Base flashings _b_, _c_, _d_, and _e_
and cap flashings _a_, _f_, and _g_ lap over the base flashings and
provide watertight construction. A full bed of mortar should be provided
where cap flashing is inserted in joints.]
A chimney must be flashed and counter-flashed (fig. 15),[3] to make
its junction with the roof watertight. When the chimney is not located
on the ridge but on a sloping roof, a cricket, _j_, is built, as
detailed in figure 16, high enough to shed water around the chimney.
Corrosion-resistant metal, such as copper, galvanized metal, zinc, or
lead, is best for the flashing and counter-flashing. When tin is used,
paint it well on both sides.
[3] See p. 26, Farmers' Bulletin 1751, Roof Coverings for Farm Buildings
and Their Repair, for method of installing flashing.
A feature, said to have originated in colonial Williamsburg as a
precaution against fire hazard, is to build the upper section of outside
chimneys 18 inches to 2 feet away from the gable ends of the house (fig.
17). This is not only a safety factor but a practical one because the
chimney can be more easily flashed, small windows can be used in the
walls of upper story rooms behind the chimney, and framing the roof is
simplified.
_Capping the Chimney_
Various methods of terminating chimneys are shown in figures 11 and 18.
Whatever one is used should be architecturally acceptable, effective in
preventing disintegration, and so made as to keep water out of the flue.
[Illustration: Figure 16.--Cricket, _j_, as seen from the back of the
chimney shown in figure 15. A section through the cricket is also shown.
Note how counter-flashing is built into the mortar joint at _l_.]
It is advisable to project the flue lining 4 inches above the cap or top
course of brick and surround it with at least 2 inches of cement mortar
finished with a straight or concave slope to direct air currents upward
at the top of the flue; the sloped mortar also serves to drain water
from the top of the chimney. (See fig. 11.) Hoods are commonly used to
keep rain out of a chimney (fig. 18, _A_ and _B_). The area of the hood
openings should be at least equal to the area of the flue and each flue
should have a separate hood. Concrete and brick caps are usually made 4
inches thick, and it is advisable to project them an inch or two to form
a drip ledge.
Many of the chimneys built today are unsightly and frequently detract
from an otherwise well-designed house. Within the last 100 years the
size and attractiveness of chimneys ordinarily built has declined. The
large old chimneys of colonial days were proportioned to suit the house
and surroundings and at the same time provide for two or more large
fireplaces. With reduction in the size of fireplaces and the substitution
of several stoves and eventually one central heating plant, the chimney
has developed into a merely utilitarian shaft.
[Illustration: Figure 17.--A house in southern Maryland in which the
space between the chimney and the house wall shows clearly. The practice
of building the chimney in this way is common in the tidewater section of
the South.]