Government specifications for fireplace mortar, which must be used in
laying all work except the fire brick, are as follows: hydrated lime,
nine pounds to each 94-lb. bag of Portland cement; mix this with three
times its volume of clean sand and enough water to make a smooth
workable mortar. Wet all stones before applying the mortar. _Don’t use
ordinary_ lime mortar for fireplaces and chimneys.
Construction of the flue is the next important matter. The flue is
installed as the chimney is built and must be planned with a fairly
close relationship to the fireplace itself. Fireplace experts estimate
that the correct ratio is 14 to 16 square inches of flue area to each
square foot of fireplace opening.
A round flue is the most satisfactory type. It offers less friction and
is easier to clean. Square or oblong flues should be slightly larger
than if a round one is used. The easiest method—and one that assures a
tight flue—is to use fire clay flue lining, filling in between it and
the stone shell with cement. Be sure that the flue is as straight as
possible and uniform in size right up to the top. Don’t taper it. If
turns must be made in it, make them gradual—never greater than an angle
of 45 degrees.
Don’t try to use one flue for fireplace and kitchen stove or another
fireplace. Make separate flues for each. They may go up the same chimney
but the two or more flues should be separated by at least four inches of
solid brick or concrete unless tile flue linings are used. When you have
reached the top, test the flue for leaks by lighting a fire and covering
the top of the chimney. Escaping smoke will reveal any leaks, which must
be repaired at once.
[Illustration: Diagram of a patented fireplace form showing how it
heats and circulates the air. The masonry is laid around the form.]
SMOKE DOME
DAMPER
DOWN DRAFT SHELF
APRON—TO BE COVERED BY MASONRY
WARM AIR OUTLET
DAMPER CONTROL
HEATING CHAMBER
BOILER PLATE FIREBOX
COLD AIR INTAKE
Location of your house will affect the action of the chimney, since
large objects such as trees or a hill nearby will cause a down draught.
If this trouble is encountered, it may be corrected by building an arch
over the chimney cap. The arch should be placed at right angles to the
cause of the down draught.
The chimney cap itself, which is an important feature affecting the air
currents that pull the smoke out, should be about 12 inches high. It can
be made of cement and stone chips, modeled by hand. Reinforcing rods or
wire mesh may be added if desired. The sides of the cap should curve
from the outside edge of the masonry proper to the top of the cap.
A good type of flashing, which must be installed to prevent leaks
between the chimney and the roof, is made of sheet lead, which is
everlasting. Galvanized iron or copper is also satisfactory. The
flashing should be cut to fit the chimney like a collar and is imbedded
in the masonry. The roofing should overlap the upper edge several
inches, and the flashing must overlap the roofing at the lower edge if
set on a slanting roof. In case the chimney comes at the ridge,
flashings are fastened over the roofing all around.
While you’re up on the roof finishing the chimney, it would be a good
idea to take the measurements of the entire top. In your leisure time,
make a fairly tight cover, shaped like a shallow box, that will fit on
top of the chimney when you close the cabin for the season. It can be
lashed down with wire or rope. The object of this will be well known to
anyone who has left a cabin chimney open for several months, because it
seems to have a great attraction for birds and squirrels—not to mention
the fact that snow and rain get in and rust the flue lining if it’s made
of metal.
In planning your cabin fireplace don’t fail at least to investigate the
several so-called “patented” fireplace forms which circulate heated air.
(See illustrations.) These are especially worth considering if you
expect to rely upon your fireplace for heating the cabin during the
colder months, and also greatly simplify the intricate job of fashioning
a fireplace.
When using these forms the fireplace is constructed with an air chamber
behind the fire box. Cold air is drawn off the floor and circulated back
in the room as warm air. This gives you both direct radiated heat and
warm air from absorbed heat. Various installations are possible, with
the added heat being used in an adjoining room—or even upstairs.
[Illustration: Another patented fireplace design showing how warm
air may be piped to adjoining rooms]
WARM AIR OUTLET
COLD AIR INTAKE