Ordinarily the fire is built on the hearth, no grate being used. Fire
regulations in hazardous localities may require firing doors, dampers,
spark screens, and a solid-plate cooking surface; otherwise these
features are not essential. Two and a half square feet of cooking surface
is desirable, while access to both sides and the end permit several
people to cook at the same time. The side walls should have fairly level
tops for pots and pans. Side walls are made 2 to 6 inches higher than the
cooking level to permit anchoring the grille; if too high, they interfere
with cooking. Commercial grilles are available, but satisfactory ones can
be made of ½-inch to ¾-inch pipe or 5/8-inch reinforcing rods. The
pipes should be 6 to 10 inches longer than the width of the firebox; they
should be spaced not more than 1¼ inches apart and have their tops
exactly level to prevent pots and pans from wobbling. Two or three pipes
can be used for a lintel over the opening into the flue if regular iron
lintels are not available. Where a solid top is desired, it should be of
boiler plate at least ¼-inch thick. Such plates must be stiffened to
prevent buckling by alternate heating and cooling; for ordinary purposes
they are merely set on top of the grid though they may be hinged at the
rear so they can be tipped back against the chimney.
The best draft is secured when the fireplace faces the direction of
prevailing breezes and is protected from strong winds which might scatter
sparks. If the fireplace is built too near shrubbery or under trees,
the heat and smoke may damage or burn the foliage. A slight rise or a
gentle slope that affords good drainage should be selected. Paving the
ground around the fireplace, with flagstones or covering it with a layer
of gravel or sand will prevent the area from becoming a mudhole or an
unsightly bare spot; also, danger of starting brush fires by sparks
falling from the firebox is lessened.
Fireboxes 12 to 16 inches wide, 16 to 24 inches long, and 6 to 8 inches
deep with the hearth at 9 to 16 inches above the ground are sufficient
for most purposes. Large fireboxes are wasteful of fuel; while, if the
grille is too high above the hearth, much of the best cooking heat from
glowing coals is lost. Most grilles are set 15 to 24 inches above the
ground, though 30 inches may be desirable to avoid the necessity of
stooping when cooking. The hearth should slope 1 to 2 inches toward the
front so that rain water will drain away.
The area of the chimney flue should be at least one-eighth the vertical
cross-sectional area of the firebox.
Fire-clay linings for the firebox and flue are not absolutely necessary
except when required by fire regulations or where hot fires are
maintained for long periods. They, or common brick linings, are advisable
for the more permanent and expensive structures or where it is necessary
to use porous stone, such as sandstone and most stratified rocks, which
absorb water and flake or chip upon exposure to fire.
Most rocks or stone that can be worked up without special tools or skill,
brick, and concrete are adaptable for the average fireplace (fig. 43).
The size of the stones determines the thickness of the walls; no wall
should be less than 8 inches thick.
Where suitable stone is difficult to get in sufficient quantity, the
exposed surface may consist of a shell the thickness of the stone and the
inner portion of the wall be made of concrete or large stone bedded in
concrete. When flue lining is necessary, it will serve as a form for the
flue: otherwise a metal stovepipe makes a practical form, or the flue can
be formed of brick laid on edge. The stones can be laid and the concrete
deposited with the least trouble by building the veneer and flue only 6
to 8 inches high at one time. All masonry should be laid in mortar, as
described on page 10. Concrete made in the proportions 1:2½:4 will serve
most purposes where wall sections are about 8 inches thick. For heavy
foundations and thick walls not subject to direct fire, 1:3:6 concrete is
strong enough.
A concrete slab 4 to 8 inches thick with the bottom 4 to 6 inches below
the surface provides a sufficient foundation for medium-sized structures
where frost is negligible and the soil is well drained and firm. If
the soil is not well drained a 6- to 8-inch layer of stone, cinders, or
sand should be provided under the slab and the surface of the immediate
vicinity graded or otherwise protected from water. It is advisable to use
¼-inch or ½-inch reinforcing rods, 6 inches apart in both directions,
one-third the distance from the top in slabs for all but the smallest
fireplaces or where frost is not severe.
[Illustration: Figure 43.--_A_, Ground plan of a stone fireplace that can
be built in different sizes to suit the landscape; _B_, vertical-section
sketch; _C_, the completed fireplace. The stones for this fireplace
were picked up at "the swimming hole." About 8 bushels were used over a
backing of concrete; 16 bushels would have been needed if the concrete
had not been used. Gravel and sand were dug from the excavation. The
chimney is battered 4 inches in the 24-inch height.]
Heavy and expensive structures, especially those having tall chimneys,
should have foundations below the surface affected by frost or erosion
and strong enough to prevent settlement or cracks. Such a foundation can
be made of concrete, with a liberal use of large stones for economy,
extending under the whole structure, or be continuous walls with a
footing similar to those used for houses. The advice and help of builders
may save money in the construction of foundations of large expensive
fireplaces, especially where climatic conditions are severe and the
bearing power of soil is not known.