_Materials to be Used_
It is not necessary to use expensive materials for fire-stops, but they
should be carefully placed. Materials like mineral wool are the best,
since they expand as the wood shrinks and fill up the space. Concrete
which is held in position by strips of metal lath is also excellent.
The concrete or mortar used can be made from refuse material, and need
not have any great strength. Old bricks are satisfactory if they are
slushed into position with mortar which fills all the crevices. Gypsum
blocks are good except for damp location, where they absorb moisture
easily and, holding it, induce dry rot in the surrounding timbers.
Asbestos board, gypsum board, and metal lath and plaster are suitable
for covering large areas, such as cellar ceilings, over the boiler.
In fact, fire-stopping can be cheaply done with odd-and-end bits of
material which usually go to waste around the building.
The details of constructing these fire-stops are best shown in the
illustrations, and no further descriptions will be necessary.
_Chimney Construction_
In view of what was said in the first part of this chapter, the
construction of a chimney by approved methods is also a safeguard
against fire. It can be considered a rule that every chimney should
be lined with a terra-cotta flue, that every chimney should be
an independent structure of its own, with walls thick enough for
stability, capable of standing upon their own foundations and not hung
from any part of the structure, that all woodwork of the building
should be framed far enough from the chimney to make no contact with
it, and, finally, that all the smoke-pipes which enter into the flues
should be proof against leakage of flames and heat of such intensity as
to cause combustion.
In the past this need of lining the flues of a chimney with
terra-cotta flue tiles was not considered important, but to-day it is
a well recognized fact that no chimney is safe without this protective
lining. There are many instances where chimneys are built without this
lining and show no fire dangers, but the action of flue gases is slow
and sure, and the mortar is attacked gradually, with the resulting
disintegration of the brickwork, through which the flames eventually
find their way to the surrounding wood timbers. It is found that even
where terra-cotta flue linings are used the hot gases from the burning
of natural gas as a fuel break down their resistance and they crumble,
so that in such cases the flue linings should be made of fire-clays.
From practical experience the minimum thickness allowable for any of
these flue linings should be 1 inch, and the joints should not be made
with collars.
When setting these linings they should be laid in cement mortar, not
in lime mortar, for this disintegrates under the action of gases from
burning wood. The joints should be struck smooth on the inside, and the
space between the lining and the brickwork filled in solid with mortar.
Wherever two flue linings are run within the same chimney space, the
joints should be staggered or offset at least 6 inches. Two linings,
however, in one chimney space should be the maximum number permitted.
Where more are required, each group of two should be separated by
brick walls of at least 4 inches, which are well bonded into the
outside walls of the chimney. This is in order to give stability to the
chimney and also prevent any fires in one flue spreading to others. The
thickness of outside walls of the chimney around the flues should not
be less than 4 inches if built of brick or reinforced concrete, but
if built of stone they should be 8 inches. Wherever there is no flue
lining of terra-cotta, such as in the smoke-chamber, the thickness of
the masonry from the interior to the exterior should never be less than
8 inches.
If chimneys are built of reinforced concrete, the reinforcements should
be run in both directions to prevent cracks during the setting of the
cement or from temperature stresses. Where concrete blocks are used,
reinforcements should run continuously around the blocks, and the shell
of the blocks should not be less than 4 inches thick.
Wherever the walls of dwellings are of brick and 12 or more inches
thick, they may be used to contain chimney flues. If it is necessary to
corbel out the flues from the wall, they should not extend farther than
4 inches from the face of the wall, and the corbelling should not be
done with less than five courses of bricks.
Next in importance to the correct lining of flues is the proper
construction of the foundation under chimneys. There are often cases
where it is necessary to cut off the chimneys below in part or in whole
to supply room on the first floor. This should be avoided as much as
possible, but if it cannot be done it should be supported by steelwork
from the ground up.
[Illustration: Fire place