The chimney must extend at least 3
feet above the highest point where it
passes through the roof. In addition,
the top of the chimney should be at
least 2 feet above any portion of the
building that is within 10 feet of the
chimney. The best draft is achieved if
the top of the fireplace opening to the
top of the chimney is at least 15 feet.
If this distance is less than 15 feet, a
larger flue size may be necessary.
CHIMNEY CAP
Every masonry chimney should have
a chimney cap to terminate the
masonry and protect the chimney.
This cap can be precast concrete or
cast in place concrete. The chimney
cap should extend beyond the exterior
face of the masonry and incorporate
a drip slot or other characteristic to
CLEARANCE TO
COMBUSTIBLES
One of the most important fire safety
requirements is the air space that
separates the chimney from the
combustible materials such as the
framing. For interior chimneys this
space must be a minimum of 2 inches.
For an exterior chimney this space
must be a minimum of 1 inch. This
space insulates the chimney and
minimizes the transfer of heat to
combustible materials. It must not
be filled except for required fire stop-
ping. When masonry fireplaces and
chimneys are part of a masonry wall,
combustible materials should reside not
less than 12 inches from the outside
surface of the flue lining or firebox.
Exposed combustible trim and the
edges of sheathing materials such as
wood siding, flooring and drywall
should be at least 8 inches from the
inside surface of the fireplace or
flue lining.
Woodwork or other combustible
materials should not be placed within
FLASHING
Base and counter flashing should be
installed at the chimney / roof surface.
The base flashing should be installed
first, on the faces of the chimney
perpendicular to the ridge line with
tabs at each corner. The flashing
should extend a minimum of 4 inches
along the roof surface. Counter flash-
ing should then be installed over the
base flashing, inserted into the mortar
joint 3/4 to 1 inch wide, and mortared
solidly into place. The counter flashing
should lap the base flashing by at least
3 inches. If the flashing is installed in
sections, the flashing higher up the roof
line should lap over the lower flashing
a minimum of 2 inches. All joints in
the same base and counter flashing
should be sealed thoroughly, with
any exposed side of the bends in the
flashing also sealed.
the building. Warm air tends to flow
out from upper rooms, while cold air
flows down to the lower rooms in the
house. If you determine that you need
a permanent source of makeup air,
contact a heating and ventilation
specialist and discuss the various
options. Fireplaces will exhaust
100-cfm to 1,000-cfm depending on
the size and intensity of the fire.
REDUCING CREOSOTE
BUILDUP
Burning wood and fossil fuels in the
absence of adequate airflow causes
incomplete combustion of the oils in
the wood, which are off-gassed as
volatiles in the smoke. As the smoke
rises through the chimney it cools,
causing water, carbon and volatiles
to condense on the interior surfaces
of the chimney flue. The black oily
residue that builds up is referred to as
creosote. Creosote deposits reduce the
draft (airflow through the chimney)
which increases the probability that
the wood fire is not getting enough
air for complete combustion. Since
creosote is highly combustible, a thick
prevent moisture penetration from the
top of the masonry wall to interior of
the chimney. The cap should never
be cast tightly against the flue liner.
A gap should be left to accommodate
expansion and contraction of the flue
lining. The gap at the top between
the cap and the flue liner should be
sealed with caulk to prevent water
or moisture from entering the
chimney. This is a maintenance joint
that should be inspected on a regular
basis to ensure water tightness. It
should be replaced when necessary.
The use of quality polysulfide, butyl
or silicone rubber caulking compound
is recommended. Oil based sealants
do not perform well in this applica-
tion. Backer rod also is suggested to
support the caulk around the flue.
6 inches of the fireplace opening.
Combustible materials that are within
12 inches of the fireplace opening
should not project more than 1/8 inch
for each 1 inch distance from such
an opening.
BALANCING THE
VENTILATION
In order for a fireplace to function
properly, a natural supply of makeup
air must be available to replace the
exhausted air from the chimney. In
older homes there is often enough
leakage around doors and windows to
provide this air. In newer, tightly built
homes another source of air must be
provided. Before lighting the fire, use
a match stick or candle to determine
if there is an updraft or a down draft
in the chimney. If there is no updraft
try cracking open a window or door
to provide a source of air to the room.
If insufficient air is present, you may
need to locate other sources of air
entering or being exhausted from the
room. The use of exhaust fans and
opening an attic or upstairs rooms
may be a source of air flowing out of
MATERIAL SPECIFICATIONS
Use only those materials conforming to the following specifications:
A. Concrete Block – conforming to ASTM C-90 Grade N, ASTM C-129.
B. Mortar Mix – ASTM C-270 Type N.
C. Concrete Mix – ASTM C-39 (for the hearth slab and chimney cap).
D. Brick – ASTM C-55 (or 216 Grade-SW).
E. Caulking – Polysulfide, Butyl, or silicone rubber.
F. Flashing – Corrosion resistant metal. No aluminum.
G. Clay Flue Liners – Clay Flue Lining Institute and ASTM C-315, C-1283.
H. Firebrick – ASTM C-27 or C-1261 low duty 2 inches thick minimum.
I. Refractory Mortar – Non-water soluble refractory mortar is best for installing both firebrick
and clay flues. Homemade mixes combining fireclay and mortar simply do not meet the code.
Refractory parge for the smoke chamber must also be non-water soluble. All of the above must
meet the high temperature requirements of the ASTM C-199 medium duty code.
J. Dampers – Should be certified to have air filtration losses not to exceed 20 cubic feet per minute
when in the closed position.
K. Steel Angle – Must meet ASTM A-36 with minimum dimensions as specified in Chart II in each
fireplace drawing.
Chimneys, con’t.
8
9
