C. If the floor in front of the fireplace is combustible, either a
raised or floor-level protective hearth extension must be
constructed. For flush-face models only, a raised hearth
extension may be flush with the fireplace hearth. To
construct a raised hearth extension with louvered
models, the fireplace must rest on a raised platform. A
raised hearth extension must have minimum dimensions as
follows:
?
and be constructed of materials with an R -factor equal to
or greater than 1.20.
?
A hearth extension installed directly on the floor must be
a minimum of 16" x 52" and be constructed of materials
with an R-factor equal to or greater than .80. If
combustible materials are used to construct the hearth
extension, they must not touch the black surface of the
fireplace. The same material that is used to protect the
top of the hearth extension must be placed between the
combustible hearth and the black face of the fireplace.
The hearth extension must be fastened to the floor to
prevent shifting and the gap between the fireplace and
the hearth extension must be sealed with a noncombus -
tible material (400
0
F. min) if a metal safety strip is not
used. These materials (listed below) may be used for a
wall shield as well.
D. The framing header may rest on top of the standoffs.
E. The minimum distance from the fireplace opening to an
adjacent combustible wall is 15½". The minimum
distance to an adjacent combustible wall may be reduced
to 12" when an approved wall shield is used on the wall.
The wall shield must be 40" x 40" and be constructed of
a noncombustible, inorganic material having a thermal
resistance of R = 1.49.
Determining the R-Values
The hearth extension must be constructed of non-
combustible materials which have a total thermal resistance
(R factor) equal to or greater than .80 for floor level hearth
extensions or 1.20 for raised hearth extensions and be a
minimum size as shown in the table on page 5. Choose the
desired materials and obtain the K value at 75° mean
temperature. The C value and the R value may be
calculated with the following formulas:
K = Thermal conductivity. K = BTUs-ins./hrs.-ft.
2
-
0
F
T = thickness C= Thermal conductance
R = Thermal resistance K/T = C; 1/C = R
Example
: 3/4" Marble with 3/8" Micore
Determine the R value for each material used as follows:
Marble:K/T= 11/.75=14.66.
1/C = 1/14.66 = 0.068 (R factor)
Micore 300: K/T = .458/.375 = 1.22.
1/C = 1/1.22=0.82 (R factor)
After the R value is obtained on each material in this hearth,
add the R values to obtain total thermal resistance (R).
Total R factor = 0.068 + 0.82=0.89
The total must be equal to or greater than specified above.
Typical materials: Micore 300 has a K of .458; Micore 230 has a
K of .43; Micore 180 has a K of .34; Ceraboard has a K of .34;
common brick has a K of 5; cement mortar has a K of 5; marble
has a K of 11; limestone has a K of 6.5; tile has a K of 12; slate
has a K of 21; Wonder Board has a K value of 3.2.
MICORE NC 180-300, manufactured by U.S. GYPSUM
CORPORATION
CONWED SPEC 300, manufactured by CONWED
CORPORATION
CERA FORM TYPE 106R board, manufactured by
JOHNS-MANVlLLE.
WONDER BOARD, manufactured by GOLD BOND
Figure 4
6
X
Y
36" models
16"
52"
42" models
20"
66"
Framing Members
