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 com-
bustible materials are used to construct the hearth exten-
sion, they must not touch the black surface of the fire-
place. The same material that is used to protect the top
of the hearth extension must be placed between the com-
bustible 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 noncombustible mate-
rial. These materials (listed below) may be used for a
wall shield as well.
D. The framing header must rest on top of the standoffs.
E. The minimum distance from the fireplace opening to an
adjacent combustible wall is 15½". The minimum dis-
tance 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 resis-
tance (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. Typ-
ical 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; mar-
ble 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
BUILDING PRODUCTS.
Figure 4
