15
ODSHR Series
20002264
8. Hearth Installation
A hearth extension is required to protect a combustible
floor in front of the fireplace. See Figure 24 for mini-
mum dimensions and mounting detail.
NOTE: Hearth Extension must not cover the air
inlet opening of a fireplace.
The hearth extension described in Figure 24 must be a
durable noncombustible material with a minimum (total)
Rt value of 1.09; see Figure 25 for examples. The
overall height (above a combustible floor), depth and
width must be as indicated, with the extension centered
to the fireplace opening.
The top of insulation must be covered with a non-
combustible decorative covering or a piece of .018”
minimum sheet metal, to protect hearth extension
material. (Fig. 24)
Secure the hearth extension to the floor to prevent
shifting, using trim molding or other similar means at
three (3) outer edges. Seal crack between the fireplace
hearth and hearth extension with a noncombustible
material. (Figs. 24, and 26 - 28)
WARNING: HEARTH EXTENSION MUST BE IN-
STALLED IN ACCORDANCE WITH FIGURE 24.
Alternate noncombustible materials may be used
providing the (total) thermal resistance (Rt value) of the
alternate material employed is greater than or equal to
R = 1.09. Thermal resistance (R) or thermal conductiv-
ity (K), may be obtained from manufacturer of the
material. Factors are related by the formula K = 1/R.
(Fig. 25)
T = given thickness
R = thermal resistance for a given thickness (T)
K = thermal conductivity
Non-combustible material with a lower R value may be
used, provided thickness of material is sufficiently
greater to maintain an equivalent (total) thermal resis-
tance (Rt).
Example of Determining Hearth Extension
Equivalents
To determine the thickness required for any new
material:
NEW
K of new material (per inch)
thickness
required +
X
of listed
thickness
K of listed material (per inch)
material
Example for Common Brick
T (new) = 5.0/0.458 x 0.50 in. = 5.46 in. (new required
thickness).
FP532b
Fig. 24 Combustible side wall protection and hearth extension dimensions.
Minimum Wall Clearances
WITH
Noncombustible
Surround Facing
WITHOUT
Noncombustible
Surround Facing
Minimum Hearth Extension Dimensions
(for On-Site Construction)
12"
20"
12"
66"
Seal cracks
between the
fireplace
and hearth
extension with
noncombustible
material
Safety strips
must overlap
¹⁄₂
" minimum
May install
noncombustible
decorative
covering
OR .018" min.
sheet metal
Fireplace
Hearth
Combustible
Floor
Minimum
Insulation
Value "R":
4" Min.
Firebox
Opening
22" - Min. clearance
to combustible
perpendicular wall
18" - Min. clearance
to combustible
perpendicular wall when
using noncombustible wall shield*
Side
Wall
Side
Wall
14"**
32
¹⁄₂
"**
18"
18"
39
¹⁄₂
"
4" Brick
(Example material)
Combustible material permitted within shaded area.
* Noncombustible wall shield requires 1" Majestic EH2416
insulation (minimum R Value = 1.85) between decorative
noncombustible rigid covering and combustible wall.
Minimum height and width is 40" x 40".
** Dimension/degree of angle will vary depending on thickness
of noncombustible surround facing.
4"
Shaded area starts
3/4" away from
edge of unit
