New Buck Corporation 85 - Manual (Page 8)

MINIMUM CLEARANCES
1. The hearth must be of masonry construction and must extend a minimum of 16” in front of
the firebox opening and a minimum of 8" to either side of the firebox opening.
2. If there is not minimum hearth protection from the front of the firebox opening and the
front of the masonry hearth, a floor protector must be used in front of the hearth to
protect combustible materials. The floor protector must be 3/8" minimum thickness non-
combustible material or equivalent.
With the floor protector or hearth extension material specified as one layer of 2" thick brick
with a K* factor of 5.0, you may use alternate materials as long as the K* factor (s) is calculated
as follows:
FORMULA: Required thickness = (K of alternate material/K of millboard) X (thickness
of millboard)
EXAMPLE: An alternate inorganic non-combustible millboard with a K* of .9375
would require a thickness as follows:
Required thickness = (.9375/5.0) x 2 in.
= 3/8 inch
K* FACTORS OF ALTERNATE MATERIALS
Material
K Required Thickness
Ceroform 126 .21 1/8"
Limestone 8.0 3.2"
Concrete 8.0 3.2"
Sandstone 13.0 5.2"
Marble 18.0 7.2"
Hearth extension or floor protector thickness above four (4) inches are generally not acceptable
due to cost, floor load capacity, and look. The alternate we recommend is using Ceroform No.
126 millboard 1/8" minimum and putting any type decorative brick or stone (any thickness) on
top of it.
* The K* factor represents the thermal conductivity of a material and is the time rate of heat
flow through a homogeneous material under steady-state conditions, through unit area, per unit
temperature gradient in the direction perpendicular to a surface.
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