16
Hearth Extensions and Wall Shields
A hearth extension must be installed with all fireplaces. It is to protect the floor in front of the fireplace from both radiant heat and sparks. For Model
Note: Any noncombustible material whose 1" k value is less than .84 or whose r value is more than 1.19 is acceptable.
If the fireplace is installed on a combustible floor, the metal safety strips (provided) must be used on the floor extending half under the fireplace and half under the hearth extension.
Secure the hearth extension to the floor to prevent possible shifting.
If the fireplace is installed diagonally across a 90° corner, the 24" sidewall clearance can be reduced to 20" and no wall protection is re- quired.
Side Wall cannot be Closer than 24" under any Circumastances
C
B
C A
Figure 44
C 
B 
C 
A
D
Figure 45
Hearth Extension Dimensions
(For Figures 44 and 45 )
A20"
B48"
C12"
D72"
Methods of Determining Hearth Extension Equivalents
To determine the thickness required for any desired material when either the k or r values are known:
TM = thickness of desired material in inches kM = k value of desired material
rM = r value of desired material T1 = minimum listed thickness
Example: Micore CV230 is to be used with the
Using the k formula:
Desired | k value of desired | Min.Thickness | |
Required = | material (per inch) x | of listed | |
Thickness | k value of listed |
| material |
| material (per inch) |
| |
TM (inches) = kM x T1
.84
TM (inches) = 0.43* x 1" **
.84
Answer using k = 0.51 x 1 = 0.51 = ¹⁄₂"
Using the r formula:
Desired | k value of desired | Min.Thickness | |
Required = | material (per inch) x | of listed | |
Thickness | k value of listed |
| material |
| material (per inch) |
| |
TM (inches) = | 1.19 |
| x | T1 | |
| rM |
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TM (inches) = | 1.19 |
| x | 1" ** | |
| 2.33* |
|
| ||
Answer using r = 0.51 x 1" = 0.51 = ¹⁄₂"
*value taken from chart
**minimum thickness per listing
NOTE: DIAGRAMS & ILLUSTRATIONS NOT TO SCALE.
Alternative Hearth Extension
and Wall Shield Materials
Listed | Values | Min. Thick | |
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Material | k | r | TL |
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Millboard | .84 | 1.19 | 1" |
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Alternative | Values | Min. Thick | |
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Materials | k | r | TM |
Wonderboard | 1.92 | 0.56 | 2 ³⁄₈" |
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Common brick | 5.00 | 0.20 | 6" |
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Cement mortar | 5.00 | 0.20 | 6" |
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Ceramic tile | 12.5 | 0.08 | 15" |
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Marble | 11.0 | 0.09 | 13" |
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Micore CV230 | 0.43 | 2.33 | ¹⁄₂" |
(U.S. Gypsum) |
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Ceraform 126 | 0.27 | 3.70 | ¹⁄₂" |
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At times, it is important to know what combina- tions of materials are acceptable for use as hearth extension. The “R” values are used to determine acceptable combinations of materi- als because “R” values are additive where r and k values are not.
“R” value = 1 = r x thickness of material k
Example: Given that the required “R” value for a suitable hearth extension used with the
1.19x 1" = 1.19. It is desired to elevate a marble hearth extension to a level of 5” or more off the floor surface. What combination of noncom- bustible materials can be used to accomplish this?
If common brick is used so that the 3 ¹⁄₂" dimension is the height, then “R” for the com- mon brick becomes:
“R”M = r x TM = 0.20 x 3 ¹⁄₂" = 0.70
Using ¹⁄₂" of mortar to set the brick, then “R” for the mortar becomes:
“R” = r x TM = 0.20 x ¹⁄₂" = 0.10
A ³⁄₄" marble slab set on ¹⁄₄" micore covers the brick, then “R” for the marble and micore becomes:
“R” = r x TM = 0.09 x ³⁄₄" = 0.068 “R” = r x TM = 2.33 x ¹⁄₄" = 0.5825
