Hearth Pad Help

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firecracker_77

firecracker_77

Minister of Fire
Dec 16, 2010
1,438
NWI office - 2 Heritages; Chicago home - Woodstock
Getting a 2nd Heritage it looks like. I want to build a hearth pad and have 2 questions.

First, how thick should the cement pad be. I'm not tearing out the carpet underneath. Underneath the carpet is a concrete slab.

Second, can I use granite or marble tiles for the hearth pad? I can get them free and that would look really nice with the stone on the stove.
 
I'm no expert on hearth pads, but every stove manual I've ever read makes a point about saying you cannot set your hearth on top of carpet.
 
firecracker
This is what I found in the 2009 IRC code book
R1001.9 Hearth and hearth extension. Masonry fireplace hearths and hearth extensions shall be constructed of concrete or masonry, supported by noncombustible materials, and reinforced to carry their own weight and all imposed loads. No combustible material shall remain against the underside of hearths and hearth extensions after construction.
R1001.10 Hearth extension dimensions. Hearth extensions shall extend at least 16 inches (406 mm) in front of and at least 8 inches (203 mm) beyond each side of the fireplace opening. Where the fireplace opening is 6 square feet (0.6 m2) or larger, the hearth extension shall extend at least 20 inches (508 mm) in front of and at least 12 inches (305 mm) beyond each side of the fireplace opening.


The following is from the International Phase I Standards of Practice for Inspecting Fireplaces and Chimneys:
Hearth extensions that have a thickness of less than 2 inches.

Hearth extensions that are less than 16 inches in front or less than 8 inches beyond each side of fireplace openings (6 square feet or less).

Hearth extensions that are less than 20 inches in front or less than 12 inches beyond each side of fireplace openings (greater than 6 square feet).

The 2006 International Residential Code (IRC) offers the following exception to the 2 inch-thick rule:​
When the bottom of the firebox opening is raised at least 8 inches (203 mm) above the top of the hearth extension, a hearth extension of not less than 3/8-inch thick (10 mm) brick, concrete, stone, tile, or other approved noncombustible material is permitted.​
 
Also, from any one of a number of hearth pad or stove manufacturers (in this case, Hearth Classics):

Why can’t I install my hearth pad directly over carpet?
When a stove is installed on a hearth pad, its weight pushes the hearth pad down directly under its footprint. If the pad isn’t sitting on a firm, hard surface, this might cause the pad to flex. Unfortunately, tile and grout aren’t flexible materials. If the stove is heavy enough, you might end up with cracked grout or tile.
A second reason is that our pads are designed to have air space beneath them to increase their thermal resistance. If installed over carpet, the carpet will fill that air space and significantly reduce the R-value. For these reasons, our warranty is voided when our pads are not installed on a firm hard surface.
If cutting the carpet away is not an option, a Low Profile base can be placed beneath the hearth pad. The Low Profile base compresses the carpet and provides a flat surface on which to place the pad. Another option is to cut a piece of 3/4” particleboard slightly smaller than your hearth pad to do the same job. The particle board should be cut so that it supports the metal frame of the pad on all sizes.
Click to expand...

http://www.hearthclassics.com/classic_edge_faqs.php
 
The heritage manual reads that covering with tile is fine. If there's no padding under the carpet then I'd use just enough thinset to cover the carpet, then lay your granite or marble.
 
The appliance manufacturer's installation documentation is the go-to reference for hearth pad requirements. I can't understand why you would want to leave the carpet under there, but so long as the hearth pad you use provides the manufacturer's required insulative properties (R-value), then it's OK, I guess. Rick
 
I personally would remove the carpet, might use it somewhere else. But Hearthstone says he's good to go so....
 
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