Guidelines for sufficient thickness and size of hearths and hearth extensions can be found in the International Phase I Standards of Practice for Inspecting Fireplaces and Chimneys and in the manufacturer’s instructions.
The following is from the International Phase I Standards of Practice for Inspecting Fireplaces and Chimneys:
The inspector should inspect for hearth extensions that have a thickness of less than 2 inches.
The inspector should inspect for 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).
The inspector should inspect for 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 inspector should inspect the hearth, hearth extension, and chambers for joint separation, damage and deterioration.
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.
Inspectors should note that carpet or tile may obscure the hearth extension so that it may be difficult to tell how thick it is.
In summary, hearths and hearth extensions are noncombustible surfaces designed to prevent fires from spreading beyond the fireplace. If they are not large and thick enough, they might not be sufficient to prevent the spread of fire.