Fireplace Hearth
To better understand function of the fireplace heart we need to get familiar with main components of the fireplace:
Firebox: the interior of the fireplace. Its function is to contain the fire and collects the smoke.
Flue: the passageway at the top of the firebox through which the smoke and gases are
exhausted.
Surround: protects the walls around the fireplace.
Mantel: the decorative component on top of surround.
Smoke chamber: connects the fireplace and the flue. At the bottom of the smoke chamber is the smoke shelf, which deflects downdrafts and prevents any rain or soot from dropping directly into the fireplace.
A fireplace hearth is the floor area of the fireplace, and extends in the front and side of the firebox opening. It is built out of noncombustible materials, such as brick or stone. Hearths and hearth extensions are designed to prevent sparks from leaving the fireplace and igniting nearby combustibles.
If the fireplace hearth is not large and thick enough, it might not be sufficient to prevent the spread of fire.
Here are some minimum sizes for fireplace hearths according to the International Phase I Standards of Practice for Inspecting Fireplaces and Chimneys:
Hearth extensions must have a thickness no less than 2 inches.
Hearth extensions must be at least 16 inches in front and no less than 8 inches beyond each side of fireplace openings that i are 6 square feet or less.
The hearth extensions must be at least 20 inches in front and no less than 12 inches beyond each side of fireplace openings that are 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.