How Big Should a Stone Hearth Be for a Wood-Burning Insert?

Follow Concrete Base

• As a general rule, install your stone hearth to follow your concrete footing, the masonry foundation on which the fireplace hearth is built. Laying stone allows you to line and potentially extend your hearth, but you should already have a foundation line to follow. Use this as a guideline to keep your hearth plans contained and easy to build; it is much more labor-effective to built on the masonry that is already present in your house, which also provides a guideline for the original, and correct, fireplace size.

Thickness

• When adding a new stone hearth to your fireplace, always make sure it is supported by non-combustible materials (another reason an original masonry foundation is useful) and built thick enough to protect underlying materials from potential heat problems. Building codes can vary, but you should plan on a hearth at least four inches thick. This does not all need to be stone -- the stone layer can be made of tile and installed on top of a new concrete extension.

Extensions

• If you plan on extending your hearth, the extension itself may only need to be a minimum of two inches thick. Some extensions are required as part of the fireplace foundation anyway. For example, the hearth should extend at least 16 inches in front of the fireplace and at least eight inches on every side of the firebox insert. Brick, concrete, stone and tile are all generally approved materials.

Additional Extensions

• The height at which you set your firebox can be important. If your firebox insert is more than eight inches above the level of the hearth, you may be able to put in a much thinner layer of stone, saving on materials and work. On the other hand, for very large fireplaces, such as those with openings more than six square feet, the extension needs to be greater than 20 inches with 12 inches on every side.