I went over a number of different manu. specs. for heatilator & majestic, two of the most common prefab units out here and here is what I found. Gas Fireplace: Both companies gas fireplaces only recommend and do not require a non-combustible hearth extension. Wood Fireplace: These require a non-combustible hearth extension. Below are non-combustible materials and the required thickness that meets the specs.(min. R value of 1.16) for the fireplaces non-combustible hearth extension. Hearth Extension Insulation Alternatives, R Value = 1.16 Hearth & Home HX3, HX4 - 1/2 inch USG Micore 300 - 1/2 inch USG Durockâ⢠Cement Board - 2-1/4 inches Cement Mortor - 5-7/8 inches Common Brick - 5-7/8 inches Ceramic Tile - 14-5/8 inches Marble - 16-5/8 in. to 23-3/8 in. These are the materials that can separate combustible material, lets say the wood flooring or wood framed hearth extenstion, from the fireplace hearth. As you can see it would take over 14 inches of ceramic tile applied over a combustible surface to meet the manu specs. - thats a lot of tile! http://www.heatilator.com/products/fireplaces/index.asp http://www.vermontcastings.com/indes.cfm