Question: I am planning to install a gas fireplace in a room with at concrete slab. I built a chase with a concrete slab. I plan to lift the fireplace about 3/4" off the slab. What is the best material to place on the slab to serve this purpose? Answer: Cement board like Dura-Rock and Wonderboard...usually sold for tile backing. It's about 1/2" thick, so you'll have to raise 1" or so by doubling the board. Many gas fireplaces are OK sitting on wood, so plywood may be able to be used. Check your manual.