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Site-built masonry is expensive and takes days or weeks to build, which is why the vast majority of new fireplaces are prefabricated, manufactured units. A typical manufactured fireplace is a double-wall metal box that connects to a metal flue pipe. Most are zero-clearance units, which is another way of saying that they sit directly on the subfloor and can be installed close to or in contact with combustible framing. But most manufacturers suggest leaving 1/2-inch clearance to the back and sides for thermal expansion. Greater clearances are required above the box and around the flue pipe. Fireplaces usually require permits and must be installed to code. The vast majority of manufactured fireplaces use natural gas,