Fireplace Doors For Zero Clearance Fireplaces ... Having a zero clearance fireplace means you have a prefabricated fire box. A prefabricated firebox is made of steel, and because they are easy to install and ventilate, they are very popular with new home builders. The term zero clearance is used to describe these fireplaces because there is zero clearance between the metal firebox and the combustible material that surrounds them. When installing a zero clearance fireplace (also called pre-fab) a builder can bring combustible material, such as wood studs, drywall and tile, right up to the metal firebox. Contrast this with a masonry fireplace that requires a mason to build and does not allow combustible material near the structure, and you can see why a builder would prefer to install a zero clearance fireplace over a masonry fireplace...