Masonry doors are designed for true brick/mortar fireplaces. They are designed to overlap the opening as much as possible. Fiberglass insulation will fill in the gap between the door and the brick/stone face. These doors help stop some of the draft associated with traditional fireplaces. PRO-TIP: Fireplaces come in a lot of sizes and so do doors. Measure both the height of the opening and the height of the steel lintel bar to get the correct fit! Pictures also help us guide you in the right direction!