Pre-fabricated doors are designed for metal fireplaces. These doors will still allow adequate air-flow around a metal fireplace during the burn process. PRO-TIP: Don’t use a masonry door on a pre-fab fireplace. Restricting the airflow around a metal fireplace can damage the fireplace, venting, or home. Metal fireplaces are usually sitting on plywood and framed with wood 2×4’s. It’s important to allow these fireplaces to ‘breathe’.