Wooden fire doors must have solid wood or mineral cores, while steel doors typically have honeycombed or insulated cores. The door should be prepped at the factory so there is no more than three-quarters of an inch of clearance between the bottom of the door and the finished floor. Also, no holes larger than 1 inch in diameter may be drilled without voiding the fire rating. Pairs of doors must be equipped with an astragal, or overlapping seam or molding. Additionally, frames must be made from at least 16-gauge steel and should be prepped at the factory to receive hardware.