In a building, all doors that open to the outdoors are called exterior doors. An exterior door frame is, therefore, the frame in which the exterior door is mounted. Exterior doors and exterior door frames are, of course, subject to the extreme conditions of sun and weather, and must be stout and weather-resistant.
An exterior door frame consists of posts, casings, jambs, and a threshold. The casings and jambs of the door frame are usually divided into head casings/jambs, and side casings/jambs. The head pieces are horizontal members at the top of the door frame, and the side pieces are vertical members on each side of the door. The threshold is the bottom piece of the frame. Dados, or notches for a sill, as well as cut-outs and holes for door hardware, hinges and locking mechanisms, are also inserted in the frame.
When undertaking an exterior door installation, great care must be taken that the rough opening, the unframed opening for the exterior door frame and door, is properly prepared so that weatherization materials can be correctly installed and/or applied. Unlike an inside door frame, which ordinarily has no casings, and little need of weatherizing, an exterior door frame requires considerable attention to the prevention of draughts and/or leaks. Both draughts and leaks will rob a building of heat in cold weather, or air conditioning in warmer weather. As well, an exterior door frame, especially a wood frame, is subject to rot if it is not properly coated and maintained.
In addition to casings and jambs, the typical pre-fabricated door frame will have a threshold as the lower piece of the frame. The threshold is often the least snug portion of the frame to the door, and must be well-sealed with special weather-stripping, or weather-stripping may be affixed to the bottom of the door. Some door frames are manufactured with weather-stripping on both the threshold and the bottom of the door. In the case of an exterior door frame located below the grade of the land, the space between threshold and door, unless properly sealed, will allow water to seep into the building between door and frame.
Most exterior doors and frames are constructed of wood because it is light-weight yet sturdy, reasonably easy to work with, and far cheaper than metal. Metal exterior doors and frames, however, are far more impervious to weather than is wood, with the added benefit of also being much harder to break down or into, in the case of criminal intent.