Proper Size Wood for a Door Header
One of the strongest architectural design elements is the arch. The vertical framing and header planks of a doorway are a prime example of an arch. The header creates a load bearing platform and transfers the weight of the structure to the vertical framing. The size and number of header planks is often determined by the size of the door opening and the weight of the door to be hung.
Door Frame Components
Rough door framing consists of king studs, jack studs, header planks, cripple studs and double plate planks. The king studs are full length and extend from the sill to the underside of the double plate framing. Shorter length jack studs are nailed to the side of the king studs facing the rough door opening. The horizontally oriented header planks which will span the top of the door frame rest on their top ends. Header planks are stood on their narrow edges and nailed together. They are also attached to the jack studs and king studs. Shorter cripple studs are installed above the header and extend to the underside of the double plate. On an exterior wall or load bearing interior wall, cripple studs will be installed on 16-inch centers. On interior walls which are not load bearing, such as closet framing, they might be installed on 24-inch centers. The builder, architect, or local building codes may dictate the framing requirements of interior and exterior walls.
If the framing being built will eventually house a closet, the header planks might be a pair of simple two-by-fours. On an exterior door frame which will house a heavy 36-inch door, the headers will likely be a pair of two-by-sixes. If the entry door is a double door or a wide sliding glass door, the header planks might be two-by-eights, or larger. The header planks in barns are often bigger still. As an example, in a pole barn with 6-by-6-inch poles, there may be three 2-by-12-inch header planks resting atop the jack studs. They are needed to support the tremendous weight of large sliding barn doors. For additional strength, a single 6-by-12-inch beam may be custom cut at a saw mill and used as a header. The dimensions of header planking or beams are dependent upon the structural elements of a building and the width of the opening to be spanned.
Door Frame Diagram
The Architectionary.com website has a labeled diagram illustrating the structural elements of a basic door frame (see References).