The header for a door is much like a bridge, transferring the weight above it to the floor and foundation below. Doors in a load-bearing wall that holds the weight of the house, created by beams and trusses, need a larger header than those in non-load-bearing walls. The header is usually made out of dimensional lumber installed on its edge. Calculating the size of the header depends on what the header needs to support.
Determine whether or not the door is under a load-bearing wall. Any exterior wall should be considered load-bearing unless a structural engineer tell you otherwise. Consider interior walls that run perpendicular to the way the floor joists run to be load-bearing as well. If you are unsure, contact a contractor or structural engineer. Columns, posts and arches can be deceptive and camouflage a load-bearing wall.
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Consult the 2000 International Building Code. Your local library or local building code department may have a copy. This book gives over two pages of examples for how to calculate the necessary headers under given circumstances.
Follow the example of other headers in your home. For instance, if you have another 4-foot-wide door frame in your home and it has a double 2x6 header, consider a similar header safe for installation.
Measure the width of your door frame. Most door frames that are 4 feet wide or less require a 2-by-6 header. Between 4 and 5 feet, the header should be built 2 inches wide and 8 inches long while a larger opening needs a header that is 2-by-12. When in doubt, use 2-by-12-inch headers.
Check your calculations with your local building code officer and apply for any required permits.