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.
Step 1
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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Step 2
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.
Step 3
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.
Step 4
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.
Step 5
Check your calculations with your local building code officer and apply for any required permits.
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