To our knowledge no additional testing has been done to justify continuous headers over multiple garage door openings.
Garage door header span.
Most door frames that are 4 feet wide or less require a 2 by 6 header.
Tables providing size selections for various beam spans and loading combinations for southern pine dimension lumber and southern pine glued laminated timber are available for the following applications.
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.
Measure the width of your door frame.
When in doubt use 2 by 12 inch headers.
Notches on the outer thirds must not exceed 1 6 of the width of the board.
Both headers and girders are beams that are oriented horizontally and transfer load down to walls piers or posts.
Portal frame details within the irc specify simple span headers with a maximum finished opening width of 18.
These details are based on testing done by the engineered wood association that included only simple span headers.