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What are jamb extensions?

Originally indicating the vertical members of the frame, the term “jamb” has come to mean the frame in which a window or door sits.  Jamb depth varies between window manufacturers — vinyl windows typically are 2 5/8” to 3 3/8” thick while wood windows range from 3 1/2” to 4 9/16”.  In turn, jamb extensions refer to wood or another material that adds width to the jamb so that the window fills the entire opening depth from the exterior to interior sheathings (often plywood or oriented strand board on the outside and drywall on the inside).  In this way, casing or trim material simply needs to be added to the vertical wall surfaces after the windows are installed.  Without jamb extensions, a skilled trim carpenter or drywall installer is required to fill the unrequited space with wood, drywall, or another material before the window can be trimmed, so having jamb extensions applied by the window manufacturer can save labor expenses on the jobsite.