What is a jamb extension?

A jamb extension is a board that fills the depth of the wall space from the inside face of the door frame to the face of the wall. This widens the frame, making it ideal for situations where the wall thickness is greater than the standard jamb width. Modern Doors typically use MDF to make their jamb extensions, which create a clean edge for the casing trim to be nailed to or inserted if this is a telescopic L-casing. With the help of extensions, Modern Doors frames can cover walls that are 4 7/8” to 6 7/8” thick.