An extension jamb is a strip of wood or a board that is used to widen, or increase the depth of the window jamb, in order to make it fit the wall in which the window is being installed.
The top or horizontal jamb of a window is called the head jamb, and the jambs on the side are called side jambs. An extension jamb can be for either or all of these jambs, in order to make the window fit correctly and tightly in the wall.
There are pre-made extension jambs on the market. This kind of extension jamb is pre-sized to work with that particular brand of replacement window.
Constructing an Extension Jamb
In cutting your own extension jamb, be sure to use wood that is clear of knotholes. Generally, you would use a board that is ¾ of an inch think. Cut your extension jamb to the proper length, test it by setting it in place, and then cut the rest of your extension jambs to that same measurement.
Nail each extension jamb in place. Use finish nails, making sure that the nail goes at least once inch into the wood.