Making and Installing Jamb Extensions on Windows

Window jambs form the frame in which the window sash slides. The top, horizontal jamb is called the head jamb; it is connected to the side jambs. Concealed behind the side jambs of old, double-hung windows are heavy, metal sash weights connected to the sash with rope-and-pulley systems.

So that interior and exterior trim fits properly, the jambs of a window must match the thickness of the wall. Generally this isn't much of a problem because walls of most newer houses are a standard thickness. There's more variation in older houses, however.

Custom windows can be built to fit the thickness of the walls, but it is very expensive. You might be able to plane down the jambs of windows that are slightly too wide for the wall. But the easiest and most common solution to this problem is to buy windows with jambs that are not as deep as the walls and then fit them with jamb extensions.

Jamb extensions are wood strips nailed to the inside edge of the jambs to make up the difference in depth between jamb and wall. They are added after installing the window. When ordering windows, order the jamb extensions at the same time, or make your own with a table saw, as explained below. Using a table saw will ensure that the jamb extensions are cut to a uniform dimension. Caution: Whenever cutting wood into thin strips, use a push stick or some other method to keep fingers well away from the saw blade.

Cutting and installing the extensions
The wood you use should be straight and relatively clear. Because jamb extensions are so slender, any knots will make them too fragile to work with. The jamb extensions can be cut as needed from an existing board. For most painted extensions, use commonly available and inexpensive stock that is 3/4-inch thick.

If you are planning to stain or use a clear finish on the jambs and trim, the jamb extensions should be made of the same wood as the windows. If you are painting everything, use an inexpensive and easy-to-work wood, such as pine.