What's to know about window jambs?

What is a window jamb?

There are two types of jambs found in windows:

1. Side jambs - are the main vertical parts that form the sides of a window frame. 
2. A head jamb - the horizontal part that runs along the top of the window frame. 

Jambs often house tracks or rails used to manipulate operational windows. They also support latches and hardware that are used to lock the window when it is closed. Jambs are not necessarily required but are commonly used by most window manufacturers.

What is a jamb liner?

A jamb liner is a strip of material that goes on the sides of the window frame to provide a snug fit for the window sash. There are several ways a jamb liner can be installed onto or around a frame depending on the type of window and the manufacturer.

What is a jamb extension?

A window jamb extension is an interior window component (in varying depths) that allows the window frame to fit the window opening. They can be included as an attachment as in the case of most window replacement projects, or they can be retrofitted after the window frame has been installed. Jamb extensions reach out from the jamb from top, sides and bottom (in most cases, depending on finishing requirements) of the window.

Depending on the thickness of a wall, a jamb can be made to fit a certain depth so that the window unit can sit flush with the interior wall, creating a snug fit and making it easier to apply the window casing (trim). Standard jamb extension depths are 15/16 inch, 1 3/8 inch, and 3 3/8 inch. Optional variable depths can range up to 8 3/8 inches (depending on the manufacturer).

Note: Window units should never be anchored through jamb extensions.

Jamb extension options you should know about

Interior jamb extensions are available in several finishes including extruded vinyl, bare paintable wood (typically pine), primed wood, foam board, veneers, vinyl-wrapped wood (white, and in various faux wood finishes) and finally wood or drywall receptors. With a faux window option the visible jamb, jamb liner, jamb extension and sash will all appear in the faux option.