What is an Extension Jamb?

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  • Written By: Donn Saylor
  • Edited By: Jenn Walker
  • Last Modified Date: 13 November 2019
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An extension jamb is a piece of wood or other material that is attached to the inside of a door or window frame to make it larger and ensure it fits into a wall. Extension jambs are frequently used in building construction when a window jamb or door jamb will not install securely because it is not wide enough to meet the dimensions of the wall. Attaching a section of wood, board, or other substance elongates the jamb and allows it to fit snugly into place.

When constructing an extension jamb, there are several key factors to bear in mind. If using a block of wood, a section free of knotholes and other blemishes should be chosen, so the integrity of the wood remains strong and solid. The gap around the window or door should also be measured very carefully. Typically, the wood is cut to a 3/4-inch (approximately 1.9-mm) thickness and then tested against the jamb. If it is the appropriate thickness, the window or door extension can then be nailed, stapled, or drilled into place, making sure the fasteners go at least 1 inch (about 25.4 mm) into the wood.


Extensions applied to door jambs should always go on the opposite side of the door from where the hinges are located. If an extension jamb is attached to the same side as the hinges, it can be next to impossible to fully open the door. This reminder can save a lot of headache after the installation process is complete.

Window jamb extensions can sometimes be trickier than door jambs. Whereas doors come in fairly standard shapes and sizes, windows follow no such protocol. When fitting an extension for a window, it is important to pay careful attention to the exact design and idiosyncrasies of the window itself, so the board can be cut to fit properly.

A variety of hardware is utilized in the creation and installation of an extension jamb. Saws, electric drills and drill bits, staple guns, staples, hammers, and nails are the basic tools necessary for accomplishing this project. Extension jamb installation does not require any specialty tools or advanced knowledge of building construction, and the whole undertaking should take around three hours.

For those not comfortable with tackling this project solo, there are door and window specialists who are professionals in proper installation. They are readily available and can be contracted for jobs big and small. Dependable contractors can be found online, in the phone book, or through word of mouth.


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