Finish Carpentry Basics:

Installing Custom
Door Extension Jambs

In This Article:

Pieces of wood are ripped to width, cut to length, and nailed in place, resting on shims if necessary.

Related Articles:
Skill Level: 2-3 (Basic to Intermediate) Time Taken: 1.5 Hours

By , Editor


A Note On Tools Used:

We used a few expensive tools on this project, but they are not absolutely necessary. The critical issue here is the ability to make straight and smooth cuts. A circular saw could be used instead of a table saw, especially if a long straight edge guide were employed to keep the cut straight.

Cutting the boards to length could be done with an old-fashioned miter box. The jambs could be nailed with regular, hand-driven finish nails. Pre-drilling the holes would help prevent splitting.

The door before we started. The walls had been painted, the floor was complete.


Note how the hinge-side jamb was installed right up against the opening in the framing. This is poor practice. We always shim each side, even if the hinge side is perfectly plumb.

The striker side had a large gap between jamb and framing. The framing carpenters should have split the difference.

The above snafu is all too common these days. It seems that just about all construction crews are in a great big hurry. And if they can take a short cut, like just slapping the pre-hung door up against the opening, and popping in a couple of nails, to save a few minutes... well they do it. But they are just passing the buck, because eventually someone has to deal with most short-cuts, and it mostly seems to be the finish carpenter. (Because he is the last guy in the construction process.)

Across the top we installed a few blocks of wood for spacers.

The top jamb was ripped to width, cut to length, and nailed in place.

But How Wide To Cut The Extension Jambs?

We cut the jambs wide enough so the front edge would extend beyond the drywall slightly, about 1/16". If the front edge is recessed too much, the edge of the drywall may prevent the casing from seating properly on the extension jamb. A little too much protrusion is better than not enough.

The Consequences Of Passing The Buck:

We used 1x8 pine, ripped to about 2" wide, for our extension jambs. Of course, 1x lumber is actually 3/4" thick. But that was too thick to fit on the hinge side.

Making this cut was a pain in the neck !!!

We had to rip one piece thinner, meaning that the board had to be placed on edge and run through the table saw. We set the cut to 1/2" wide.

The thinner piece slipped in behind the hinges without a problem, and was nailed in place.


The extension was installed on the striker side. We had to slide some shims under it to position it properly.

The door with the extension jambs installed.

Completing The Job: Casing


The three pieces of casing were cut and laid out on the garage floor.

We drove 1" brad nails into the corners, one in each piece, to hold the unit together while we installed it.

Then we carried the unit into the house, and held it in place. We had made some pencil marks earlier, 3/16" in from the edge of the extension jambs.


We tacked the casing in place with 1" brad nails. The brads are driven into the thin edge of the casing, so they go into the extension jambs.

We drove brads about every 10 to 12 inches.


Once the inside edge is secured with small brads, we drove 2" finishing nails at the outer edge. These nails go through the drywall and into the wood framing.

We usually drive nails about every 1 or 2 feet, just enough to keep the casing from being flexed if someone was to grab it.

We try to avoid putting in too many nails. Some people like to remove their door and window casing when they repaint their walls, and we try to attach our trim with as few nails as possible. Eventually, someone is going to remove the trim for some reason, so why over-nail it?  And we never attach our trim with construction adhesive (like Liquid Nails).

The completed project.

The homeowner will be staining the bare wood to match the casing.

See another, more detailed article about installing door casing.



Tools Used:

  • Table Saw
  • Power Miter Saw
  • Pneumatic Brad Nailer
  • Pneumatic Finish Nailer
  • Assorted Carpentry Tools

Materials Used:

  • Lumber, Clear Pine, 1x8x8'
  • Nails


Back To Top Of Page 

Before You Hurt Yourself,  Read our Disclaimer.

Search Page

Home  What's New  Project Archives  H.I. World

 Rants  Contact Us






Copyright © 1999, 2005

Written October 23, 1999 
Revised January 10, 2005