Basic Trim Carpentry:

Installing Hardwood
Interior Door Casing

In This Article:

Some careful measurements are made, the side and top casing pieces are cut to length, then the 3 pieces are nailed together. The assembly is installed with finish nails.

Related Articles:
Skill Level: 2-3 (Basic to Moderate) Time Taken: 45 Minutes

By , Editor


The homeowner had stained the jambs on this pre-hung door to match the oak casing.

The casing came in 7 foot lengths. Each door used 2 full pieces for the sides and half of the third piece for the top.


Note the profile on this "Colonial" casing.

The first step is to mark the reveal on the door jamb, that is, the amount of the jamb edge that will be visible. I used a reveal of 1/4".

To mark the reveal I used a Stanley adjustable try-square. The white knob loosens a clamping screw which lets the 12 inch ruler slide for adjustment. This tool is indispensable.

Notice the marks on the jamb. It is only necessary to mark the top corners.

Next I stand a piece of casing up against the door jamb. Before this I made sure the bottom end had a good square cut.

Sometimes the drywall is not flush with the door jamb. If the wallboard sticks out too far, there will be a gap between the casing and the door jamb, or the casing will sit on a weird angle. See how I cut away the drywall when it sticks out.

With the casing held in place, I mark it at the horizontal line I made earlier. Then I repeat the process for the left hand trim piece.

Before going outside to the miter saw, I measured the width of the door opening.

In this case the measurement I got was 30-1/8". I was not measuring from one vertical pencil mark to the other.

I cut the trim on a miter saw. The blade on this saw is a combination blade, meant for fast cutting. It can leave a rough edge, so I cut very slowly. A fine tooth blade would be better.

When the cuts are done, I lay out the pieces on a firm, flat surface. In this house, the best place is the front entry with it's ceramic tile floor.

Cutting the top piece: The casing on this project was 2-1/4" wide. For the top, I first make one of the miter cuts. Then I measured along the top (the long edge) 35-1/8". This dimension comes from adding the 30-1/8" door opening plus 1/2" for the two reveals, plus 4-1/2" for the two casing widths. These numbers worked out easy because for each of the top pieces I simply had to measure the door opening and add 5".

With the casing pieces laid out on a smooth surface, I aligned the corners carefully and drove 1 inch brad nails into the outer corners.


The nailer has to be carefully positioned or the brad will come out the back side, or worse, the front face. Both pieces of trim must be held securely or the impact of the nailer will move them.

I drive just one brad into each side of the miter joint.

What I do is rest one knee on each piece and let my weight hold the trim from moving. I find wearing kneepads to be helpful. Clamping the trim to a workbench may also work.

Note that the brads are driven only at the outside of the corner, and there is nothing to connect the inner area of the miter joint. When the casing is assembled this way, it is very fragile and must be handled with extreme caution. The purpose of these nails is to keep the miter joint aligned properly if the wall surface is uneven. From my experience, it takes longer to install casing when they are not first connected, because the miter joints rarely line up well.

The assembled casing, ready for (careful) transport to the door.

The casing is set in place.


Nailing The Casing In Place:

With the casing assembly in place I attach it with 1 inch brad nails. I start at a top corner, making sure that the edge of the casing lines up with the pencil marks I made earlier. I drive the brads into the door jamb, so they must be kept close to the inside edge of the casing.

  • I first secure one corner, by driving one brad on each side of the joint.
  • Then I secure the other corner in the same way.
  • I drive a few more brads into the top section.
  • Then I fasten each side piece, working from top to bottom. I straighten out any warpage as I fasten.
  • I typically use 5 or 6 brads on the top, and 8 to 10 brads on each side.


After the small brads, I use 2" finish nails (which may require a larger nailer) to fasten the outer edges.

I typically drive 2 or 3 nails in the top and 4 or 5 in each side. Too many nails can make the casing very difficult to remove.


That's all there is to it. With this pre-stained and varnished casing, there is no more work required, except maybe for filling in some tiny nail holes. Most of the holes made by the pneumatic nailers are so small that they need no filling.

Finish carpentry is satisfying work because the results are highly visible.

With paint-grade casing I usually paint the trim first. After installation I fill the nail holes and touch up the paint. A time saving approach some builders use is to paint the walls and trim with the same paint, which would have to be a satin or semi-gloss paint. Flat paint on doors and trim will collect dirty finger prints and is hard to clean. 

For a different approach to window and door casing, see how we created an elegant Victorian casing from scratch.



Tools Used:

  • Power Miter Saw
  • Pneumatic Brad Nailer
  • Pneumatic 2" Finish Nailer
  • Tape Measure
  • Adjustable Try-Square

Materials Used:

  • Colonial Casing
  • Nails


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Copyright © 1999, 2005

Written September 15, 1999 
Revised January 10, 2005