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Installing interior door Trim, or case moulding, around a door provides the finishing touch to a door.

Interior Door Trim Installation is not too difficult for a DIY homeowner to do, as long as he or she has the right tools.  I highly recommend the use of a Miter Saw and Nail Gun.  In addition, you will also need a level, square, tape measure, pencil, shims, wood or caulk filler, center punch, 4 penny and 6 penny nails, rag, and some paint/stain. If extension jambs are need to be installed a table saw or skill saw with a fence will also be required.

The instructions included in this article focus on trimming out an interior or exterior door that has door jambs that already extend out flush with the finished sheet-rocked wall. If the door installed was meant for 2”x4” versus 2”x6” wall construction, extension jambs will need to be installed first. See the end of this article for installation of extension jambs.

Typically the only trim required for installing interior door trim is case moulding. For this particular project I used 2.5” colonial case moulding.

If extension jambs are also required, then 1”x N” Select quality boards will also be required. I typically use pine material for extension jambs if I am painting the trim, however you can also use other materials such as Poplar, Oak, or Maple if you plan on staining the wood or putting on a clear finish.

Measuring for Interior Trim Case Moulding

The colonial case moulding trim required for this project consists of three (3) pieces. There are two vertical members that will reside on each side of the door, and a top piece that will sit on top of the two vertical members.

I used two 8’ colonial case moulding trim pieces for the vertical members and one 4’ trim piece for the horizontal top piece.

Pre-Paint or Stain Case Moulding

Prior to installing the case moulding, it is best to pre-paint/stain it. This will provide a cleaner finished look and save time.

Then, using your miter saw make a 45^o angle cut on a piece of colonial case moulding trim board that is at least several inches longer than the measurement you just made. Note that the thicker portion of the cut colonial casing trim should extend beyond the thinner portion of it.

Now, using your measuring tape, measure from the inside edge (thinner section) of the colonial casing piece you just cut, to the required distance as you just measured on the door frame. Make a small mark on the wood and then cut it at a 90^o angle (straight cut)

After making the cuts, touch up the ends with paint/stain and place it into position. The lower top edge point should be lined up with the cross hair mark. If it is, tack into place with 6-penny nails on the outer portion of the trim piece. If not, re-trim it.  Make sure you position the nails approximately 1” in from the edge to prevent splitting.

Use a level to ensure the trim piece is plumb while installing the nails. Note do not sink nails.

Repeat the same process for the left hand side of the doorframe.

Installing the Top Colonial Case Moulding Trim Piece

With the vertical members tacked in, now install the top colonial case moulding trim piece.

Start by measuring the distance between the inner corners of the left and right vertical colonial case moulding trim board members. Record this measurement.

Next, make a 45^o angle cut on a piece of case moulding trim that is several inches longer than the measurement you just made. Again, the thicker portion of the colonial case moulding should extend beyond the thinner portion (inner edge) of the colonial case moulding trim.

Next, measure from the inner edge of the just cut colonial case moulding trim board the distance you recorded between the two vertical colonial case moulding trim board members. Make a small mark on the board. This mark represents the inside edge/corner of your next cut.

Now reposition the angle of your saw so that you can make another 45^o angle cut in the opposite direction of the previous cut on the board.

Again, touch up the edges with paint/stain and position the top trim piece above the two vertical members.

Apply 6-penny nails on the outer edges of the trim piece (both edges, and one in the middle).

Final Tasks

Once the trim is all tacked in, apply 4-penny nails on the inner side of all the trim pieces. Again make sure you do not place nails to near the ends of the trim pieces, otherwise you may get some splitting.

Finally, sink and caulk the nail holes, and touch up with a little paint and your done.

Extension Jamb Installation (If required)

When installing Extension Jambs start out by installing the top piece first. First measure the depth of the required extension jamb (from the existing door jamb to flush with the interior wall). 

Use a table saw and rip a section of 1” x N” trim board to this width.

Next, measure the width of the rough doorframe opening at the top of the doorframe. Use a miter saw/chop saw to cut the board you just ripped to this length.

Place the created extension jamb into the top portion of the doorframe. It should fit snuggly up against the existing doorframe jamb and should be flush with the interior sheetrocked wall. Once the extension jamb is located into its proper position, nail into place with 6 or 8-penny nails. Again, do not sink them.

Repeat this process for the two vertical extension jamb pieces.

Shims and/or small blocks of wood may be required to adjust the top/side extension jambs so that they are flush with the existing doorjamb, and are square and level.

Once all the extension jambs are tacked in, and are square and level, sink the nails. Then plug and paint them and begin the case moulding trim installation.