Installing Door Trim

How to Install Door Trim like a Pro

By Mark J. Donovan

Door trim provides a finished dressing around a door frame. Installing door Trim is easy to do, however its best if you have access to a miter saw. For installing door trim, you will also need a level, hammer, nail punch, measuring tape, pencil and a square. Materials required for installing door trim include the door casement moulding, wood filler, stain or paint, 4 and 6/8 penny finish nails and a few shims. A nail gun is a great alternative to the hammer and nail punch as it will automatically countersink the nails into the door trim.

Finally, if extension jambs need to be installed a table saw or skill saw with a fence will be necessary.

Extension jambs are only necessary if you installed, for example, a door meant for a 2”x4” wall, into a 2”x6” wall. In this situation filler pieces of wood, known as extension jambs, would need to be installed to extend the door frame outwards so that it is flush with the width of the door frame. If, however, you installed the proper width door for the particular thickness of the wall, extension jambs are not necessary when installing door trim.

If extension jambs are required, use 1”x N” “Select” quality boards. Typically I use pine for extension jambs if I plan to paint the door trim, however if you plan to stain or apply a clear finish to the wood door trim you may want to use a wood material such as Oak, Maple, or Poplar for the extension jambs.

There are numerous types of door casement moulding that can be used for installing door trim, however for the instructions in this article I used 2-1/2” colonial casement moulding.

Measuring Required Door Trim Moulding

Depending upon the door you may need 3 or 6 pieces of door trim. If it is an exterior door that you are installing door trim, you will only need 3 pieces of trim for the interior of the door. If on the other hand you are installing door trim on an interior door you may need 6 pieces of trim to cover both sides of the door. For each side of the door that needs trim you will need two long vertical members and one shorter horizontal member to go above the top of the door.

I typically select 8 foot lengths for the long vertical members and 4 foot lengths for the horizontal members to give me some safety margin and for trimming the material square.


Pre-Paint or Stain Door Trim

Before installing door trim I find it quicker to pre-paint or stain the door trim first. This way I only need to make a few minor touch ups to the trim after it has been installed and thus save a lot of time.

Installing Vertical Door Trim Pieces

Using your pencil make a couple of small cross hair marks, on each side of the door frame, on the top extension jamb and just above the left and right side extension jambs. These crosshair marks locate the height of the inside edge of the vertical door trim pieces. They also specify where the door trim pieces are to be positioned on the vertical extension jambs. Please note that the crosshair marks should be made such that there is a door jamb reveal of 1/8th to 1/4th inch thick.

After making the crosshair marks, measure the distance between the floor and the center of the right cross hair mark and make a note of it.

Next, use the miter saw to make a 45o angle cut on a piece of door trim moulding that is at least several inches longer than the measurement you just recorded. Note that the thicker portion of the cut door trim (in the case of colonial casement trim) should extend beyond the thinner portion of it.

Now, measure from the inside edge (thinner section) of the door trim piece you just cut, to the required distance as you just measured on the door frame. Use your pencil and make a small mark on the door trim piece and then cut it at a 90 degree angle.

Here is a 6 panel wood door.

After cutting the door trim piece touch up the ends with paint or stain and place it into position up against the door jamb. The lower top edge point should be positioned up against the crosshair mark. Use the level to make sure it runs straight and true down the length of the door jamb edge. Once properly positioned nail it into place with 6-penny nails on the outer portion of the door trim piece, however do not sink the nails. If the trim piece is slightly too long, trim it accordingly before nailing it into place.

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

Installing Top Door Trim Piece

With the vertical members tacked into position it’s now time to install the top door trim piece.

First, measure the distance between the inner corners of the right and left vertical trim pieces and make a note of this measurement.

Then make a 45o cut on one of the 4 foot lengths of door trim stock. Again, the thicker portion of the door trim, if using colonial casement moulding, should extend beyond the thinner portion (inner edge) of the colonial case moulding trim.

Now measure from the inner edge of the piece of door trim you just cut, the distance you measured between the two vertical door trim pieces. Make a small mark on this door trim piece. This mark represents the inside edge, or corner, of your next cut.

To make this final cut, reposition the angle of your miter saw so that you can make another 45o cut, but in the opposite direction of the previous cut on this horizontal door trim piece.

After making the cut, again touch up the edges with stain or paint and then place the piece above the doorframe and between the two vertical door trim pieces.

Make sure the corners meet up perfectly. You may need to use a few small pieces of shims behind the corners to ensure that the joints are perfectly even along their entire seam. Also use your level to make sure the top piece is straight and level. Once the top door trim piece has been properly positioned nail it into place with 6 penny nails on the outside edge.

Then go back and install 4-penny nails on the inner side of all the door trim pieces, nailing them into the edges of the door jambs. Make sure, however, not to position the nails too near the ends of the trim pieces to avoid any splitting of the door trim pieces.

Sink and fill the nail holes.

Finally touch up the door trim pieces with stain or paint to complete the process of installing door trim.

For information on how to install interior door trim, see the “Installing Interior Door Trim” eBook from  The “Installing Interior Door Trim” eBook provides step-by-step instructions on how to install interior door trim. Pictures are included for every key step in the process.

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