Installing Door Trim
Basics
The trickest part of installing new trim is to keep the trim level and plumb in relation to the door frame.
Begin by marking a 'reveal' edge line around the frame after the old trim is removed. This will give you a guideline to install the new trim. Using a combination square, adjust the end to 1/4" in. Place it flush against one side of the doorjam and using a pencil up against the end of the ruler, slide the handle and pencil down at the same time along the edge of the jamb, marking the reveal line. Mark the head jamb and other side jamb in the same way.
To mark the reveal I use an adjustable try-square. The white knob loosens a clamping screw which lets the 12 inch ruler slide for adjustment. This tool is Next stand a piece of casing up against the door jamb. Make sure the bottom has a clean level cut and is straight. Sometimes the drywall may stick out a little from the trim. Take a sharp knife and cut this area out so it is not protruding and the trim will lay flat agains the jamb. Cutting the top piece.
If the casing on this project, for example, was 2 1/4" wide, then begin with the top casing piece; measure along the top (the long edge) 35 1/8". This dimension comes from addition the 30 1/8" door opening plus 1/2" for the two reveals, plus 4 1/2" for the two casing widths. These numbers work out easy because for each of the top pieces, simply measure the door opening and add 5".
With the casing peiecs laid out on a smooth surface, align the corners carefully and drive 1 in brad nails into the outer corners. The nailer has to be carefully positioned or the brad will pop 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.
Drive in just one brad into each side of the miter joint. Note: 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 care. The purpose of these nails is to keep the miter joint aligned properly if the wall surface is uneven. It may take longer to install casing when they are not first connected because the miter joints rarely line up well.
With the casing assembly in place attach it with 1 inch brad nails. Start at a top corner, making sure that the edge of the casing lines up with the pencil marks made earlier. Drive the brads into the door jamb, so they must be kept close to the inside edge of the casing.
First secure one corner, by driving one brad on each side of the joint.
Then secure the other corner in the same way.
Drive a few more brads into the top section.
Then fasten each side piece, working from top to bottom. Straighten out any warpage as they are fasten.
Typically use 5 or 6 brads on the top, and 8 to 10 brads on each side.







