How to Add Molding to Untrimmed Doorways

Measure the door opening's width. If the door opening has a door frame installed in it, subtract 1/2 inch from the width measurement to allow for a 1/4-inch reveal on either side (a portion of the door frame's edge that will be left visible).

Set a miter saw to a right-hand 45-degree cut. Set a piece of molding on the miter saw table and position its left edge underneath the saw blade. Cut about 1 inch off the molding's end.

Transfer the measurement from Step 1 onto the piece of molding you just cut. Measure from the cut edge's bottom edge down. Make a small mark on the molding. Adjust the miter saw to a left-hand 45-degree cut. Line up the mark you made on the molding with the saw blade and cut through the molding.

Set the molding on the wall over the doorway. If the wall has a high point that pushes the molding out slightly, set the molding aside and gently tap the high point down with a hammer. Set the molding back over the doorway. If there is a door frame, hold the molding high enough for 1/4 inch of the frame's edge to remain visible underneath the molding's bottom edge.

Use a level to check that the molding is plumb over the doorway. Drive finishing nails through the molding, near the top and bottom edges, with a pneumatic nail gun. Space the nails approximately 12 inches apart.

Repeat Step 2 with a new piece of molding. Measure the length of the door opening on the left-hand side, from the installed molding's top edge to the floor. Transfer this measurement onto the piece of molding you just cut, measuring from what will be the molding's outer edge, and make a mark.

Shift the miter saw to a straight cut. Line up the saw blade with the mark you just made and cut through the molding. Hold the molding on the doorway's right-hand side, butting up the two mitered ends to form a 90-degree angle. Place a level on the molding to check that it is plumb. Drive finishing nails through the molding to hold it in place.

Set the miter saw to a left-hand 45-degree angle cut. Cut about 1 inch off the end of a third piece of molding. Measure the doorway's length along the right-hand side, the same way you did on the left-hand side. Transfer this measurement to the new piece of molding, measuring along what will be the molding's outer edge, and make a mark.

Move the miter saw blade to a straight cut. Cut through the mark you just made on the molding. Butt the molding's mitered end together with the upper molding piece's mitered end. Place a level against the molding and make any necessary adjustments. Drive finishing nails through the molding.