How to Trim the Pieces for an Open Doorway

Mark the wall next to the passageway where you intend to place the inner corners of the trim. Place the trim flush with the opening or, to create a wider and more dimensional trim affect, position the trim slightly to the outside of the opening. A 1/4- to 1/2-inch reveal is commonly used. Measure and mark a faint line parallel to the upper vertical edges of the passageway. Measure and mark a level line on the wall above the passageway. The width of the reveal should be the same on the sides and top of the passageway.

Measure and note the horizontal distance between the intersecting lines outside the upper outer corners of the passageway. This will be the length of the shortest edge of the header, or top trim. Measure and note the vertical distance between the intersecting lines to the baseboard or shoe molding. This will be the length of the short edges of the side trim pieces.

Set the miter saw to make a 45-degree cut to the right. Place the backside of the molding on the saw table with its thick edge against the guide fence. Position all the moldings this way on the saw. Holding the trim firmly against the guide fence, trim off the left end of the molding. Rotate the saw blade to make a 45-degree cut to the left. Measure and mark the molding to the length of the header trim. Make the mark on the front of the molding on its thin edge.

Position the molding on the saw table. Cut through the molding on the mark. Set the trimmed header molding piece aside. With the saw blade still turned to the left, trim the top corner of the left vertical molding piece. Measure and mark the molding to the distance between the upper corner and the baseboard molding. The measurement and mark are made on the front of the thin edge of the trim. Rotate the saw blade to make 90-degree cuts. Trim the lower edge of the left side molding.

Turn the saw blade to the right to trim the upper end of the right-side molding piece. Trim off the left end of the molding. Measure and mark the molding to the distance between the upper corner and the baseboard molding. Rotate the saw blade to make 90-degree cuts. Trim the lower edge of the right side molding.