How to Case a Doorway

Hold a framing square against the doorframe's corners, verifying the doorframe rests square in the wall's door opening. If the doorframe does not sit square in the door opening, loosen the doorframe's mounting screws with the proper screwdriver and force a shim between the out-of-square part of the doorframe and the door opening. Tighten the mounting screws with the screwdriver, and recheck the doorframe for square by using the framing square.

Lay out a 3/16-inch reveal along the doorframe's exterior edge with a tape measure; make pencil marks near each door frame corner and every 18 to 24 inches around the perimeter of the doorframe.

Measure the height of one of the doorframe's vertical sides, using the tape measure. Add the width of the door casing to the frame's height measurement, and transfer the total to a piece of door casing. Place a pencil mark at the appropriate spot on the door casing's thick edge. Repeat this step for the door casing's second vertical side.

Adjust a miter saw's blade guide to the 45-degree saw position, following the miter saw manufacturer's instructions. Many miter saws use thumb screws to adjust the saw position.

Place one of the door casings in a miter saw, keeping the casing's thick edge facing the miter saw's blade guide. Slide the door casing along the miter saw until the pencil mark aligns with the blade guide. Cut the door casing with the miter saw.

Slide the second vertical door casing piece into the miter saw, keeping the door casing's thin edge facing the saw's blade guide. Adjust the door casing until its measurement mark meets the blade guide. Cut the door casing.

Hold one of the door casing's vertical pieces against its respective spot on the doorframe, keeping the mitered cut facing up and the door casing's thin edge even with the 3/16-inch reveal marks. Tack the door casing to the doorframe with a 4d finish nail, using a hammer to drive in the nail. Place one 4d nail about 8 inches from the floor and a second 4d nail about 8 inches from the mitered cut. Tack the second vertical piece of door casing against the doorframe, using the same method as used for the first piece.

Measure the horizontal distance between the tops of the door casing's two vertical pieces. Transfer the measurement to the thick edge of a piece of door casing, using a pencil to place a measurement mark at the correct spot on the casing's thick edge.

Position the door casing on the miter saw with the measurement mark on the casing's thick edge against the blade guide. Cut the door casing, using the miter saw. Flip the piece of door casing around, placing the casing's thin edge against the blade guide. Slide the door casing until the factory-cut end meets the blade guide. Cut the casing with the miter saw.

Apply a liberal amount of wood glue to the cut ends of both pieces of vertical door casing.

Hold the door casing's horizontal piece against the doorframe, aligning the horizontal casing's cut ends with the vertical casing's cut ends. Tack the horizontal piece of door casing to the doorframe with 4d finish nails, using the hammer to drive in a nail about 8 inches from each cut end. The thin edge of the horizontal piece of casing should align with the reveal marks on the doorframe's horizontal piece.

Drive additional 4d finish nails around the perimeter of the door casing with the hammer, keeping the thin edge of each casing piece in line with the doorframe's reveal marks. Space the 4d nails every 12 to 24 inches.

Clean the excess wood glue from the door casing's corners, using a rag.