How to Install a Casing for an Exterior Door

Hold a carpenter's square in both upper corners of the door to check the door jamb for square.

Measure the height of both sides of the door from the top of the side jamb to the floor and the width of the jamb across the doorway. Cut casing to fit the measurements.

Make 45-degree miter cuts on the top edges of the side casing and on both edges of the top casing. If the corners aren't square, determine the angles at which to cut the casing. Measure this with an angle finder.

Lay the pieces of casing on the floor and fit them together the way they'll go when they're installed. Measure the distance from the top edge of the assembled casing down the side pieces and mark the casing to match the height of the door.

Cut the bottom of the casing at the marks with the hand saw.

Glue the casing together at the mitered joints with carpenter's glue. Clamp the casing together with corner clamps and allow the glue to set for the time specified on the packaging.

Drill two pilot holes through the top edge of the casing, through the miter joint and into the side casing on both sides of the frame.

Screw the pieces together with 3 1/2-inch-by-6 flat-head wood screws on both sides. Remove the corner clamps.

Place the assembled casing into position around the door. Center the casing around the door, then drive finishing nails partway into the door jamb in the upper corners to hold it in place.

Nail the top piece of the casing into the door jamb by driving finishing nails into it every 6 inches.

Nail the side pieces of the casing into the door jamb with finishing nails every 6 inches, starting with the nails you drove halfway into the casing to hold it in place.

Drive nails into the studs on the sides of the door frame and the header across the top along the outer edge of the casing. Use one nail every 6 inches.

Drive all of the nails into the casing 1/16 inch below the surface with a nail set. Fill the holes with wood putty and sand it down.

Caulk any gaps between the casing and the wall, using a caulk gun.

Finish the casing. If you're using stain, apply the stain with a paintbrush. If you're using paint, apply a coat of primer and allow it to dry, then apply the paint. Use different paintbrushes for this, or clean the primer off the paintbrush before you add the top coat.