Split-jamb doors have a two-piece jamb that is separated lengthwise and sandwiches the wall between the two pieces. The door casing comes attached to each edge of the jamb. Each half of the jamb is installed separately, starting with the side that holds the hinges.
Hammer, Phillips screwdriver, 24" level, 24" framing square, nail set, wooden shims, 8d and 16d finish nails, 3" screws, putty knife and wood filler.
Verify the width and height of the rough opening is 1/2 inch larger than the width and height of the split-jamb door frame (discounting the pre-attached casing). This will allow you to shim and square the door frame easily. Make sure the threshold is level, the hinge side of the rough opening is plumb, and the head is square to the hinge side of the rough opening. Adjust the framing if necessary.
Determine if you’re installing a right- or left-handed door. When inside a room, the door should swing open in the direction that makes the best use of room space, such as toward a wall where it will be least intrusive to the flow of traffic.
Remove any staples, strapping, nails or plastic clips that hold the door frame and door casing together. Working with a partner, separate the door frame carefully. The split-jamb system will separate into a Door Unit and a Casing Unit.
The two parts of the door frame join together with a tongue-and-groove connection, which you’ll reconnect from each side of the wall to install the door.
Position the door unit into the opening and square it against the hinge side of the jamb.
Make sure the hinge side of the jamb is plumb and level so the door will operate correctly. It might help to drive 8d finish nails (or trim screws) through the jamb near the top corners to secure the door in place while you adjust it.
Block and shim the jamb at the hinge locations. Double-check to make sure that the door is still square and plumb. Adjust the shims as needed. Use tapered shims inserted opposite of each other so that the jamb doesn't twist or bow.
Test the fit of the casing unit before driving the final fasteners.
Inspect for any final adjustments to plumb, level and the fit of the split-jamb frame.
Replace one screw per hinge with a longer 3" screw to make the door installation more secure and to reduce the number of nail holes.
Then, nail through the final shim locations with 16d finish nails. Follow the same steps to shim the jamb on the lock side of the door, and to fasten the door header, securing the shims with finish nails. Cut away excess shim material. Test by opening and closing the door, and adjust if needed.
Slide the second part of the door frame into place. Use a rubber mallet to tap the tongue-and-groove connection together if necessary. Nail through both halves of the jamb into the center stop. Refer to the manufacturer’s instructions for further nailing guidelines for your door, such as recommendations to nail the trim on both sides of the door every 12 to 16 inches using 4d finishing nails. Fill all nail holes with putty.
Pro Tip: Pre-hung door frames extend below the door slab to allow for different floor heights. The jambs can be trimmed as needed before beginning installation, but make sure to leave adequate space to allow air to pass under the door according to ventilation codes.
Editor’s Note: Special thanks to Scott Webb and Richard Hamilton for contributing to this article.