How to Install Interior Doors the Easy Way

Whether installing doors into a new construction scenario or changing your doors to create an updated look, this method allows for quick and easy installation with the most common type of finishing – mitered corners.

You will need the following tools and supplies:

1. A 4′ Level
2. About 30 8d finish nails
3. About 40 6d finish nails
4. A hammer
5. A nail set or punch
6. One package of shims
7. Enough casing (trim for doors) to wrap the sides and top of both sides of the door
8. A miter saw
9. A pencil
10. A prehung door sized to fit your opening (see below)

Introduction to Prehung Doors: A prehung door is a door slab pre-installed into the jambs (frame) with hinges, door stop moulding, and is ready for door hardware. Each part of a prehung door needs to be sized to fit into your particular door opening. The slab of the door (the part of a door that actually swings and allows passage) is about 2 inches smaller in width than the door rough opening. The width of the jamb should come close to the depth of the rough opening (the width of the rough framing plus thickness of both finish surfaces – usually drywall). Most prehung doors set into rough openings with 2×4 framing and 1/2 inch drywall on both sides have 4-9/16 inch jambs. Prehung doors set into rough openings with 2×6 framing and 1/2 inch drywall on both sides have 6-9/16 inch jambs. Other scenarios are considered special and will require either custom treatment or special order from the vendor who supplies your doors. Once the correct door size and jamb width is determined and the prehung unit is on site, you are ready for installation.

Installation Instructions:

1. Cut and install two legs of casing – Remove the packaging from the prehung door and set flat it on the floor near the installation area hinges up. Set a length of casing on the hinge side of the door jamb such that the end of the casing is even with the bottom end of the door’s jamb. While holding the casing, make a mark on the inside edge of the casing 3/16″ past the point where the head jamb meets the leg jamb. Using the miter saw, cut a 45 degree angle with the mark being the short point of the angle. Once cut, you will have a length of casing with one end square and one end with a 45 degree angle whose shortest length is 3/16″ longer than the distance from the bottom of the door to the leading edge of the head jamb. Nail this length of casing to the edge of the door jamb using about eight 6d finish nails or a 16 or 18 gauge brad nailer leaving a 3/16″ reveal between the inside of the jamb and the leading edge of the casing. Tip – no need to get out a tape measure for the 3/16″. A little bigger than 1/8″ and a little smaller than 1/4″ is good enough. If hand nailing, it is sometimes a good idea to pre-drill pilot holes into the trim so it does not split when nailing. Repeat this process for the other leg jamb.
2. Hang the door – Carefully lift up the prehung door and set the door into the opening. The casing you installed in step one will prevent the door from falling through the opening and assure that the jamb is close to flush with the drywall on the opposite side of the wall in which the door is being installed. Adjust the installation height depending on your finish flooring. If carpet is to be installed on both sides of the door, it is usually a good idea to put 3/8″ to 1/2″ blocks under the bottoms of the door jambs so the door slab does not drag on carpet as it is opened or closed. If your finished flooring is already installed, set the bottoms of the jambs directly onto it. Using the 4′ level against the hinges, assure that the door sits plumb in the opening. Also, assure that the door is close to centered in the opening to allow for any adjustments. Once good, put a 8d nail (or 15 gauge or 16 gauge pneumatic trim nail) through the hinge side casing and into the framing at a height about even with the top hinge, then put another nail about even with the bottom hinge through the same piece of casing. These two nails should hold the door from falling. At this point, some adjustments can be made to assure that the reveal between the door slab and the door jamb is even on both sides and the top of the door. Once this is assured, four to five 8d nails should be driven in evenly spaced through both pieces of casing into the rough framing. The door should now be firmly seated in the opening.
3. Adjustments and shimming – At this point, the door can be opened and closed. On the side of the door opposite the hinges, verify that the door slab makes contact with the door stop moulding evenly from top to bottom. If it does not, you are able at this point to push or pull the door’s frame to fix this. While on the side of the door opposite the hinges, you must shim the door also. Shims ought to be installed in three places on each side of the door – ideally at heights equal to that of each hinge. Only put enough shims between the rough framing and the door jambs to fit a bit snugly. Too many or too few shims will result in poor operation of the door. Nail the shims into place through the faces of the door jambs as you go with two 8d finish nails per shim location. Verify proper door operation, contact with door shop moulding, and slab to jamb reveal with each shim set installation.
4. Final trim installation – Install the head piece of casing on the hinge side of the door and all three pieces of casing on the side of the door opposite the hinges – 6d finish nails at the thinner side of the casing to the jambs and 8d finish nails through the thicker side of the casing and into the drywall and rough framing. If you need advice on making perfect miter cuts, search for advice on perfect miter cuts in another article. After all trim is installed, ensure all nails are set past flush at all locations by using the nail set to allow for putty.
5. Finishing touches – At this point, the nail holes can be puttied, door hardware installed, and door components painted or stained. The installation is now complete.

After you have a couple door installations under your belt using this method, it is possible to install doors in less time than it takes to read this article. I hope this gives you a better understanding of what is involved in installing interior doors.