Ask the Builder: Key to installing a prehung door is good shimming

DEAR TIM: While on vacation, I plan to install a few new interior doors around my house. I’m using prehung doors. I’ve never hung a door before and could use some tips. I’m lost on what to do when shimming a door. My wife told me she will bake you a cake if your advice gets me through this endeavor. – Peter K., Philadelphia

DEAR PETER: I’ll see the chocolate cake and raise you a pint of mocha chip ice cream from my favorite parlor in Cincinnati. While I appreciate your wife’s generous offer, it’s not necessary. I’ll do my best to guide you on how to install all the doors in a day or less. That way you and your best friend can go do some fun things instead of slaving away on your vacation.

The first thing to do is carefully remove the door casing trim from around your existing doors. This allows you to confirm the rough openings. This is vital to ensure you get the right doors. Measure accurately and have these measurements handy when you order the doors.

Usually the width of the rough opening between the wall studs is a half-inch wider than the outside overall width of the new door frame. This extra space gives you plenty of room to shim the door. The height of the rough opening is often 84 inches from the rough subfloor, but since your house is not new construction that’s hard for you to determine.

Just make sure that from the finished floor to the top of the rough opening you have around 83 inches, especially if the rooms have wall-to-wall carpeting. Be sure to check the width of the doorjamb. If you live in a home that has 2-by-4 wall studs and 1/2-inch drywall, you’ll discover the jamb width is most likely 4 9/16 inches.

Take photos of the doors and show them to the person who takes your order. Stand back from the door so you can see the entire door and part of the wall. It’s important that you get the “handing” of the door correct. The photos will tell the person at the lumber yard what you need.

Modern doors are made nearly perfectly square. The door frame is supposed to be parallel to the door once it’s installed with the same-sized gap between the door and the frame on all three sides. This means the door frame will also be square once installed.

I always check to see if the rough opening stud on the hinge jamb is plumb. If it’s not, I add shims to the rough stud to make it plumb. If I have to add shims, I do it at the places I intend to put nails through the door frame when installing the door. More on that in a moment.

When you add tapered shims, you need to use two at a time and overlap them so the thin end of one is on top of the thick end of the other. When you then slide one shim across the other, the thickness of the two shims changes equally across the overlap.

With the rough jamb plumb, it’s easy to get a head start on an easy install. Place the door into the opening and open the door. Place a few shims under the end of the door so there’s no stress on the door frame. Without this support, the weight of the door will pull the top of the frame away from the rough opening.

I prefer to use 10-penny finish nails to start the installation process. I nail my first one about an inch above the top of the top door hinge on the door frame. This stabilizes the door and allows you to work with no fear of the door falling out of the opening. Carefully hit the nail, making sure you do not miss, which would cause an unsightly blemish on the wood doorjamb. You’ll need to use a nail set to drive the head of the nail below the surface of the wood frame.

You will need a minimum of six nails on each side of the door frame. These nails should be 1 inch above each hinge location and level with these spots on the latch side of the door frame. Put a nail on either side of the center door stop.

When shimming the door, the goal is to make sure that, once the frame is nailed, the gap between the door and the frame is equal all around the door. Most prehung doors come with a 1/8-inch gap between the door and the frame on the hinge side and the top. You’ll discover it’s fairly easy to attain this gap around the entire door.

As you shim, the shims will stick out beyond the wall and the door frame. Do not be concerned about this. You’ll cut these off after the installation is complete. You can use a sharp razor knife, but the safer tool is a vibrating multitool that resembles what a barber uses to cut your hair.

To prevent the door from rubbing the frame in the future, I always install two hidden screws that penetrate the door frame and go at least 1.5 inches into the rough framing. I place these screws under the top hinge of the door. This step is only done after you’re completely happy with the gap and all the nails are set below the surface of the door frame.

Be sure you support the door with shims under the open door when you go to unscrew the top hinge plate from the door frame. Drill pilot holes and be sure the screws are as far away from the edge of the door frame as possible so they bite into the rough doorjamb.

The screws must penetrate through shims so they don’t stress the door frame. Countersink the screw heads a small amount so they don’t interfere with the hinge when you put it back in place.

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