Hang a Pre-Hung Door So It Doesn't Drift Open at 2 a.m.

Quick answer: Get the hinge side dead plumb in both directions before you touch anything else. Keep the door shut and clipped until the jamb is shimmed, and swap one top-hinge screw for a 3-inch screw into the stud.

You know the door. It creeps open on its own at night. It latches only if you lift the handle and lean. It rubs the top corner every time the humidity changes. Nobody built that door wrong on purpose. They nailed the wrong side first.

Check the hole before you buy the door

Measure the rough opening, which means the framing with the old jamb pulled out. A standard pre-hung unit wants an opening about 2 inches wider than the slab and about 2-1/2 inches taller. A 32-inch door goes in a hole roughly 34 inches wide and 82-1/2 inches tall.

Take three width measurements, at the top, middle and bottom. If the bottom is half an inch narrower than the top, a stud is bowed, and you want to know that now, not with a 60-pound door balanced on your knee.

Then check these:

• Wall thickness. Most pre-hung jambs are 4-9/16 inches, built for a 2x4 wall with 1/2-inch drywall on both sides. Old plaster walls often run thicker. Order a wider jamb or plan on jamb extensions.
• Floor level. Lay a level across the opening at the floor. Shim the low end until the bubble centers and measure the shim. That number matters later.
• Trimmer studs. Hold the level on both studs at the sides of the opening, against the edge and against the face. Don't panic if they lean. You'll shim past it. You just need to know which way.
• Handing. Stand on the side the door will swing toward you. Hinges on your left means a left-hand door. If you're unsure, sketch the opening on your phone and show the guy at the store.

Tools and parts, with rough costs

• Pre-hung interior door: about $80 to $180 for hollow-core, $200 to $450 for solid-core
• Shims: $5 to $10 a bundle. Buy two bundles.
• 4-foot level, $25 to $50. A 6-foot level makes the hinge side easier and runs $60 to $100.
• Finish nailer and 2-1/2-inch nails, or a hammer and nail set for about $15. Plenty of rental counters carry nailers.
• 3-inch wood screws, under $10 a box
• Casing: figure $25 to $60 a side for primed finger-joint pine
• Lockset: $15 to $40
• Utility knife, flat pry bar, a handsaw or oscillating tool, and a miter saw or a $20 miter box for the casing

Safety before the first shim

If a light switch or outlet sits within a few inches of the opening, the wire feeding it runs right where you'll drive nails and screws. Turn that circuit off at the panel and confirm it's dead with a tester before you start.

Wear safety glasses with a nailer. Get a second person for a solid-core door. They're heavy, they're awkward, and they love to tip.

Two situations aren't a weekend project. If you're widening an opening in a load-bearing wall, that's a new header, and it needs a licensed contractor and usually a permit. And the door between an attached garage and the house falls under fire-separation rules. Most codes require a solid-core or fire-rated door there, and some areas require a self-closer too. Codes vary by area, so call your local building department before you buy that one.

Settle the floor before the door goes in

Remember the number from the level? That's how much higher one side of the floor sits than the other.

Trim that amount off the bottom of the jamb leg on the high side. Leave the door clipped shut while you cut so nothing shifts. Now the head jamb will sit level even though the floor doesn't.

Skip this and you'll spend an hour shimming a crooked head and wondering why the gap at the top looks like a ramp.

Hinge side first, and nowhere else

Leave the shipping clips or the nail through the stop in place. The door stays closed. A pre-hung unit comes square from the factory, and a closed slab holds the jamb in that shape while you work.

Stand the unit in the opening and center it. Push it in until the jamb edges sit flush with the drywall on both sides.

Now shim the hinge side. Use shims in pairs, thick ends pointing opposite directions, so they slide into a flat, parallel pack. Put a pack behind each hinge. Start at the top hinge, then the bottom, then the middle.

Put the long level on the hinge jamb and get it plumb along its face. Then check it plumb in the other direction, across the edge of the jamb. That second check is the one people skip, and it's the reason doors drift. A jamb leaning even a little toward the room turns the door into a pendulum.

When both directions read plumb, nail through the jamb at each shim pack, two nails per spot, tight against the stop so the holes are easy to fill.

What if the wall itself leans? Then the jamb and the wall disagree, and you pick plumb. The casing on one side might sit a little proud of the drywall at the top. You can plane it or caulk it. A door that swings on its own is a much bigger problem than a tapered caulk line.

The string test nobody bothers with

Tack a thin string from the top corner of the hinge jamb to the bottom corner of the latch jamb, on the face. Run a second one across the other diagonal.

Where they cross, they should just kiss. If one string pushes hard into the other, or there's a gap, a jamb leg is twisted out of plane. You'll see it later as a door that hits the stop at the top before the bottom. That's the door that never feels quite shut.

Fix it now by tapping the bottom of the latch jamb in or out before you nail that side. Two minutes, two strings, and you're done.

Latch side and the reveal

The reveal is the gap between the door and the jamb. You want a consistent 1/8 inch down the latch side and across the top.

Shim the latch jamb at the top, at latch height, and near the bottom. Adjust the packs until the gap runs even all the way down. Don't drive the shims hard. Too much pressure bows the jamb inward and the door binds in the middle.

Nail through the shims. Then go back to the top hinge, take out one screw that goes into the jamb side, and drive a 3-inch screw through that hole into the trimmer stud. It pulls the jamb tight to the framing and carries the weight of the door so the top corner doesn't sag over the next five years.

Cut the clips. Swing it.

Read what the door tells you

The door will tell you exactly what's wrong if you watch it.

• Swings open or closed by itself: the hinge jamb isn't plumb across its edge. Pull the nails at that side, adjust, re-nail.
• Rubs the top latch corner: the top hinge is loose from the framing. That 3-inch screw fixes most of these. Snug it a little more.
• Gap wide at the top, tight at the bottom on the latch side: the head isn't level. Go back to the floor measurement.
• Hits the stop at one end first: twisted jamb. The string test would have caught it.
• Latches but rattles: bend the small tab on the strike plate toward the stop with a flat screwdriver.
• Won't latch at all: the latch is hitting above or below the strike hole. Move the strike, or file the opening a sixteenth at a time.

Score the shims flush with a utility knife and snap them off.

Casing and the last ten percent

Cut the casing with a 3/16-inch reveal back from the inside edge of the jamb. Mark that line with a combination square on all three sides so your miters land in the right place.

Glue the miters. Same principle as when you reglue a wobbly chair: glue only holds clean wood to clean wood, so wipe off the dust first. A glued miter stays closed. One held only by nails opens up the first dry winter.

If pulling the old casing tore the drywall paper, fix it before the new trim goes on. A seal coat of primer keeps the patch from bubbling. Patching drywall without making it worse walks through it.

Install the lockset. Close the door with two fingers.

If it clicks shut and stays exactly where you left it, you didn't get lucky. You did the hinge side right.