Why does my door swing open or closed on its own?

The hinge jamb is out of plumb, so gravity is pulling the door toward one side. Correct the hinge jamb rather than trying to compensate elsewhere.

What reveal should I aim for around a door?

A consistent gap of roughly an eighth of an inch at the top and along the strike side is a common target, with the bottom clearance set for the finished floor covering.

Can I hang a prehung door alone?

Yes, though a second pair of hands helps with heavy or solid-core doors. Working alone, tack the unit in place lightly first, verify everything, and only then drive fasteners home.

Hanging a Door Without Losing the Afternoon

A door that is hung right closes with a light push, stays where you leave it, and shows the same gap all the way around. A door that is hung wrong will fight you for years. The difference is set in the first twenty minutes.

Start with the opening, not the door

Before the door comes out of the packaging, check the rough opening.

Plumb. Hold a level against both king studs. Out of plumb on the hinge side is the single most common cause of a self-swinging door.

Level. Check the floor across the opening. Most floors are not level, and knowing by how much lets you plan which side gets shimmed up instead of discovering it halfway through.

Twist. Sight across the opening from one side. If the two sides of the wall are not in the same plane, the jamb will twist and the door will hit the stop on one corner and gap at the other. A string across the corners or a straightedge held diagonally will show it.

Width and height. A prehung unit needs roughly half an inch of shim space around the jamb.

Fixing an opening is much easier before the door is in it. If a king stud is badly out, it is worth correcting.

Hanging a prehung unit

Remove the packaging but leave the shipping blocks or the retaining clips holding the door closed in the jamb until the unit is set. They keep the reveal while you work.
Set the unit in the opening from the side the door swings toward, and tip it into place.
Shim behind the top hinge, then check the hinge jamb for plumb in both directions: side to side, and front to back against the wall plane.
Tack the hinge jamb through the shims at the top hinge with a finish fastener, leaving it slightly proud in case you need to adjust.
Shim and check at the bottom hinge and the middle hinge, keeping the jamb straight rather than bowed. A straightedge along the jamb catches this.
Once the hinge jamb is plumb and straight, drive the fasteners home. Replacing one screw in each hinge with a long screw that reaches the framing keeps a heavy door from sagging over time.
Now remove the shipping blocks, close the door, and set the strike jamb to the door, shimming until the reveal is consistent from top to bottom. Fasten through the shims near the strike plate location, where the jamb takes the impact.
Check the head reveal and adjust before the last fasteners.

The order is the whole trick: hinge jamb to the world, strike jamb to the door.

Hanging a slab in an existing jamb

Different job, and mostly about transferring hinge locations.

Measure the existing hinge positions on the jamb and mark them on the slab, accounting for the reveal you want at the top. Scribe and mortise the hinge leaves so they sit flush with the edge; a hinge sitting proud pushes the door out of the opening and closes the reveal on the strike side.

Trim the slab for width and height before mortising, taking material from the hinge side where possible so the strike edge bevel stays intact. Most doors have a slight bevel on the strike edge so the leading corner clears as it swings; keep it.

Common problems and what actually causes them

Door swings open or shut on its own. Hinge jamb out of plumb.
Reveal wider at the top than the bottom on the strike side. Hinge jamb tilted, or hinges sitting proud of the mortise.
Door hits the stop at the top and gaps at the bottom. The jamb is twisted, usually following a twisted opening.
Latch does not catch. Strike plate misaligned; move the plate or file the opening, and check the jamb is not bowed.
Door sags over time. Hinge screws not reaching framing. Replace one screw per hinge with a long one.
Door binds in humid weather. Reveal was too tight to begin with, or the slab is unsealed on the top and bottom edges.

The finishing details people skip

Seal all six sides of an exterior slab, including the top and bottom edges, or it will take on moisture and move. Set the bottom clearance for whatever flooring is going in, not for the subfloor you see now. Check the operation before the casing goes on, because casing hides shims and makes adjustment a much bigger job.

Hang it once, carefully, and it will be right for as long as the house stands.