A crack running across an old plaster ceiling has a way of looking much worse than it usually is. Most of the time, it's cosmetic, the kind of thing plaster does as houses breathe through the seasons. Every so often, though, that crack is the visible end of something actually failing behind the wall. Telling the two apart, before calling anyone or reaching for the spackle, saves a lot of unnecessary demolition.
How the wall is actually built
Traditional plaster in a house of this age isn't one material, it's a three-coat system applied wet over horizontal wood strips called lath. As the wet plaster gets pushed through the small gaps between lath strips, it squeezes out the back and curls over, forming what are called keys. Those keys are the entire structural connection holding the plaster to the wall or ceiling. There's no adhesive involved, no drywall screws. It's just plaster, physically hooked behind wood, by the hundreds, across the whole surface.
The two things that actually go wrong
Real plaster failure comes down to one of two mechanisms. Either the keys themselves snap, from decades of vibration, drying, or minor building movement, or the lath pulls away from the framing behind it, usually because the nails holding it have loosened over a century of use. Either way, the plaster is no longer properly anchored, and it's being held up by little more than the paint layer and habit.
Separate from that, and far more common, is simple surface cracking from ordinary shrinkage and humidity or temperature swings. Plaster shrinks slightly as it cures, and it keeps expanding and contracting slightly with the seasons for its entire life. That produces thin, hairline cracks, often radiating from a door or window frame, that open a little wider in the dry winter months and close back up in the summer. Cosmetically annoying. Structurally meaningless.
How to tell which one you're looking at
A thin, hair-width crack that tracks along a door frame or corner, and has been there for years without changing much, is almost always the harmless kind. A wide crack, one you could work a coin into, especially if it's jagged, diagonal, or paired with a section of ceiling that feels soft or springy when you press on it, is a different story. That's a sign of key failure or lath separation, and it needs the substrate stabilized before any cosmetic repair will actually hold.
The same goes for what people tend to call popping plaster: a patch that bulges out slightly, sounds hollow when tapped with a knuckle, or makes a faint crunching sound under gentle hand pressure. That crunch is usually broken keys shifting around behind the surface. A section like that is no longer really attached, even if it looks fine from across the room.
What the direction of a crack tells you
Direction isn't a diagnosis on its own, but it narrows things down quickly.
Vertical cracks are the most common and usually the least worrying. They tend to show up at inside corners, where two walls meet and move slightly differently, and running straight up or down from the corners of doors and windows. A hairline vertical crack in the middle of a wall often sits over a stud or a seam in the framing. Unless it's widening, it's typically seasonal.
Horizontal cracks deserve a closer look, mostly because of how the wall is built. Lath runs horizontally, so a long, fairly straight horizontal crack can be tracing a row of lath where the keys have let go. Press gently on either side of it. If the plaster moves, or one side sits slightly proud of the other, the keys are the likely cause. On ceilings, cracks that run parallel to the lath are the ones that tend to precede sagging, especially under rooms that see a lot of foot traffic.
Diagonal cracks, particularly ones that start at the corner of a door or window and angle outward, point toward movement in the framing or the house itself. A diagonal crack that's noticeably wider at one end than the other is worth watching over a few months. A light pencil mark at each end, with the date, is a cheap way to see whether it's still growing.
Then there's the fine, irregular web of cracks some older walls develop, sometimes called map cracking or crazing. That's almost always confined to the finish coat or the paint layers on top of it, and it's cosmetic.
The mistake that makes it worse
The single most common way people damage otherwise sound plaster is patching it with modern gypsum-based compound, the fast-setting stuff sold for drywall repair. The two materials don't move the same way. Lime plaster and modern gypsum compound expand and contract at different rates with temperature and humidity, so even a technically solid patch tends to crack loose along its own seam within a season or two, and the National Park Service specifically warns against using high-strength gypsum patches on historic lime plaster for exactly this reason. It's a compatibility problem, not a workmanship one.
For genuine structural failure, plasterers use metal plaster washers, screwed through the loose plaster into the lath or framing behind it, to physically pull the material back into place before any surface work happens. Skipping that step and just skim-coating over a section with failing keys doesn't fix anything, it just hides a wall that's still not attached to the house.
When it really is bigger than the wall
Occasionally a crack is a symptom of something further out, a settling foundation, an overloaded floor joist, or water damage from a slow roof or plumbing leak working its way through the framing. These usually show up as diagonal cracking around doors and windows, or cracking paired with staining, sagging, or a door that's suddenly hard to latch. In those cases, patching the plaster before addressing the actual source is wasted work, since the same crack tends to reopen in the same place once the underlying cause is dealt with.
In houses with stone foundations, water is often part of the story. The guide on fieldstone foundation moisture covers what's going on down there.
None of this is an argument for tearing out plaster at the first sign of a crack, which is unfortunately the default move for a lot of renovation crews unfamiliar with the material. Plaster outperforms drywall on sound dampening and durability, and most of what looks alarming about an old wall is just the ordinary sound of a house that's been standing for a hundred years.