Almost every older home in Massachusetts, New Hampshire, and Maine has a ceiling crack somewhere. Most homeowners either ignore them for years or panic about them. Both reactions are usually wrong, because the shape and location of a crack tell you a great deal about what's causing it.
Hairline cracks that run along a seam in a straight line are the most common and the least alarming. On a drywall ceiling this is typically a taped joint that has moved slightly with seasonal expansion and contraction. The house swells in humid summers and shrinks in dry heated winters, and a joint that wasn't taped with enough compound behind it eventually shows a line. It's cosmetic. It needs the joint re-taped and re-coated properly, not just filled.
Spiderweb or map-pattern cracking across a broad area of old plaster is different. That's typically the finish coat crazing as it ages, and it's largely a surface condition. A skim coat over the full ceiling resolves it — spot-filling each crack will not, because the pattern will keep spreading around your patches.
The one to take seriously is a ceiling that sounds hollow when you tap it, or that has visibly dropped away from the plane, or that shows a crack with one side sitting lower than the other. In traditional lath-and-plaster, that means the keys — the plaster that squeezed through the lath gaps and hardened behind it — have broken. The ceiling is being held up by friction and habit. This one does not fix itself and it does eventually come down, usually in sheets. It can be re-secured with plaster washers and screws into the joists, then skimmed flat, but it needs attention rather than another coat of paint.
Cracks with brown or yellow staining are telling you about water, not structure. Somewhere above there is or was a leak — a roof, a bathroom, a failing supply line, or an ice dam backing water under the shingles. The staining will bleed straight back through fresh paint unless it's sealed with a stain-blocking primer, and more importantly the leak has to be fixed and the cavity dried before anything is closed up. Repairing the ceiling over a live leak just buys you a few months.
Ice dams deserve their own mention here because they're a regional specialty. Snow melts on the warm upper roof, runs down, and refreezes at the cold eave, and the resulting dam pushes water back up under the shingles and into the wall or ceiling below. The damage often shows up along exterior walls and in top-floor rooms, and it recurs every hard winter until the insulation and ventilation causing it are addressed.
The short version: straight-line cracks are usually joints, web patterns are usually surface aging, sagging means the plaster has let go, and stains mean water. If you're not sure which one you're looking at, send us a photo or call (978) 751-0616 and we'll come look at it.