Hairline cracks in plaster: cosmetic, or a sign of something worse

A hairline crack you cannot fit a fingernail into, running along a plasterboard joint or angling out from the corner of a door or window, is ordinary movement and is a filling and painting job. The cracks worth worrying about are wide enough to get a fingernail into, run diagonally or in a stepped line across a wall, keep reopening after a proper repair, or arrive alongside doors that have started sticking and floors that have gone out of level.

Almost everything people ask us about falls into the first group. The point of this piece is to help you be sure which group you are in before you spend anything.

What causes hairline cracks in a plasterboard wall?

The joints between sheets are the weak line, so that is where cracking appears first. Behind the lining, the timber framing moves. It shrinks as it dries out over the first years of a building's life, and then it keeps moving seasonally as indoor humidity swings between a damp closed-up winter and a dry heated room. Thermal expansion adds to it. Minor settlement adds to it. So does the ordinary low-level seismic movement you get in Wellington, most of which you never feel but which is quite capable of opening a taped joint.

Corners of doors and windows crack most often because the lining is weakest where a sheet has been cut around an opening, and stress concentrates at the corner. A single crack running up at forty-five degrees from a door head is close to a standard feature of New Zealand houses.

Which cracks are cosmetic and which are not?

What you see	Most likely cause	Verdict
Fine straight line following a sheet joint, ceiling or wall	Joint movement, framing shrinkage	Cosmetic. Fill, tape, paint.
Short fine crack from a door or window corner	Stress concentration at an opening	Cosmetic. Repair properly and it usually stays shut.
Hairline crack that opens in winter and closes in summer	Seasonal humidity and thermal movement	Cosmetic. Needs a flexible detail rather than more filler.
Crack at the junction of ceiling and cornice or scotia	Two elements moving independently	Cosmetic, but it wants sealant, not hard filler.
Wide enough to get a fingernail into, or wide at one end and tapering	Real structural movement	Investigate before repairing.
Diagonal or stepped, running across several sheets	Foundation or frame movement	Investigate. Get a builder or engineer.
Several cracks appearing together in one part of the house	Ongoing movement in that area	Investigate. A pattern matters more than a single crack.
Cracks plus sticking doors, sloping floors or gaps at skirtings	Structural	Stop. Filling this hides a problem you need to know about.

Why does the crack come back after I fill it?

Usually one of three things. The first is that filler alone was pushed into a joint that still moves, and filler has no tensile strength across a moving line. The second is that the movement has not stopped, so any rigid repair will fail again on the same schedule. The third is that the repair was fine but the paint over it was not, so what you are seeing is a flashing patch or a shrinkage line rather than the original crack.

A repair that lasts on a plasterboard joint means raking the crack out, re-taping the joint with paper tape and jointing compound, feathering the compound out wide enough that the build is invisible, sanding, and spot-priming before the topcoats. That is a stopping job rather than a filling job, and it is the difference between a repair that stays gone and one that comes back.

Flexible sealant has its place, but that place is the junction between two different materials or two elements that are meant to move: where scotia meets the ceiling, where an architrave meets the wall lining, where a wall meets a chimney breast. Sealant in the middle of a plasterboard sheet is a patch, not a repair, and it will always show.

Why is my patch a different sheen to the rest of the wall?

Because bare jointing compound is porous and the surrounding wall is not, so the topcoat soaks in differently and dries to a different sheen. This is the single most common complaint after a do-it-yourself crack repair, and the fix is to seal the patch before painting and then coat the full wall from corner to corner rather than spotting it in. Trying to touch in a patch on a wall that has been up for ten years almost never matches, because the existing paint has aged and the new tin has not.

Do cracks in a Wellington house mean earthquake damage?

Mostly no. Hairline cracking at joints and door corners accumulates in every house here over time, and on its own it is not evidence of damage from any particular event. What to do about cracks that appear after a shake, including the insurance side of it, is covered in ceiling cracks after an earthquake.

What about the plaster in an older villa?

Solid plaster and lath-and-plaster behave differently to modern lining and need a different eye. Tap across the wall with your knuckles: a solid note means the plaster is still keyed to the lath behind it, and a hollow drummy note means it has come away. Drummy areas will keep cracking and eventually let go regardless of what you fill them with. Fine crazing across a solid plaster surface is usually a surface phenomenon and can be sealed and coated. A crack with one side sitting proud of the other is not cosmetic.

On a villa with widespread crazing or many old repairs, a skim coat over the whole wall often gives a better result for less money than chasing thirty individual cracks and then repainting anyway.

When is filling a waste of money?

When the wall is still actively moving, because you are buying a repair with a known expiry date. When the cracking is dense enough that skimming or re-lining is the sensible answer. And when there is a structural question you have not answered yet, because a filled crack tells you nothing while an open one is still giving you information.

Otherwise, fill it, tape it, prime it and paint the wall properly, and it will be gone for a long time. If you want it done as part of an interior repaint, or you simply want someone to look at a crack and tell you honestly whether it matters, get in touch.