Which wall cracks matter, and which do not

The cracks that are just the house breathing

Drywall is a rigid sheet screwed to framing that is not rigid, so when the framing swells through a humid Georgia summer and dries once the heat runs, the board goes with it. That stress finds the weakest line available: the short piece of board above a door, the butt joint where two sheets meet, the taped inside corner where a wall turns.

• A hairline running diagonally from the top corner of a door or window
• A straight vertical hairline mid-wall, where two sheets of board butt together

Plaster on lath lets go, it does not split

In the older housing around Rome the wall behind the paint is not board. It is wet plaster pressed onto thin wood lath, nailed across the studs with a gap left between each strip. Plaster squeezed through those gaps slumped over the back of the lath and set hard, and those hooks, the keys, are all that holds the wall up. Nothing is screwed to anything.

So plaster does not split, it lets go. A key snaps, the piece it held drops a fraction, and the surface tears along the easiest line, which is why plaster cracks wander and branch instead of running straight the way a board seam does. The failure is wider than the line, because keys break across an area bigger than you can see.

Lay a palm flat either side of the crack and press. Plaster that moves under light pressure, or a knuckle that gets a hollow drum instead of a solid tap, means the keys behind that patch are gone. Then look into the crack. A white hairline in the finish coat is cosmetic. One showing the gray base coat, or showing lath, has gone through the wall.

That is why filled plaster cracks come back: filler grips two pieces that are both still loose, so the next season opens the same line. What holds is fixing the plaster back to the lath first, then taping across so the repair carries a little travel. Where the loose area runs wide, skimming the whole wall is the honest answer, because a patch in old plaster shows as a shape under raking light.

• Hollow sound or movement under a flat palm: keys broken well past the visible line
• A bulge with give in it, on a ceiling especially: keys gone wide, with weight overhead

The cracks that are telling you something

What changes the reading is direction, displacement, and what else went wrong at the same time. A crack running diagonally through masonry follows the mortar joints, the weakest path through a wall being pulled out of shape. A step you can feel means two pieces of building no longer sit in the same plane.

None of it means a house is coming down. It means something moved, and the question is whether it still is. Soil that swells wet and shrinks dry carries whatever bears on it, and the corner that moves is usually the corner roof water has been landing at.

Openings corroborate. One door dragging through a wet month is about that door and its hinges. Four along the same wall, gaps all tapering the same way and trim opening at the miters, is the frame moving.

• Stair-step cracking through mortar joints in brick or block: a chimney, a foundation wall, a crawlspace pier
• A horizontal crack partway up a block foundation wall, or a wall bowing inward

What to check, and in what order

Movement over time is the whole answer, and a pencil gathers it better than any tool.

• Find out what the wall is, because board and plaster want different repairs. Tap it: board sounds hollow between studs, plaster stays dense
• Find the crack with a fingernail before your eye does. A nail that catches nothing is hairline, the normal category
• Run a finger across it rather than along it. Flat is a finish problem, a lip means the two sides have moved
• Date it: a pencil tick across each end, the date beside it, a photograph with a coin for scale
• Leave it through a humid summer and a wet winter, then look again. A year of no change is what an engineer asks for first

Who to call for the serious kind

With masonry stepping, displacement, a bowing wall, or openings failing together, the first call is a licensed structural engineer rather than a foundation repair company. An engineer sells you an opinion and walks away. A repair contractor sells the repair, which does not make them dishonest but does attach the opinion to something. Take the dated photographs. What comes back is a written scope that competing bids can be priced against.

Foundation and load-bearing work sits on the far side of what we take on. This side of the line is the rest of it: re-securing loose plaster and skimming it flat, patching drywall so a repair holds, cutting out and re-taping a seam that keeps opening, and putting the paint back.

Repairs a homeowner can make hold

Patching a stable hairline in drywall is a genuine homeowner job. Open the crack with the corner of a utility knife so compound has something to key into, bed mesh tape over it if it has been back before, then thin coats, each wider than the last. Sand, prime, paint.

Plaster is homeowner work too. Plaster washers are a hardware store item, a thin perforated disc that goes on a drywall screw. Driven through sound plaster into a lath strip, they pull the loose material back tight and sit below the surface so compound covers them. Set them across the whole soft area, not only along the crack, then tape and skim. Paint over plaster in a house built before 1978 may hold lead, so sand wet.

Use caulk, not compound, where a wall meets a ceiling and at inside corners. Those joints move and compound is rigid, so rigid filler there cracks again by next season.

Not a homeowner job: anything with a step in it, stair-step cracking through masonry, a foundation wall cracked horizontally or bowing, a plaster ceiling with give in it overhead, or any crack you have watched grow. Filling those hides the evidence.

Cosmetic, worth doing properly, or worth an engineer

It can wait. A hairline at the corner of a door or window, no step in it and no change over a year, is cosmetic. Patch it when you next paint the room.

Soon. A crack that keeps reopening in the same place, plaster gone hollow around it, or a wall and ceiling joint that opens every winter. Do those properly once instead of filling them every spring. Roof water landing against the foundation is in the same bracket.

Now. Stair-step cracking through mortar, displacement across a crack, a horizontal crack or a bow in a foundation wall, several openings failing together, or floors starting to slope. That is an engineer. So is a plaster ceiling that has begun to sag, for a plainer reason: weight over a room in use.

Change over time decides it, not width. A wide crack that has sat still for a decade is a scar. A hairline that has doubled since spring is a question.

Questions that come with this one