Is the crack in my wall cosmetic or structural?

What is a crack?

It is a narrow opening in a wall or ceiling. It may stay in the plaster and paint only, or it may run through the wall and follow movement in the structure. How it looks is not enough to tell which of the two you have.

Likely causes

Fine cracks usually come from the drying and shrinking of plaster, from small expansions with heat and cold, or from a joint between different materials, such as brick and concrete. The more worrying ones are usually linked to movement of the building, such as ground settlement, long-term water ingress or nearby building works. In Portugal, many older homes have thick stone or rubble walls, and many flats have brick infill in a concrete frame; both can show hairline cracks where materials meet. That is why we always say "likely cause".

How to tell them apart

• Hairline and not moving: usually just the plaster or paint.
• Diagonal, stepped, or running from the corners of windows and doors: deserves attention.
• Getting wider: the most important sign, even if it is thin.
• Wide, with one side standing out from the other, or plaster flaking at the edges: ask for an assessment.
• Together with doors and windows starting to stick: may point to movement.

To find out whether it is growing, mark the ends of the crack in pencil with a small line across it, and photograph it with the date. Look again in a month. If the ends have passed the lines, it has grown.

What to do now

Most fine cracks that appear in new or freshly painted homes come from the materials settling and stabilise on their own. In older homes, a crack that reappears in the same place after being repaired deserves more attention, because the movement that caused it may be continuing. Keeping dated photos also helps a professional assess it faster.

1. Photograph the crack close up and from a distance, with a ruler or a coin beside it for scale.
2. Mark the ends in pencil and note the date.
3. Check for damp, stains or sticking doors near the crack.
4. Look again in a month and compare.
5. Do not repair a crack that is growing yourself: covering it hides the sign and postpones the problem.

A fine, stable crack can be filled and painted once you have confirmed it is not growing.

When to call a professional

Call a structural engineer (engenheiro civil) if the crack is diagonal, stepped, wide or growing, if it appears at several points of the home at once, or if doors and windows that stick come with it. For a finish-only defect, a painter (pintor) or a builder (pedreiro) will sort it out. If plaster or loose pieces might fall, keep away from the area and ask for an urgent inspection. If you rent, tell your landlord (senhorio) in writing and send the dated photos.