Hairline crack or crack: what is the difference?
A hairline crack is a fine opening that usually affects the surface of the element or its finish (plaster, paint, tiling). A crack is a larger opening that goes right through the element.
Size matters, but it does not tell the whole story: a hairline crack in a beam can be more significant than a crack in a partition wall. To assess it, you need to know where it is, what shape it has and whether it is still growing.
Why they appear
Many hairline cracks are not structural: they are caused by mortar or concrete shrinking as it dries, by thermal expansion due to temperature changes, by damp or by small movements in partition walls. They are usually fine, stable and merely cosmetic.
Others indicate that the structure is working beyond what was expected. When a material bears stresses above its elastic limit, it deforms permanently, and if they keep increasing it eventually fails. The most common causes are:
- Differential settlement of the foundation, due to ground that gives way unevenly.
- Unforeseen overloads or changes of use.
- Design, calculation or construction errors.
- Corrosion of concrete reinforcement.
- Material fatigue caused by repeated loads.
Signs that it may be structural
It is advisable to have a technician inspect the crack if you notice any of these signs:
- Diagonal cracks, at around 45°, in walls and partitions, especially near the corners of doors and windows: these are typical of foundation settlement.
- Cracks in beams, columns, floor slabs or load-bearing walls, and not just in the finish.
- Cracks that follow the line of the reinforcement, with rust stains or pieces of concrete breaking off.
- Cracks that widen or lengthen over time.
- Visible deformation: sagging floor slabs, sloping floors, doors and windows that no longer close properly or walls that are out of plumb.
How to monitor a crack
The most important thing is to know whether the crack is active (still moving) or stabilised. While you wait for the technician, you can do the following:
- Photograph it with a size reference (a ruler or a coin) and note the date.
- Mark its ends with a pencil and the date, to see whether it lengthens.
- Repeat the photographs from the same spot every few weeks.
- Do not cover it up or paint over it before the diagnosis: you would erase the information the technician needs.
In the technical study, tell-tales or crack monitors are installed to measure precisely whether the crack opens over time.
What a pathology study is
A structural pathology study has three parts:
- Identification of the defective behaviour: inspection, data collection and, if necessary, testing.
- Investigation of the causes, in the structure and in the ground.
- Proposed intervention: repair, strengthening or, if the problem originates in the ground, work on the foundation.
Repairing the crack without correcting its cause only means it will reappear. This is what we do in our structural pathology service.
What if the home is new?
If the building is less than ten years old, damage affecting the structure may be covered. The Spanish Building Act (Ley de Ordenación de la Edificación, LOE) sets a ten-year period, from the handover of the works, for damage caused by defects in the foundation or structure that directly compromise its strength and stability, and requires decennial insurance for residential buildings.
There is a two-year time limit for claims from when the damage appears, so it is best not to wait. We explain this in the guide structural damage and the decennial guarantee.