Almost every building develops cracks. Most are cosmetic. A few are warning signs of structural trouble. The skill lies in telling them apart, and engineers do that by looking at three things: width, pattern and whether the crack is growing.
Step 1: measure the width
A widely used engineering classification grades cracks by width:
| Width | Category | What it usually means |
|---|---|---|
| Under 0.1 mm | Negligible | Hairline, cosmetic |
| Up to 1 mm | Very slight | Fill during repainting |
| 1 to 5 mm | Slight | Repair and monitor |
| 5 to 15 mm | Moderate | Needs an engineer's inspection |
| Over 15 mm | Severe | Get professional help promptly |
A simple way to measure: a standard credit card is about 0.8 mm thick. If the card slips in easily, the crack is wider than 1 mm.
Step 2: read the pattern
Fine map-like cracks in plaster
A spider-web pattern of hairline cracks is usually shrinkage in the plaster, from drying too fast or a mix that was too rich. It is cosmetic.
Horizontal crack where the roof slab meets the wall
A roof slab expands in the heat of the day and contracts at night. On the top floor, this movement often cracks the plaster along the junction. It is usually thermal and not dangerous, and good roof insulation or waterproofing reduces it.
Vertical crack at a column and wall junction
Concrete and brick move differently with temperature and moisture. Without a mesh strip over the joint when plastering, a straight vertical crack appears. It is common and cosmetic.
Diagonal cracks from the corners of doors and windows
These deserve attention. Diagonal and stepped cracks, especially wider at one end, often point to uneven settlement of the foundation. Check whether doors have started to jam.
Cracks along a beam or column with rust stains
This is the most serious common pattern. When reinforcement corrodes, the rust takes up more space than the steel and splits the concrete cover. It usually starts in wet areas such as bathrooms, terraces and coastal buildings, and it gets worse with time.
Cracks in the middle of a beam or slab soffit
Cracks underneath the middle of a slab or beam, especially with visible sagging, can indicate overloading. Get an engineer to look at it.
Step 3: check if it is growing
Draw pencil marks across both ends of the crack and write the date beside them. Better still, stick a small glass slide or a strip of plaster across the crack. If the marks extend or the strip breaks over weeks, the crack is active. Active cracks need their cause fixed before any repair will last.
How cracks are repaired
- Hairline cracks: fill with a flexible crack filler before painting.
- Wider plaster cracks: cut into a V groove, fix a fibre mesh and replaster.
- Structural cracks: after the cause is fixed, engineers may use epoxy injection, jacketing or other strengthening.
- Corrosion cracks: remove loose concrete, clean and treat the steel, then restore cover with polymer-modified mortar.
Frequently asked questions
Can paint alone hide cracks?
Only hairline ones, and only until the next season of expansion and contraction. Anything wider will reappear unless it is properly filled.
Do new houses crack?
Yes. The first year brings shrinkage and minor settling cracks. Most are cosmetic and are best repaired after the first full cycle of summer and monsoon.