Somebody points at a crack on your wall and says it needs an engineer. Somebody else says fill it with putty and forget it. Both sound confident.
There is an objective way to settle this, and it starts with a ruler.
Why Crack Width Is the First Thing to Measure
Crack width is the single most useful diagnostic you have. It correlates reasonably well with cause, and it directly determines what repair method will hold.
Use a crack width gauge if you have one. If not, a business card is roughly 0.3 mm and a standard visiting card edge is close enough for a rough call. Measure at the widest point, not the average.
Second most useful thing: mark the ends with a pencil and date it. Come back in three months. A crack that has grown is a different problem from one that has not, regardless of width.
Hairline Cracks: Under 0.3 mm
These are cosmetic. Almost every plastered wall in India develops them within a few years, usually from plaster shrinkage or minor thermal movement.
They do not threaten the structure. They do let water in during driving rain, which is why they are worth sealing rather than ignoring.
Repair: a crack-bridging exterior paint handles these directly. No filling needed. Two coats of an elastomeric or good acrylic exterior will span them.
Shrinkage Cracks: The Most Common Type
Typically 0.3 to 1.5 mm, random and map-like in pattern, often across the whole plastered surface rather than in one line.
Cause: plaster that dried too fast, an over-rich cement mix, or inadequate curing. Very common on work done in summer without proper water curing.
Repair: rake the crack open slightly to a V section, dust it out, fill with acrylic crack filler or polymer-modified mortar, sand flush, then prime and paint. Wide areas of map cracking may need a full skim coat.
Thermal Movement Cracks
These are usually straight, often horizontal or vertical, and reappear in the same place after every repair. Common at the junction between different materials, brickwork meeting RCC being the classic example.
Cause: concrete and brick expand at different rates. The joint between them moves every day, and rigid filler cannot survive that.
Repair: this needs a flexible material, not a rigid one. Rake the joint, fill with a polyurethane or acrylic sealant that can flex, then use fibreglass mesh tape over it before skimming. Rigid filler here guarantees the crack returns.
Structural Cracks: When to Stop and Call an Engineer
Stop work and get a structural engineer if you see any of the following:
- Crack wider than 3 mm
- Diagonal cracking, particularly running from a corner of a window or door opening
- Cracks that pass through concrete members, not just plaster
- Stepped cracking following the mortar joints in brickwork
- Any crack that has visibly widened in the last six months
- Cracks accompanied by a door or window that has started sticking
- Cracks visible on both faces of the same wall in the same position
None of these are painting problems. Filling and coating them hides evidence that somebody needs to see.
Repair Method for Each Type
| Crack width | Likely cause | Repair method | Can paint handle it? |
|---|---|---|---|
| Under 0.3 mm | Shrinkage, thermal | Crack-bridging exterior paint | Yes |
| 0.3-1.5 mm | Plaster shrinkage | V-groove, acrylic filler, sand, prime | After filling |
| 1.5-3 mm | Movement, poor mix | Rake, flexible sealant, mesh tape, skim | After treatment |
| Over 3 mm | Possible structural | Engineer assessment first | No |
| Any width, growing | Active movement | Engineer assessment first | No |
Can Paint Bridge a Crack?
Within limits, yes, and this gets oversold.
A quality exterior acrylic will span static hairline cracks up to about 0.3 mm. Elastomeric coatings, applied at proper film thickness, manage up to roughly 1 mm and can accommodate a little movement.
Neither will handle a crack that opens and closes seasonally by more than about a millimetre. And neither works if applied thin. Elastomerics in particular need heavy application to deliver the flexibility they claim, which is why they have lower coverage per litre than standard exteriors.
How to Monitor a Crack Over Time
If you are unsure whether a crack is active, monitoring is cheap and conclusive.
Mark both ends with a fine pencil line and write the date beside it. For a more precise reading, fix a small tell-tale across the crack, either a proprietary crack gauge or simply a blob of plaster of Paris bridging it. A rigid tell-tale that fractures confirms movement.
Check after three months and again after a full monsoon cycle, since thermal and moisture-driven movement peaks at different times of year. A crack that has not moved across both checks is stable and can be treated as a finishing problem. One that has moved needs assessment before any money goes into repainting.
What This Costs to Fix
Rough figures for planning: sealing hairline cracks with crack-bridging paint adds nothing beyond the paint itself. Filling and finishing cracks in the 0.3 to 1.5 mm range runs Rs 40 to Rs 90 per running metre. Flexible sealant with mesh tape on moving joints runs Rs 120 to Rs 250 per running metre. Structural investigation by an engineer typically costs Rs 5,000 to Rs 15,000 for a residential assessment, which is inexpensive relative to what it can prevent.
Frequently Asked Questions
Are hairline cracks in walls normal?
Yes. Hairline cracks under 0.3 mm are common in Indian plaster within a few years and are usually caused by shrinkage or thermal movement. They are cosmetic but should be sealed because they let water into the wall during driving rain.
When is a wall crack serious?
Treat a crack as serious if it is wider than 3 mm, runs diagonally from a window or door corner, passes through concrete members, follows brick mortar joints in a stepped pattern, or has widened over the past six months.
How do I fill exterior wall cracks?
Rake the crack open into a shallow V, remove dust, fill with acrylic crack filler for static cracks or flexible sealant for moving joints, sand flush, then apply exterior primer before painting.
Can paint cover wall cracks?
A good exterior acrylic bridges static hairline cracks up to about 0.3 mm, and elastomeric coatings up to roughly 1 mm at full film thickness. Anything wider must be filled first.
What causes cracks in new plaster?
Usually inadequate water curing, an over-rich cement mix, or plaster drying too rapidly in hot weather. These produce random map-like shrinkage cracking across the surface rather than single defined lines.
