1. Not all cracks are equal
Cracks vary enormously in significance:
Cosmetic cracks — Hairline cracks in plaster or paint film. Unsightly but structurally irrelevant. Often due to normal drying, minor settlement, or thermal movement.
Monitoring cracks — Wider cracks or distinctive patterns that might indicate movement. Worth watching, but not necessarily urgent.
Significant cracks — Cracks suggesting structural issues: foundation problems, overloading, material failure. These need investigation and may need intervention.
The challenge is distinguishing between them. A crack that looks alarming might be harmless; a subtle pattern might indicate serious problems. Context, pattern, and professional assessment matter.
2. Thermal movement
Buildings expand and contract with temperature changes. Materials have different expansion rates. This creates stress at junctions and weak points.
What it looks like:
- Cracks at corners of windows and doors
- Cracks at junctions between different materials
- Cracks that open and close seasonally
- Long, relatively straight cracks
Is it serious?
Usually not structurally serious — the building is doing what buildings do. But external thermal cracks can allow water ingress and should be repaired. The repair needs to accommodate ongoing movement (flexible fillers, not rigid ones).
What to do:
Monitor for a year to understand the movement cycle, then repair appropriately. If cracks are extensive, consider whether expansion joints are needed.
3. Settlement and subsidence
All buildings settle slightly after construction as loads consolidate. This is normal. Subsidence — ongoing downward movement due to foundation problems — is more serious.
Normal settlement:
- Typically occurs in first few years after construction
- Small, hairline cracks that stabilise
- Evenly distributed, not concentrated
Subsidence:
- Cracks that continue to grow
- Diagonal cracks from corners of windows and doors
- Cracks wider at top than bottom (or vice versa)
- Doors and windows sticking
- Floors becoming uneven
Common causes of subsidence:
- Clay soils shrinking in dry weather
- Tree roots extracting moisture from soil
- Leaking drains washing away soil
- Inadequate foundations for soil conditions
- Mining or underground workings
Is it serious?
Subsidence is serious. It can affect structural stability, cause progressive damage, and be expensive to remedy. If you suspect subsidence, get professional investigation promptly.
What to do:
Commission a structural investigation. Monitor crack movement (crack gauges over several months). Investigate soil conditions and drainage. Consider tree management if relevant.
4. Heave
The opposite of subsidence — upward movement of foundations, usually due to soil expansion.
What causes it:
- Clay soils swelling after wetting (following dry period or tree removal)
- Frost heave in cold conditions
- Underground water sources
What it looks like:
- Cracks often wider at bottom than top
- Upward displacement visible
- Typically affects part of building rather than whole
Is it serious?
Yes — heave can cause significant structural damage. The forces involved are substantial.
What to do:
Investigate the cause. If tree removal is suspected, consider the timeline. Professional structural assessment is essential.
5. Overloading
Buildings are designed for specific loads. Exceed those loads, and structural elements can fail or deform, causing cracking.
What causes it:
- Adding heavy equipment (HVAC units, heavy storage)
- Change of use (office to archive storage, for example)
- Structural alterations that redistribute loads
- Cumulative additions over time
What it looks like:
- Cracks in load-bearing elements
- Visible deflection (sagging beams or floors)
- Cracks in masonry under concentrated loads
- Cracking around points of load application
Is it serious?
Potentially very serious. Overloading can cause progressive failure. The element that’s cracking may not be the problem — it may be taking load from something else that’s failing.
What to do:
Assess current loading against building capacity. Structural engineer investigation. Remove or redistribute excessive loads. Strengthen if necessary.
6. Moisture movement
Materials absorb and release moisture, changing dimension as they do. This creates stress and cracking.
What causes it:
- Wetting and drying cycles
- High humidity environments
- Water ingress
- Condensation
What it looks like:
- Cracking concentrated around wet areas
- Pattern related to moisture source
- Associated damp staining
- Cracking in timber, plaster, or other moisture-sensitive materials
Is it serious?
The cracking itself is often minor, but the moisture causing it may indicate serious problems (water ingress, drainage failures, condensation issues).
What to do:
Address the moisture source. Cracking is the symptom; moisture is the disease.
7. Material degradation
Materials deteriorate over time, and some forms of deterioration cause cracking.
Carbonation in concrete:
Concrete relies on alkalinity to protect reinforcement. Carbon dioxide from air penetrates and neutralises the concrete (carbonation). If it reaches the reinforcement, the steel corrodes and expands, cracking the concrete.
Sulfate attack:
Sulfates in groundwater or soil can attack concrete, causing expansion and cracking.
Frost damage:
Porous materials that absorb water can crack when that water freezes and expands.
Corrosion of embedded metals:
Steel beams, lintels, or fixings that corrode expand and crack surrounding masonry.
Is it serious?
Material degradation is often progressive. What’s visible is usually less than what’s affected. Professional investigation determines the extent.
What to do:
Investigate the mechanism. Address the cause (moisture ingress, exposure conditions). Repair may range from surface treatment to significant structural intervention.
8. Poor original construction
Sometimes cracks result from defects in the original construction:
- Inadequate foundations for the loads and soil conditions
- Poor-quality materials
- Inadequate reinforcement
- Bad workmanship
- Design errors
These defects may manifest years after construction.
Is it serious?
Can be. Depends entirely on the nature of the original defect.
What to do:
Investigation to understand what’s wrong. Solutions depend on findings — from monitoring to major structural work.
When to get professional help
Definitely get help if:
- Cracks are growing or changing
- Cracks are wide (over 5mm)
- Diagonal cracking from corners of openings
- Cracks through bricks/blocks, not just mortar
- Associated symptoms (sticking doors, sloping floors)
- Building was recently altered or loaded differently
- Trees are nearby on clay soil
Probably fine to monitor:
- Hairline cracks in plaster that haven’t changed
- Seasonal cracks that open and close predictably
- Cracks along joints between different materials
Remember: You’re not expected to diagnose this yourself. If in doubt, get professional assessment.
Key Takeaways
- Not all cracks are serious — but some are, and the difference isn’t always obvious
- Pattern and progression matter — diagonal cracks, growing cracks, and associated symptoms are warning signs
- Many causes exist — thermal movement, settlement, heave, overloading, moisture, degradation
- Context is everything — the same crack can mean different things in different buildings
- When in doubt, investigate — the cost of assessment is trivial compared to missing a structural problem
Need Help?
If you’ve got cracks that concern you — or you’re not sure whether they should concern you — we can help. Building pathology investigation determines what’s happening, why, and what should be done about it.
Related Services:
- Building Pathology & Defect Analysis — Professional investigation and diagnosis
- Pre-Acquisition Surveys — If you’re buying and have noticed cracking
- Planned Maintenance Schedules — Systematic monitoring and maintenance
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