A commercial building needs a structural inspection if you see any of these: cracks that are new, growing or wider than about 3 mm; diagonal cracks from the corners of doors and windows; sagging slabs or beams; spalling concrete with exposed rusting steel; rust stains running down concrete; doors and windows that suddenly stick; floors that slope or bounce; tilting walls or columns; and persistent leaks into the structure. One sign alone may be minor. Several together, or any that keep getting worse, need a registered structural engineer. Painting over them hides the evidence without fixing the cause.
1. Cracks that are new, growing or wide
Hairline cracks in plaster are common and usually harmless. Watch for cracks that:
- Are wider than about 3 mm.
- Get wider over weeks or months.
- Run through the blockwork, not just the plaster.
- Appear on both sides of a wall in the same place.
A simple way to monitor: draw pencil marks across the crack with the date and width, or fix a crack gauge. Our guide on which cracks are serious explains the patterns.
2. Diagonal cracks at doors and windows
Diagonal cracks running from the corners of openings often point to foundation movement or a missing or failed lintel. On black cotton soil, cracks that open in the dry season and close in the rains are typical of soil movement. See building on black cotton soil.
3. Sagging slabs and beams
Any visible sag in a slab or beam, or cracks running across the underside of a slab, needs attention. Causes include overloading (for example heavy files, water tanks or machinery that were not in the design), early removal of props during construction, or corrosion of reinforcement.
4. Spalling concrete and exposed steel
When reinforcement rusts, it expands and pushes concrete off. You see pieces falling from columns, beam edges or slab soffits, with brown bars showing. It is common in coastal buildings, in bathrooms and on balconies. Spalling gets worse over time and reduces the strength of the member. See building near the coast and reinforcement steel.
5. Rust stains on concrete
Brown streaks on concrete surfaces can be the first sign that steel inside is rusting, before any spalling. Find out why water is reaching it.
6. Doors and windows that start to stick
If doors that used to close easily now jam, or windows will not open, the frame around them may have moved. Check whether it is one opening or many, and whether it is getting worse.
7. Sloping, bouncing or cracked floors
Floors that slope noticeably, feel springy, or show cracks running across the building can point to foundation settlement or overloaded slabs. Put a marble or a level on the floor and see.
8. Tilting walls or columns
A wall or column that is visibly out of plumb is serious. Check with a long spirit level or a plumb line. Boundary and retaining walls are especially prone to this. See retaining walls on sloping plots.
9. Persistent leaks into the structure
Long-term leaks from roofs, flat slabs, tanks or bathrooms soak concrete and blockwork, rust steel, and weaken timber. Fixing the leak early is cheap. Repairing the damage years later is not. See roof leak repair and waterproofing roofs and walls.
Other times to get an inspection
- Before buying or leasing a building.
- Before adding a floor, a heavy water tank or heavy equipment.
- After a fire, flood, vehicle impact or earthquake tremor.
- When changing the use, for example from house to office or school.
- For older public buildings, as part of a regular check every few years.
What happens during an inspection?
- The engineer reviews any drawings and history.
- They inspect visually and measure cracks, levels and plumb.
- They may test concrete (rebound hammer, cores, cover meters) or dig trial pits at foundations.
- They report on causes, risks and recommended repairs, with priorities.
When it is urgent
Move people away and call an engineer straight away if you see rapidly widening cracks, pieces of concrete falling, a slab that has dropped, or walls leaning outwards. Safety comes before cost.
What do typical repairs look like?
| Problem found | Typical repair |
|---|---|
| Fine shrinkage cracks in plaster | Rake out, fill, repaint |
| Cracks through blockwork from movement | Find and fix the cause, then stitch the wall with bars or mesh and replaster |
| Missing lintel | Insert a new lintel with temporary support |
| Spalling concrete with rusted bars | Break out, clean or replace steel, apply protective coating and repair mortar |
| Sagging slab from overloading | Remove the load, add props, then strengthen with steel or new beams |
| Foundation settlement | Investigate the soil, then underpin or improve drainage around the building |
| Long-term leaks | Fix the waterproofing, then repair damaged plaster, steel and finishes |
The engineer decides which repairs suit your building. Simple cosmetic fixes for structural problems only hide them for a while.
Frequently asked questions
Who should inspect a building's structure?
A structural engineer registered with the Engineers Registration Board (ERB). See what a civil engineer does.
How much does a structural inspection cost?
It depends on the building size and tests needed. A visual inspection and report is usually a small fraction of the cost of the repairs it can prevent.
Are hairline cracks dangerous?
Usually not. Hairline plaster cracks from shrinkage are common. Monitor them, and call an engineer if they grow or appear with other signs.
Can spalling concrete be repaired?
Yes. The damaged concrete is broken out, the steel cleaned or replaced, treated, and new repair mortar applied. The source of moisture must also be fixed.
Can I keep using the building while repairs are planned?
Usually yes, unless the engineer says otherwise. In some cases areas are closed or propped until repairs are done.
Should I buy a building with structural cracks?
Not without an engineer's report and a repair estimate. Use the findings to negotiate the price or walk away.
Worried about a building?
Vika Construction carries out structural repairs and renovations for commercial and institutional buildings. Contact us to arrange an assessment with our engineers.
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