Cracks are the first sign building owners notice, but not every crack is dangerous. This article explains how to read cracks one factor at a time, with the crack mapping and crack monitoring procedures used in real inspection work.
What is the difference between structural and non-structural cracks?
A structural crack is a crack in a member that carries and transfers load: columns, beams, slabs and shear walls. A non-structural crack occurs in materials that carry no load, such as masonry partition walls, plaster and finishes.
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| Topic | Structural crack | Non-structural crack |
|---|---|---|
| Where found | Columns, beams, slabs, shear walls, beam-column joints | Masonry partitions, plaster, ceilings, finishes |
| Typical appearance | Diagonal at about 45 degrees, vertical at midspan, or horizontal at the column base; often through the full thickness | Map cracking in a net pattern, cracks along material joints; shallow, surface only |
| Main causes | Overloading, differential settlement, shear, earthquake forces | Plaster shrinkage, temperature, dissimilar materials moving differently |
| Trend | Often keeps widening while the cause remains | Usually stable once the material has finished shrinking |
| Urgency | Requires assessment by an engineer | Surface repair is fine, but record it for monitoring |
Note that a crack in a masonry wall "does not necessarily mean it is unimportant". Masonry walls tend to crack before the main structure, so they act as an indicator that the surrounding structure is moving, especially stepped cracks running along the mortar joints.
What does crack direction tell you?
The direction of a crack indicates the type of force acting. Cracks form perpendicular to the tensile stress that breaks the concrete or wall, so they can be used to form a hypothesis about the cause before any testing. There are 5 common patterns:
- Diagonal cracks at about 45 degrees at beam ends or on columns — usually associated with shear. This is the pattern that deserves the highest priority, because shear failure happens quickly with little warning.
- Vertical cracks on the beam soffit at midspan — flexural cracks. If they are fine and evenly distributed, this is normal behavior for reinforced concrete, but a wide crack or a single deep crack must be inspected.
- Horizontal cracks at or around the column base — usually associated with bending at the column base from lateral load (wind or earthquake), or with corroded reinforcement expanding and pushing the concrete cover off along the bar. Inspect immediately. Note that excess compression shows up as vertical cracks parallel to the column axis with spalling, not as horizontal cracks.
- Stepped cracks in masonry walls — the classic pattern of differential settlement. The direction in which the crack opens wider usually points to the side that has settled more.
- Map cracking in a net pattern on plaster — caused by shrinkage of the plaster or surface concrete; usually does not affect structural strength.
If cracks appear together with sloping floors, sticking doors and windows, or gaps between the floor and walls, consider a foundation cause. Read more in 5 Signs of Foundation Problems.
How wide does a crack have to be before it needs repair?
Under BRE Digest 251 guidance, cracks wider than about 1 mm start to need filling, and from about 5 mm upward an engineer should inspect. Width is used as an initial screening criterion, measured with a scaled magnifier (crack width gauge) or a crack card. The guidance classifies building damage into categories 0–5.
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| Level | Approx. width | Description and action |
|---|---|---|
| 0 — Negligible | Up to 0.1 mm | Hairline cracks; no structural repair needed |
| 1 — Very slight | Up to 1 mm | Can be repaired when repainting; record the location |
| 2 — Slight | Up to 5 mm | Must be filled; doors and windows may stick; have a qualified person assess |
| 3 — Moderate | Approx. 5–15 mm | Partial opening-up and repair needed; have an engineer inspect |
| 4 — Severe | Approx. 15–25 mm | Major repair; structural strengthening may be needed |
| 5 — Very severe | More than 25 mm | Stability at risk; requires assessment and possibly partial rebuilding |
From a durability standpoint, ACI Committee 224R guidance gives tolerable crack widths by exposure condition: about 0.41 mm for dry air or interior conditions, about 0.30 mm for humid air or soil, and as tight as about 0.15–0.18 mm for exposure to deicing chemicals or seawater, because wide cracks let moisture and chlorides in to corrode the reinforcement.
A 45-degree diagonal crack on a column only 0.5 mm wide may be more dangerous than a 2 mm map crack in plaster. Always consider location, direction and change over time as well. The figures above are screening guidance, not criteria for judging the safety of a particular building, which must be assessed by the responsible engineer.
How is crack mapping done?
Crack mapping is recording every crack on the building plans to see where cracks are concentrated and whether they form a consistent pattern. The procedure generally has 6 steps:
- Prepare floor plans and elevations of the building. If there are no construction drawings, sketch a rough plan with a north arrow.
- Survey every floor systematically, marking crack locations on the plan and giving each crack an ID, e.g. C-01, C-02.
- Record data for each crack — maximum width, length, direction, member (column/beam/wall) and any water staining or rust.
- Always photograph with a ruler or crack card placed next to the crack, so sizes can be compared later.
- Mark the crack tips with pencil lines and write the date beside them, to see later whether the crack has grown longer.
- Compile a crack map and data table for comparison with the next survey.
A good crack map reveals patterns immediately. For example, if cracks fan out from the same corner of the building on every floor, this usually points to settlement of the foundation at that corner rather than a plaster issue.
How long should cracks be monitored?
Monitor for at least about 3 months; it usually takes about 6–12 months to cover both the rainy and dry seasons. Crack monitoring means measuring crack width repeatedly at intervals to tell whether a crack has "stopped" or is "still moving", which is the information that most directly decides the repair approach, because filling a crack that is still moving usually leads to it cracking again.
- Instruments — tell-tale crack monitors fixed across the crack, or pairs of studs read with a mechanical strain gauge (Demec gauge), which is more precise; where continuous data are needed, automatic logging sensors may be used.
- Reading frequency — initially every 1–2 weeks, reduced to once a month once readings are found to be stable.
- Suitable duration — at least about 3 months, and it should cover both the rainy and dry seasons, so it usually takes about 6–12 months, because settlement and expansion from temperature and moisture change with the seasons.
- Record alongside — date, time, temperature and weather, because cracks that move with daily temperature are usually due to material expansion, not settlement.
A simple interpretation: if readings fluctuate up and down around the initial value with temperature but show no cumulative upward trend, the crack is stable and can be repaired. If readings keep increasing in a single trend, the cause is still active; it must be found and fixed first, and only then should the crack be repaired.
When foundation settlement is the suspected cause, crack width readings alone are often inconclusive, because you do not know how much the building has actually moved. Installing settlement points and tiltmeters alongside provides movement figures to compare directly with crack behavior. Read how to choose instruments and lay out measurement points in Settlement Points and Tiltmeters for Monitoring Settlement and Tilt of Adjacent Buildings.
Found a suspicious crack? What should be tested next?
The next step is testing to establish the facts, choosing the method according to the question that needs answering. When mapping and monitoring indicate that a crack is likely a structural problem, 6 test methods are commonly used:
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| Question | Test method | Reference standards |
|---|---|---|
| Does the concrete have voids or cracks extending deep inside? | Ultrasonic Pulse Velocity (UPV) | ASTM C597 |
| Is the concrete surface hardness uniform? | Rebound Hammer (screening only) | ASTM C805 |
| What is the actual compressive strength of the concrete? | Core Test: drill and test samples | ASTM C42 |
| Where is the rebar, and is the cover sufficient? | Rebar Scanner / GPR | BS 1881-204 (cover meter) / ASTM D6432 (GPR) |
| How deep is the existing foundation (no construction drawings)? | Parallel Seismic Test | ASTM D8381 |
| Are the soil layers beneath the building causing settlement? | Soil investigation and laboratory testing | ASTM D1586 (SPT) |
In practice, testing usually starts with non-destructive methods such as UPV and Rebound Hammer to screen suspect areas first, followed by Core Tests only where necessary, because coring leaves holes that must be repaired and costs more. Read a comparison of the two approaches in NDT vs. Core Test: How to Choose.
Which cracks require calling an engineer immediately?
The following 7 signs are urgent warnings. Stop using the affected area and contact an engineer without waiting for monitoring results:
- Diagonal cracks on a column, or diagonal cracks at a beam end where it meets a column
- Spalled concrete exposing reinforcement, especially on columns or beam soffits
- Cracks that widen noticeably within a few days or weeks
- Cracks that pass through the full thickness of a wall so that light shows through
- Noticeably sloping or sagging floors, with gaps between the floor and walls
- Creaking or cracking sounds from the structure during normal use
- New cracks appearing immediately after an earthquake, nearby pile driving, or excavation next to the building
What Are the Costs and Timeline?
The cost of crack surveys and monitoring depends on building size, number of stories, number of monitoring points and number of reading visits, so there is no fixed flat rate. Contact us for a free quotation within 24 hours. If further testing is needed, non-destructive structural testing and Core Tests are priced separately; see Structural Testing Prices 2026.
In terms of time, surveying and mapping cracks in a small to medium building usually takes 1–2 days on site, while crack monitoring runs for the planned schedule, typically about 3–12 months. The summary report with the engineer's opinion is issued at the end of the monitoring period.
Price note: This price excludes travel and equipment mobilization costs, which are charged extra depending on the distance to site.
Summary
Reading cracks correctly means looking at 4 things together: location (is it on a load-bearing member?), direction (45-degree diagonals are the most concerning), width (used to screen the damage category) and change over time (still moving or stopped). The last one can only be obtained by monitoring the crack.
SPN Soil Engineering provides crack surveys and monitoring, non-destructive structural testing and existing foundation investigation, with reports certified by a civil engineer licensed by the Council of Engineers Thailand (COE), with signature and registration number. Contact us for a free quotation within 24 hours.