Branching crack on the surface of an existing concrete floor with a tape measure laid across it to record position and width
Real site photo, crack point 1 in the bay between column lines 2 and 6: a branching crack in the top screed, photographed against a tape measure to show its true size.

What causes cracks in a concrete floor?

Concrete floor cracks have many causes, from natural shrinkage of screed or concrete to structural problems such as a slab carrying more bending than it can take or soil settling beneath it. What separates the causes is the pattern, location and width of the cracks when they are plotted on a plan against the columns and beams.

2D diagram of four concrete floor crack patterns and their usual causesFour panels show floor crack patterns seen from above. One: grid or map cracking in a screed or concrete topping, usually from shrinkage; check width and growth. Two: cracks parallel to a beam on the top surface near the beam, indicating negative bending where the slab spans or hangs from the beams; a structural check is needed. Three: diagonal cracks radiating from a column or footing corner, from differential settlement or shear; a structural check is needed. Four: a gap between the slab edge and a wall or column, from soil settling under the slab while the piled structure does not; core to check for voids under the slab. Reading concrete floor cracks: four common patterns Conceptual · plan view 1 · Grid / map cracking Seen in screeds or toppings Usual cause: shrinkage Check width and growth 2 · Parallel to a beam (top face) Near the beam, across the span Negative bending over the beam Structural check needed 3 · Diagonal around column/footing Radiating from a corner Differential settlement or shear Structural check needed 4 · Gap at slab edge Slab dropped below structure Soil settled, piles did not Core to check for voids Preliminary guide only; the cause must be confirmed by an engineer with cores and crack monitoring
2D diagram (plan view): four common concrete floor crack patterns, their usual causes and what to do next (conceptual diagram, not to scale).

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Crack patternUsual causeWhat to do next
Grid / map cracking in the top screedShrinkage of the screed or concrete toppingMeasure the width and track whether it grows
Parallel to a beam, top surface near the beamNegative bending over the beam where the slab spans or hangs from itStructural check by an engineer
Diagonal, radiating from a column or footing cornerDifferential settlement or shear around the columnStructural check by an engineer
Gap between slab edge and wall or columnSoil under the slab settled while the piled structure did notCore the slab to check for voids

Classifying by pattern is only a starting point; the same pattern can have different causes, so it must be confirmed with further checks such as cores and crack monitoring over time. For cracks across a whole building, see which structural cracks are dangerous.

How do you survey floor cracks and prepare a crack map?

A crack map turns “cracks you can see” into “data you can check again later”. The steps SPN uses on site are:

  1. Divide the floor into bays by column and beam lines: name each bay by its column lines, for example “bay 2–6”, so every point has a clear reference.
  2. Walk the whole floor and number the points: choose representative cracks in every bay so the survey covers them all.
  3. Measure positions from the column lines: record offsets in two directions, for example “line 2 +2.4 m / line B +8.9 m”.
  4. Photograph against a tape measure: one wide shot to show the location and close-ups with the tape across the crack; in the case study three photos were taken per point.
  5. Measure the width with a crack gauge: read the widest part of the crack and note its direction, length and form (straight, grid, branching).
  6. Plot the map and summarise in a table: put every point on the plan with the beams, columns and other test locations to see whether the cracks are local or widespread.
Straight cracks crossing at right angles on a concrete floor surface, with a tape measure laid horizontally
Point 2, bay 2–6: straight cracks crossing in a grid.
Branching crack on a concrete floor surface with a tape measure laid horizontally to record its position
Point 4, bay 6–10: tape laid across the line of the crack.

Case study: a 23-point crack map on a ground floor more than 140 m long

This is a real SPN job from 2026 (the building and its original drawings are not disclosed). The ground floor of an old 5-storey building in Bangkok was being converted into usable space. The slab runs the full length of the building, with ground beams on piles every 12 m. The SPN team surveyed 23 crack points covering 12 floor bays, together with 13 concrete cores, 10 rebound hammer points and checks of the soil below. The crack map below was redrawn by SPN showing only the column lines, beams and survey points.

Crack map of 23 points on the ground floor slab, column lines 1 to 48, with ground beams and floor baysPlan showing the 23 crack survey points on the ground floor slab running the length of the building, split into two strips for column lines 1 to 24 and 24 to 48. Ground beams sit on lines 2, 6, 10 through 46 at 12 metre spacing. Cracks were found in all 12 bays surveyed, from bay 1 to 2 through bay 42 to 46, mostly wider than 2 millimetres. Positions are approximate. Crack map, 23 points · ground floor slab, column lines 1–48 Approximate positions · not to scale B C 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 1–2 Bay 2–6 Bay 6–10 Bay 10–14 Bay 14–18 Bay 18–22 22–26 → 1 2 3 4 5 6 7 8 9 10 11 17 18 19 B C 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 ← 22–26 Bay 26–30 Bay 30–34 Bay 34–38 Bay 38–42 Bay 42–46 46–48 12 13 14 15 16 20 21 22 23 Crack survey point (number) Ground beam, 12 m spacing Floor slab (void found at every core)
2D crack map (plan view): 23 crack survey points on the ground floor, column lines 1–48 in two strips. Red bars are ground beams every 12 m. Positions are approximate; not to scale.
23
crack survey points, 3 photos per point
12/12
floor bays surveyed, cracks found in every one
> 2 mm
width of most cracks, measured with a crack gauge

What does the crack map show?

The first thing the map shows is that the cracks are not concentrated in one place but spread through every bay along the whole building. They are straight cracks crossing in a grid plus branching cracks in the top screed, mostly wider than 2 mm. This pattern says the problem is not local damage, such as a heavy object being dropped, but the behaviour of the whole slab, so the next step was to look under the floor.

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Floor bay (column lines)Crack pointsCore results in the same bay
1–218, 19No cores
2–61, 2, 35 cores, upper concrete 0.20–0.37 m, void 0.08–0.45 m
6–104No cores (3 rebound hammer points)
10–1417No cores
14–185, 6, 71 core, concrete 0.20 m, void 0.15 m
18–228, 9, 105 cores, concrete 0.22–0.34 m, void 0.05–0.13 m
22–2611, 121 core, concrete 0.19 m, void 0.06 + 0.05 m
26–3013, 141 core, concrete 0.26 m, void 0.14 m
30–3415, 16No cores
34–3820No cores
38–4221No cores
42–4622, 23No cores

Set against the crack map, every bay that was cored had a void under the slab 0.05–0.45 m high, and none of the 13 cores contained reinforcement. The report therefore assessed that the cracks may be related to settlement of the soil under the slab, which reduces the continuity of the slab. Core results, strengths and the 3D model calculation are covered in existing concrete floor inspection: what to check.

Site photos of cracks from each floor bay

The photos below are taken from survey points in different bays, each with a tape measure for scale. The cracks include both straight cracks crossing in a grid and branching cracks, and they run from one end of the building to the other.

Two straight cracks crossing on a concrete floor with a tape measure laid across them
Point 19, bay 1–2 (end of the building): cracks crossing at right angles.
Straight crack meeting a branching crack on a concrete floor, with a tape measure placed where they cross
Point 10, bay 18–22: a straight crack meeting a branching crack, in the bay where cores found a void.
Branching cracks running in several directions on a concrete floor, with a yellow tape measure for scale
Point 11, bay 22–26: branching cracks mid-bay.
Straight crack on a grey concrete floor with a tape measure laid across it
Point 21, bay 38–42: straight crack close to a beam line.
Ground floor concrete slab of a building under renovation, a long continuous floor with a glazed wall
Ground floor during renovation: one continuous slab along the building, before the crack survey.
Long continuous cracks crossing on a concrete floor, photographed wide to show the whole crack pattern
Point 9, bay 18–22, wide view: long continuous cracks crossing across the slab.

After finding floor cracks, what else should be checked to find the cause?

A crack map shows “what happens at the surface”, but finding the cause needs to know “what is underneath”. In the case study the SPN team followed up on the same days with:

SPN technician drilling through an existing floor with a diamond core drill while a colleague supervises
Coring through the floor to see its thickness and any void below.
SPN technician in a hard hat pressing a rebound hammer onto the concrete floor through a grid template
Rebound hammer test on the floor using a grid template to place the impacts.
Three SPN technicians running a Kunzelstab test, driving a steel rod into the soil through a hole cored in the concrete floor and reading the penetration with a tape
Kunzelstab test (KPT) through a core hole to check whether the soil under the slab is dense or soft.
Concrete core from the existing floor laid on a tape measure, showing a mortar layer and concrete with mixed aggregate
A core from the existing floor: several layers of mortar and concrete, and no reinforcement.
Looking down a core hole in an existing floor, with the concrete layer at the top and a dark gap below, the void between the slab underside and the soil
A core hole that breaks into the void under the slab: the answer a surface crack map cannot give.

How wide does a floor crack have to be before it is a concern?

There is no single number for every case. For reinforced concrete under service loads in dry air, ACI 224R-01 gives about 0.016 in., or 0.41 mm, as a reasonable crack width, and recognises that some cracks can grow wider over time. It is a useful reference, but always read it together with other factors:

  • Trend over time: a crack that is getting wider or longer is more worrying than a wide crack that has stopped; track it with a crack gauge or crack meter.
  • Location relative to the structure: cracks near beams, columns or slab edges are read differently from cracks mid-panel.
  • Other symptoms: settlement, sagging, a hollow sound or a gap at the slab edge point to a problem under the floor.
  • Which layer is cracked: in the case study most cracks wider than 2 mm were in unreinforced screed toppings, so their width did not directly show the slab strength, but it did mean the floor had to be checked from below.

What should you do if you find floor cracks?

  1. Do not fill or skim them yet: covering cracks before the cause is known makes it impossible to tell whether they are still growing.
  2. Photograph them against a tape and note the date: mark the crack ends with a pen to see whether they lengthen.
  3. Reduce load on that part of the floor: avoid heavy or concentrated loads such as document shelving, safes or machinery until the cause is known.
  4. Have an engineer survey and map the cracks: with cores where voids or settlement are suspected, so the repair matches the cause.

If the floor is to be renovated or change use, its load capacity should be assessed too; see how it is proven with real loads in what a floor load test is.

How long does a floor crack survey take and what does it cost?

It depends on floor area, number of points and the other tests done alongside. In the case study the survey of 23 crack points was carried out together with concrete coring, rebound hammer testing and soil checks under the slab, taking 3 days on site. The results were issued in a report certified by a licensed civil engineer (Council of Engineers Thailand) with signature and registration number.

For pricing, send crack photos or a plan via LINE @spnth or request a free quote within 24 hours. Prices exclude travel and equipment mobilisation, which are charged by distance to site. See all services on our non-destructive testing and structural inspection page.

Summary

Concrete floor cracks tell you more than they seem to. Recorded systematically on a crack map referenced to the columns and beams, with tape-measure photos and measured widths, they show whether a problem is local or affects the whole slab.

In the case study, a 23-point crack map spread across all 12 bays led to coring, which found a void under the slab at every core. The lesson: do not fill cracks before the cause is known, and when cracks run across the whole floor, have an engineer check what is underneath.