BUILDING INSPECTION GUIDE
Internal Slab Cracking
Cracking to a concrete floor slab can occur for a range of reasons. The crack pattern, width, location and any associated movement or distortion help provide context when assessing the condition during a building inspection.
UNDERSTANDING THE DEFECT
Why Do Concrete Slabs Crack?
Concrete naturally undergoes changes as it cures and dries, and some cracking can occur without indicating significant structural movement. Other cracking may develop because of ground movement, differential settlement, changes in soil moisture or stresses within the slab.
The appearance of a crack alone does not establish its cause or significance. A fine isolated crack can have very different implications from cracking that is accompanied by displacement, changes in floor level or related cracking elsewhere in the building.
For this reason, slab cracking should be considered together with the surrounding structure and accessible site conditions.
WHAT WE CONSIDER
Signs That Can Help Explain Slab Cracking
A slab crack is more useful diagnostically when it is considered as part of the overall condition of the building rather than as an isolated defect.
The visible width, direction and length of cracking can help establish the nature of the defect.
A difference in level across a crack may be more significant than a crack with no visible displacement.
Sloping, uneven or locally distorted floors can provide additional evidence of movement.
Related cracking to internal or external walls can indicate that movement is not limited to the floor finish.
Binding or distorted doors and windows can sometimes accompany broader building movement.
Cracked tiles or separation in floor finishes may reveal the location or extent of movement beneath.
POSSIBLE CONTRIBUTING FACTORS
What Can Contribute to Slab Movement?
Concrete slabs interact with the ground supporting them. Changes in soil moisture, reactive soils, localised settlement, drainage conditions and nearby vegetation can all influence the supporting ground in some circumstances.
Construction factors can also affect how cracking develops. Concrete shrinkage, joints, reinforcement, slab detailing and the way loads are transferred through the structure may all be relevant depending on the type of building and defect.
This is why the surrounding building and site conditions need to be considered before conclusions are drawn from a visible slab crack.
WHAT WE CHECK
Slab Cracking During a Building Inspection
Where slab cracking or possible floor movement is visible, we consider the defect together with other accessible parts of the building and surrounding site.
The aim is to identify whether the cracking appears isolated or whether other visible conditions suggest a broader movement pattern.
PUTTING THE FINDING IN CONTEXT
The Pattern Matters More Than One Crack
An isolated slab crack may have limited significance, while cracking combined with displacement, uneven floors or related wall movement can warrant closer consideration. The overall pattern is important.
Where slab cracking is significant, displaced, widespread or associated with other signs of structural movement, further investigation by an appropriately qualified structural engineer or other relevant specialist may be recommended.