Cracks in the walls: Why buildings move – and when you should worry

Understanding settlement, seasonal movement, and foundation-related issues

Gravity makes everything change shape if you give it enough time; all buildings move. In most cases, this movement is completely normal and doesn’t affect the structure’s safety or performance during its lifespan. Yet for many homeowners, the appearance of cracks or slight unevenness can be worrying. Knowing why buildings move — and when that movement becomes a sign of something more serious — can help you respond appropriately and avoid unnecessary concern.

Initial Settlement: A Natural Part of Construction

When a new building is constructed, the weight of the structure places fresh pressure on the soil beneath it. Unless that soil is exceptionally dense and strong — such as solid granite — the building will experience some degree of initial settlement as the ground adjusts and compresses.

This downward movement typically causes only minor brickwork cracks or small cosmetic changes. Buildings founded on granular, non-cohesive soils settle quickly, often completing most of their movement by the time construction finishes.

Clay soils behave differently. From my experience, they compress initially in the first 3 at a larger pace, however slowly, and can continue for many years — sometimes even 20 years or more — until the soil is fully consolidated. Even then, this long-term, low-level movement is usually harmless.

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Seasonal Movement: Buildings Respond to Weather Too

Movement doesn’t stop once a building is complete. Throughout its entire life, a structure continues to respond to seasonal changes — especially if it sits on clay-rich soils. In Australia, clays can range from moderately reactive to extremely reactive, and under AS 2870, soils are classified based on how much they shrink and swell as moisture levels change.

When soils become wet, they expand; during summer and hot, dry periods, they contract. These natural cycles lead to seasonal cracks that can open and close depending on the time of year. In most climates, this level of movement remains within acceptable limits.

AS 2870 Soil Classifications and Expected Surface Movement (Ys)

Class	Description	Ys (mm)
A	Non-reactive	~0
S	Slightly reactive	0–20
M	Moderately reactive	20–40
H1	Highly reactive (low)	40–60
H2	Highly reactive (high)	60–75
E	Extremely reactive	>75
P	Problem site	Determined by geotechnical investigation

Sydney Soil Behaviour — Regional Overview

Sydney sits on a highly variable geological landscape, and soil reactivity changes dramatically across regions. Below is a quick overview of what is typically found in each major area:

Region	Typical AS 2870 Classification	Notes
Eastern Suburbs	A / S	Shallow sandstone, very stable
North Shore	A / S / M	Residual clays mixed with sandstone.
Western Sydney	M / H1 / H2 / E	Highly reactive Wianamatta Shale
Hills District	M / H1	Clay slopes, influenced by tree roots
Penrith / Nepean	S / M / P	Alluvial soils, floodplain deposits
Inner West	S / M / P	Mixed clays and areas of historical fill
CBD / Barangaroo	P	Reclaimed land, engineered fill

In regions with highly shrinkable clays or extreme seasonal variation, movement can be more noticeable. Cracks may widen during dry summers and close again through wet winters. Although this behaviour is often harmless, excessive or sudden movement — or cracks appearing in unusual patterns — may indicate an underlying issue that requires further investigation. 

When Cracks Become a Cause for Concern

Most building movement is minor. But during inspections or routine checks, certain types of cracking can indicate something more serious.

Movement often becomes visible along natural weakness lines in the structure:

• around windows and doors,

  

• at the junctions between extensions,

• Along long, unbroken wall sections.

  

If the pattern or severity of cracks suggests more than seasonal or initial settlement, further investigation is needed. Significant cracking in an external wall, for example, may point to a foundation-related problem and may even allow water to penetrate the building envelope—an issue that can escalate quickly if not addressed. (see CSIRO BTF 18 )

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