Differential movement (footings)

Differential movement is the structural defect mechanism in which one part of a building’s footing moves vertically relative to another part, causing twisting and racking forces in the building above. On a reactive site, the south end of a slab might rise 30 mm in winter (wet) and the north end stay flat, producing 30 mm of differential between the two. The wall above the moving end cracks; doors stick; tiles crack along the line of maximum differential.

Differential movement is the primary structural failure mode that AS 2870:2011 footing designs are engineered to resist. The standard’s deemed-to-comply tables, site classification by characteristic surface movement (ys), and beam-and-mesh detailing all exist to keep differential movement within tolerance.

The two main causes:

Reactive soil under part of the building. A site with non-uniform soil profile (clay one half, sand the other) reacts differently to moisture change. The clay portion swells in winter, the sand portion does not.
Non-uniform load. A point load (a steel column on a pad footing) settles into reactive soil at a rate different from a distributed-load wall on strip footings. Even on uniform soil, the load difference creates differential.

Common visible symptoms (the defect chain):

Hairline cracks in masonry walls, typically running diagonally from corners of openings.
Cracks in plasterboard at internal corners and door head heights.
Doors that bind on the frame in one season and open clear in another.
Floor tiles cracking along a continuous line.
Cornice gaps appearing seasonally between wall and ceiling.

How AS 2870 resists it:

Stiffened raft slabs distribute load across a large area: a 5,000 ys reactive site under one corner is averaged across the whole slab footprint.
Articulation joints in masonry walls let two segments move independently without cracking the masonry.
Beam-and-mesh in stiffened rafts gives the slab tensile strength to bridge a soft spot.
Edge thickening at the perimeter resists corner heave.

Builder action:

Read the engineer’s site classification and slab design before site-start. The slab will not accept on-site modifications.
Maintain drainage around the perimeter post-build: poor drainage adds water to the reactive zone and worsens movement.
Educate the owner about gardening near the building: planting thirsty trees within 1 to 3 m of a footing extracts moisture in summer and worsens the cycle.
Where differential movement appears within the warranty period, document it photographically, measure it at the worst point, and engage an engineer to re-assess before patching cracks.

Also known as: differential settlement; differential heave; non-uniform foundation movement.

Category: Structure.

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