In short: Reactive soil does not cause damage on its own. Moisture change does, and most of the moisture change around a Craigieburn home is created after handover by its owners.
The mechanism
Reactive clay swells when wet and shrinks when dry. A slab sitting on it moves with it. If the whole site moves evenly, very little happens. Damage comes from differential movement - one part of the house wetter or drier than another for long enough.
What creates the difference
Garden beds against one wall. A downpipe discharging beside the house. Irrigation on the north side only. A neighbour's paving changing where water goes. Almost everything on that list is added after the builder has left, which is why cause matters as much as symptom.
Reading a crack
Fine hairline cracking in plaster is usually cosmetic. Cracks that step through brickwork, widen at one end, appear at openings or come with doors that stop closing are telling you something structural is moving. The Guide to Standards and Tolerances 2015 sets the line between the two.
What to do about it
Fix the cause before the crack. Photograph with something for scale, wait a few weeks, photograph again. Movement that is progressing justifies a defect investigation; movement that has stopped usually justifies a patch.
Written by the Owner Inspections team from inspections carried out in Craigieburn. Licensed building practitioners, Building & Plumbing Commission registration CDB-U 53425.
