Cracks and sticking doors: reactive clay through spring and summer

If doors start sticking, windows get tight and hairline cracks open above doorways as the weather warms, the most likely explanation is not the house failing. It is the ground beneath it drying out and shrinking. Large parts of Australia sit on reactive clay, and reactive clay moves with the seasons.

Understanding this saves a great deal of unnecessary alarm — and, occasionally, catches something that genuinely does need attention.

What reactive clay does

Clay soils absorb water and swell. As they dry, they shrink. The movement is vertical and uneven, and it lifts and drops sections of a house's footings through the year.

In an Australian climate the cycle is predictable. Through winter the ground is wet and swollen, and the house sits slightly higher. Through spring and summer the ground dries and contracts, and it settles. The movement is rarely uniform, because one side of a house is usually shadier, closer to a tree, or better drained than the other.

Soil classifications used in construction — from stable sites through to highly reactive ones — exist precisely because this is expected. A house on a reactive site is engineered to tolerate a degree of movement.

What normal seasonal movement looks like

• Fine cracks in plaster, usually at the corners of door and window openings
• Doors that stick at a particular time of year and free up again later
• Small gaps opening and closing at cornice junctions
• Hairline cracking in external render or mortar
• Symmetry across seasons — the same cracks reappear, then partly close

The defining feature of seasonal movement is that it reverses. Cracks that open in February and close in July are behaving as designed.

What is worth investigating

• Cracks that keep widening and never close
• Cracks wider than a few millimetres, or that run diagonally across brickwork in a stepped pattern through mortar joints
• Any crack you can see daylight through, or that passes through bricks rather than around them
• Doors or windows that will no longer close at all
• Visibly sloping or bouncing floors
• Cracking concentrated at one corner of the house

The useful discipline is measurement rather than memory. Photograph cracks with a ruler or a coin beside them, dated. Repeat every few months. A pattern over a year tells you far more than an opinion formed in one afternoon, and it is exactly what an engineer will want if you do escalate.

What makes movement worse

Most of the aggravating factors are water related and within your control.

• Uneven watering. Keeping one side of the house irrigated while the other dries out creates differential movement, which is what actually damages buildings. Even moisture matters more than wet or dry.
• Trees close to the house. Large trees draw substantial moisture from clay and can dry the soil under a corner of the footings. Removing a mature tree can be as disruptive as leaving it, because the soil then rehydrates and swells. Get advice before doing either.
• Downpipes discharging beside the house, or stormwater lines that leak. This is the single most common cause of localised movement.
• Plumbing leaks under the slab or in the subfloor.
• Paving and garden beds that trap water against the footings, or that stop the ground drying evenly.

What to do this spring

1. Check every downpipe discharges into stormwater, not onto the ground beside the house.
2. Look for damp patches that persist near the building through dry weather — a possible leak.
3. Aim for even moisture around the perimeter rather than heavy watering in one area.
4. Photograph and date any cracking now, so you have a baseline before summer.
5. If cracking is progressive or severe, engage a structural engineer rather than starting with an underpinning contractor.

That last point matters. An engineer assesses the cause and is not selling the remedy. Getting the diagnosis independent of the repair is the same principle that applies everywhere else in building work.

Where to go next

Drainage is the common thread through most of this. Read next: the end-of-winter roof and gutter check, or Preventative Maintenance.