Cracks in walls in Canberra: when to worry

Is the crack in my wall serious?

Usually not, if it's hairline and stays the same. A crack matters more when it's wide, growing, or part of a pattern that shows the footings have moved.

Engineers grade wall cracks with the damage categories in AS 2870, the Australian standard for residential slabs and footings. Width is the starting point. The number, pattern and location of the cracks matter just as much.

Four patterns tell an engineer most about what's happening in a brick wall:

Inside, the same logic applies to plaster. Openings concentrate stress, so movement tends to show first at the corners of doors and windows.

Which cracks are cosmetic, and which need an engineer?

Most fine cracks that appear once and stay put are cosmetic. The signs in the right-hand column are the ones worth a visit.

Why do Canberra houses crack?

Because the ground under them moves, and in Canberra it has had plenty of reason to. Two things stand out: the soils, and the swings in rainfall.

The soils

The vast majority of Canberra's suburbs are built on the Williamsdale soil landscape. The ACT's soil mapping lists its limitations as a "widespread foundation hazard", widespread complex soils and widespread seasonal waterlogging. On the footslopes of Tuggeranong, Woden and Weston Creek, the Burra landscape adds sodic subsoils (clay high in sodium, which breaks down when wet) and seasonal waterlogging.

A mapped hazard isn't a site classification, and the soil can change within a single block. Our guide to Canberra soils and footings explains what the maps can and can't tell you.

The rain

Reactive clay swells when it's wet and shrinks when it dries. Canberra's rainfall over the past decade has pushed it hard in both directions.

Annual totals at Canberra Airport. Mean annual rainfall there is 615.4 mm (1939–2010) and 636.7 mm (2008–2026). Source: Bureau of Meteorology annual climate summaries and station statistics.

Three dry years to 2019, three very wet years to 2022, then two drier ones. Clay under Canberra homes has been shrinking and swelling hard.

What makes footings move?

Most movement has more than one cause, and each cause points to a different fix.

• Seasonal clay movement. Footings rise and fall as the clay wets and dries. Cracks that open and close with the seasons often point here.
• Leaks and drainage. A leaking pipe, a blocked stormwater line or a downpipe emptying beside a wall soaks and softens the ground under one part of the house.
• Trees. Roots draw moisture from the clay near a footing, so that side shrinks. Before you dig near a large tree, note that excavating deeper than 100 mm over 4 m² or more in a protected tree's zone is prohibited groundwork under the Urban Forest Act 2023, and needs approval first. See building near protected trees.
• Fill. Poorly compacted fill keeps settling under a footing long after the house is built. If your block was built up with fill, the site classification should say how it was placed.

How do I monitor a crack before calling an engineer?

Record it, so you can tell whether it's moving. A few dated photos tell an engineer more than a description.

1. Photograph it with a date and a scale

   Take one photo close up with a ruler or coin beside the crack, and one from further back that shows where it is on the wall.

2. Mark the ends

   Draw a short pencil line across each end of the crack and write the date beside it. If the crack grows past the line, you'll see it.

3. Measure the widest point

   Use a crack-width gauge or a ruler, and note the reading with the date.

4. Note doors and windows

   Write down any that stick, won't latch or show uneven gaps, and when it started.

5. Check for leaks and water

   Look at taps, downpipes, gutters and stormwater pits near the crack, and for damp patches or pooling beside the walls.

6. Check again, and compare

   Look again every few weeks, and after a long dry spell or heavy rain. Don't fill the crack yet: filling hides whether it's moving.

Should I get a second opinion on a repair quote?

Yes, if the quote came before anyone found the cause. Contractors who sell underpinning or resin injection often assess for free because they sell the fix. Many cracked houses need the leak, the drainage or the tree dealt with first, and some cracks need only patching once the cause is fixed.

We review the quote, your photos and any reports, and tell you in writing whether the proposed work matches the problem. An independent underpinning second opinion starts at $390 + GST ($429 inc GST). Our guide to underpinning costs in Canberra shows published price ranges and when you may not need underpinning at all.

When should you call a structural engineer?

Call an engineer when a crack is wider than about 5 mm, keeps growing, or comes with sticking doors, sloping floors or a leak. Call one too before you pay for underpinning or resin injection.

• Desktop review from photos: from $390 + GST ($429 inc GST). A first view without a visit, or a second opinion on a quote.
• Crack and footing movement report: from $1,090 + GST ($1,199 inc GST). An engineer visits, inspects, and gives a written report with the findings, the likely cause and what to do next.
• Structural inspection and report: from $1,090 + GST ($1,199 inc GST), when the cracks are one part of wider concerns about the house.

Prices are the same across Canberra, from Belconnen to the Burra footslopes of Tuggeranong, and over the border in Queanbeyan. Send close-up and wider photos of the cracks, their widths, when you first noticed them and your suburb. We're engineers only, so our advice doesn't depend on selling you the repair.