Cracks in walls: cosmetic or structural?

The short version

• Victoria's building regulator says wall cracks under 1 mm wide are generally part of normal foundation movement.
• Cracks between 1 mm and 5 mm should be monitored over a full year. Cracks over 5 mm are considered significant.
• Width is only one clue. Sticking doors, sloping floors and cracks that return after patching point to movement worth investigating.

Cracks are one of the first things buyers notice at an open for inspection. You spot one above a doorway and start wondering whether the whole place is sinking. Usually it isn't, but sometimes it is, and the crack on its own won't always tell you which.

Here's how I think about cracks when I'm walking through a house, and what the official guidance in Victoria says about them.

How do you tell if a crack in a wall is structural?

A structural crack is one caused by the building moving enough to affect how it performs, and you usually tell it apart from a cosmetic crack by looking at the whole house around it. A thin line in the plaster where a wall meets the ceiling is often just the building settling and timber drying out over the years. A crack that's wide, keeps reopening after it's been patched, or comes with other signs of movement is a different story.

When I find a crack, I'm asking a few questions:

• How wide is it, and is it wider at one end? A crack that tapers can show which way something is moving.
• Does it run through brickwork as well as the plaster inside?
• Do the doors and windows nearby stick or sit out of square?
• Are the floors sloping or bouncy? Consumer Affairs Victoria notes that this can mean stumps need replacing.
• Is the mortar between bricks falling out? Consumer Affairs Victoria says this fretting can point to major structural problems.
• Has it been patched and painted, then cracked again in the same spot?

One answer rarely settles it. Three or four of them together give a much clearer picture.

What crack width is acceptable in a house?

The acceptable crack width in a house wall, according to Victoria's Building and Plumbing Commission, is generally under 1 mm, which the regulator treats as part of the normal foundation movement a house may experience. Its guidance on foundation movement sets out rough bands:

• Under 1 mm: generally considered normal movement
• 1 mm to 5 mm: monitor in all weather conditions over a full year
• Over 5 mm: considered significant and outside the tolerances for footing movement

The same guidance says foundation movement can also cause bulging walls and doors or windows that stick. For anything significant, the commission recommends an investigation by an endorsed civil or structural engineer.

If you see a crack at an open, take a photo with a ruler or tape measure beside it. That gives you, and your inspector, a record of the width on that day. It doesn't replace a proper look at the rest of the house.

Why do houses on clay soil crack?

Houses on clay soil crack because reactive clay swells when it gets wet and shrinks when it dries out, and that moves the footings unevenly. The Building and Plumbing Commission explains that soil tests classify how reactive a site is, from class A (sand and rock, little or no movement) up to class E (extremely reactive clay).

Moisture is the big driver. Research from Swinburne University, presented to the Australian Geomechanics Society, linked significant damage to houses in Melbourne's western suburbs built during the millennium drought. It pointed to abnormal moisture conditions, such as extreme weather or a leaking pipe, as a cause of uneven movement under slabs.

That's why I pay attention to what's happening around the outside of a house. Garden beds hard against the walls, paving that falls towards the house, a dripping tap or a big tree close by can all change the moisture under the footings. Older timber houses on stumps, like the post-war weatherboards you see around Preston and Glenroy, can also move as stumps age, which shows up as cracks and uneven floors inside.

What should you do if the house you want to buy has cracks?

If the house you want to buy has cracks, get them looked at by someone who understands how houses are built before you commit. You can't monitor a crack for a year when the auction is in three weeks, so you need an informed read on the likely cause now.

A pre-purchase building and pest inspection will record cracks, their location and what I think is behind them. Having built houses myself, I can usually tell you whether a crack looks like old, stable movement or something active. If it looks significant, I'll say so plainly and recommend a structural engineer, which is also what Consumer Affairs Victoria suggests for large cracks.

A few other things are worth doing:

• Ask the agent whether cracks have been repaired before, and when
• Look for fresh patching or paint in one patch of wall, which can hide a crack
• Check how the yard drains after rain if you get the chance

Keep in mind that a visual inspection only covers what can be seen on the day, and an inspection report isn't a guarantee. If you're not sure what a standard inspection covers, I've written about what a pre-purchase building inspection checks.

If you've spotted a crack in a house you're keen on, take that photo with the ruler and give me a call before you book. I'm happy to talk through what it might mean and whether an inspection or an engineer is the better first step. You can also read how an inspection works from booking to report.

Common questions

Are diagonal cracks above doors and windows a bad sign?

Not always. Openings are natural weak points in a wall, so movement tends to show at their corners. A fine diagonal crack can be minor, but a wide one, or one with a sticking door, should be checked.

Can I just patch and paint over cracks before selling?

You can fill minor cracks as normal maintenance, but patching doesn't fix the cause. If the house is still moving, the crack will usually come back, and a buyer's inspector may notice fresh repairs.

My house is less than 10 years old and cracking. Who do I call?

The Building and Plumbing Commission suggests contacting your builder for advice first if your home is less than 10 years old.

Does a building inspection measure every crack?

A visual inspection notes cracks that can be seen and comments on their likely significance. It doesn't monitor them over time or test the soil, which is engineering work.

Sources

• Building and Plumbing Commission - Minimising foundation movement and damage to your house
• Consumer Affairs Victoria - Inspect properties before you buy
• Australian Geomechanics Society - Ground movement and soil structure interaction: recent findings (Karunarathne and Gad, Swinburne University)

General information only, based on a visual inspection perspective. It is not legal or financial advice, and every property is different. An inspection report is not a guarantee that a property is free of defects.

About Dale

Dale Phelan owns DSP Building Inspections and does every inspection himself. He started out as a carpenter, became a registered builder (Domestic Builder Unlimited, DB-U 61057), and has project-managed, built and developed more than 35 homes around Melbourne. He is still actively building, so his reports explain the likely cause of each defect and how urgent it is. More about Dale