Wall cracks: structural or cosmetic, and how to tell

What you'll notice

Movement in a house shows up in more ways than a line on a wall. Start with what you see and feel, then follow each sign to the section that explains it.

Use width as one observation in a larger pattern

Measure the crack where it is safely accessible, then identify what is cracked: paint, plaster, render, mortar, brick or another structural material. Those are different observations. A narrow crack in a consequential location can need investigation, while a wider gap might be an intended joint. No width alone certifies safety, cause or repair cost.

QBCC's subsidence guidance considers cracking alongside bulging walls, distorted openings, drainage and changes around the building. This guide uses that contextual approach. The table below is Homechecker's practical triage, not a reproduction of an Australian Standard's damage categories or a compliance test.

The damage bands used in Australia

Australian footing practice describes wall damage in categories that pair an approximate crack width with what you would typically see. CSIRO's homeowner guide to foundation maintenance and footing performance sets these out alongside AS 2870, the standard for residential slabs and footings. The summary below is in our words; the source documents carry the full definitions.

Two cautions keep the table honest. First, a category describes the damage present, not its cause, and not whether the movement has stopped: a stable 3 mm crack and a 3 mm crack that grew last winter are different problems. Second, the count matters as well as the widest crack, and so do associated symptoms such as sticking doors or sloping floors. The UK's Building Research Establishment uses a similar banding and makes the same point in its guide to assessing cracks in houses.

How to measure a crack properly

Measure at the widest point with a crack-width card or a fine rule, note which side of the wall you measured, and photograph the card against the crack. Pencil a small mark at each end of the crack with the date, so later growth in length is visible. Repeat the same measurement in the same place every few weeks, and after a notable dry spell or heavy rain. Glass or plastic tell-tales fixed across a crack give a more precise record if an engineer recommends them.

Common crack patterns and what they raise

Modern brickwork also has articulation joints: deliberate vertical gaps filled with flexible sealant, built in to let long walls move without cracking. They are design features, not defects, though the sealant needs renewing and a joint that has been mortared over can force cracking elsewhere. If you are unsure whether a straight vertical line is a joint or a crack, check whether it runs cleanly through the full height with sealant rather than mortar. For how the wall system changes the reading, see brick veneer vs double brick.

Inside the house: plaster and plasterboard cracks

Most cracks people notice first are inside, in the wall and ceiling linings. What those linings are made of depends a lot on the era. Older homes often have lath and plaster: wet plaster pushed onto thin timber strips, common before the Second World War. Many interwar and postwar homes have fibrous plaster sheets, and most homes from the later decades use plasterboard. Each one cracks in its own way, so it helps to know which you have before reading anything into a line. The inside-the-house guide covers the linings themselves in more detail.

Render, concrete slabs and tiles

Cement render often develops fine, map-like crazing as it dries and ages. On its own that is usually a surface matter, though it can let water in over time. A crack in render that follows a straight or stepped line is different: it may be tracing a mortar joint or a movement in the masonry underneath, so read it the way you would read the brickwork. The outside walls guide covers render and cladding further.

Concrete slabs shrink as they cure, and fine shrinkage cracks are common. They are often hidden under floor coverings and may only show when carpet is lifted, or as a crack running across a line of floor tiles. A fine, stable slab crack is a different matter from one with a step across it, a crack that keeps widening, or tiles lifting in a pattern. Garage slabs, paths and driveways are usually separate from the house slab, so a crack in a path says little about the footings under the house.

Read the crack and its surroundings together

Use a wider photograph to show the room or elevation, a middle-distance view to locate the crack around an opening or junction, and a close view with a scale. Note whether the line follows a finish joint, mortar joints or passes through units. Keep its location identifiable so another person can compare the same place later.

Look for associated changes: a door that has recently begun binding, a gap at an extension junction, a displaced sill or a floor that appears uneven. Report what changed and when. A long-standing sloping floor and a new sticking door are not interchangeable evidence. Do not force an opening or enter an unstable area to complete a checklist.

The wall system changes the question

In brick veneer, the visible masonry and the internal frame have different roles. In cavity or solid masonry, a cracked wall may also carry building loads. YourHome identifies these different masonry arrangements. Confirm the actual construction, including later additions, before deciding what the visible crack might affect.

An internal lining crack can occur over a joint without establishing damage in the structure behind it. Equally, a clean internal face does not prove the outer masonry is adequately supported or restrained. Ask what was accessible on both sides and what remained concealed. The wall-system guide develops the load-path and moisture questions.

Reactive clay: why Australian houses move with the seasons

Many Australian suburbs sit on clay soils that swell when they get wet and shrink when they dry out. QBCC explains that if one part of the ground under a house moves more than another, the building may crack. Dry seasons can dry clay out quickly, and wet seasons can soak it just as fast. Sandy and rocky sites are usually far less prone to this.

Because of this, cracks in many homes behave seasonally. They often open during a long dry spell, then partly close again after sustained rain. That pattern is useful evidence, which is why a crack log that spans the seasons tells you so much more than a single photograph. The CSIRO guide to foundation maintenance and footing performance sets out how soil moisture and footings interact, and what owners can do to keep moisture around the house steady.

Engineers design footings to the soil they are built on. Under AS 2870, the standard for residential slabs and footings, a site is classified by how much its ground surface is expected to move: A and S for little or slight movement, M for moderate, H1 and H2 for high, E for extreme, and P for problem sites that need special design. Newer homes usually have this classification on the soil report or footing drawings. Many older homes were built before it existed, so the original footings may not have been designed for the movement the site can produce.

Several mechanisms can produce similar-looking cracks

Footing movement is one possibility

Ground moisture changes, drainage problems and changes around footings can be relevant. YourHome discusses reactive soils, settlement and drainage among possible causes. A site classification or suburb's reputation does not diagnose the movement in one home. Ask for the actual footing arrangement, any geotechnical information and the condition evidence.

Separate local components from wider movement

A crack near a window raises questions about the opening support and local masonry as well as the foundations. A crack at an addition raises questions about the junction between construction phases. Repeated cracks in a finish raise questions about the substrate, joint and repair method. Keep plausible explanations separate until the inspection provides evidence to connect them.

Do not prescribe tree removal, underpinning or drainage excavation from a photograph. QBCC warns that planting and removing trees can both affect footings. A response intended to help can change the moisture regime again. Engineering, arboricultural or drainage advice may be needed together where those factors are implicated.

How to monitor wall cracks without delaying an assessment

Dated repeat observations can show whether a crack has changed, but monitoring is useful only when the situation is suitable for it. Ask the professional what to record, how often and what change should trigger action. One photograph labelled 'old crack' supplies no measured history. Fresh filler or paint can make comparison harder without proving that anyone intended to conceal a problem.

Keep a fixed location and a consistent measurement method. Record repair dates, significant rainfall, drainage works and changes to nearby trees or structures in your home record. A crack that opens and closes seasonally still needs its effects understood; seasonal behaviour is not automatically harmless. Short monitoring during a house purchase may not span the conditions that matter. If the decision deadline arrives first, the uncertainty remains explicit.

A simple crack log you can keep yourself

Give each crack a short name, such as Kitchen door head or North wall, laundry corner, so every entry refers to the same place. Mark a reading point on the crack with a small pencil tick, and mark both ends of the crack with the date. Measure at the reading point each time, photograph it with a crack-width card or ruler in the frame, and note anything else that changed. A spreadsheet or a page in your home record is enough.

Take a reading every few weeks, and again after a long dry spell or heavy rain. Where time allows, keep going for a full year, because one season shows only part of the picture. Send the log to a building professional sooner if a reading widens, the crack grows past its end marks, new cracks appear nearby, doors or windows start to stick, or water gets in. The example above would justify a call: the crack widened and lengthened while a nearby door got worse.

Brief the professional with a question they can answer

A competent building inspector can record the accessible pattern and recommend specialist follow-up. Ask a structural engineer to assess significance and likely mechanism where structural movement, support or stability is in question. The engineer may need floor levels, access, drawings, soil information or service testing. A conclusion can be provisional if that evidence is unavailable.

Price the established scope and its remaining uncertainty

Crack filling, local masonry repair, opening-support work and footing intervention solve different problems. A quote for one should not be used as a budget for another. Ask whether the estimate includes investigation, cause treatment, access, associated services and reinstatement. If further opening-up may change the scope, record that dependency rather than hiding it in a single reassuring number.

Review previous work the same way. 'Underpinned' or 'restumped' needs a location, date, design and scope. A repair to one corner is not evidence that every footing was renewed. Compare the documented area with today's symptoms and use the interwar guide where mixed supports or later additions complicate the picture.

What each repair actually fixes

Repairs to cracking fall into a few families. Some deal with the appearance, some reconnect damaged material and some address the cause of the movement. Knowing which is which makes quotes much easier to compare.

A sensible order is usually to deal with the cause first, give the house time to settle, then repair the finishes. Filling cracks before the cause is understood often means doing the work twice and losing the evidence a professional would want to see.

Worked example: repaired cracking at an extension

Illustrative reasoning example, not an inspected property. A 1960s home has new paint over a stepped crack at a rear extension and a door that binds. The seller provides an invoice for local brickwork repair two years earlier. Record the repaired area, the present crack and the binding door separately; the invoice does not establish the cause or prove that the repair failed structurally.

Compare earlier photographs, establish the original and extension supports, and brief an assessment of the junction and associated distortion. A stable joint with an unsuitable finish repair leads to a different scope from continuing movement with support damage. Until the evidence distinguishes those possibilities, neither a filler allowance nor an underpinning budget is a confirmed answer.