Cracking in brickwork, and what it actually tells you

Cracking in brickwork, and what it actually tells you

I get called to look at cracked brickwork more than almost anything else. Usually the owner has already decided one of two things: it is nothing, or the house is falling down. It is almost always neither.

A crack is information. Read properly it tells you what moved, which direction it went, and whether it has stopped. That is the whole assessment.

Width is the first filter

Australian practice classifies cracking by width, and the categories exist because the response changes at each step.

Hairline cracking under about 0.1mm is negligible. Fine cracking up to around 1mm is generally aesthetic damage. Once you are between roughly 1mm and 5mm you are into damage that needs filling and probably some investigation. Between 5mm and 15mm the wall may need sections replaced, and doors and windows are likely sticking. Above 15mm, and certainly above 25mm, you are looking at structural work and the building needs an engineer on site rather than a report from photographs.

The reason width works as a first filter is that it correlates with how much the supporting ground or structure has actually moved.

Direction tells you the mechanism

Width tells you severity. Direction tells you cause.

Vertical cracking, particularly near openings or at the ends of walls, is often shrinkage or thermal movement. Brickwork expands and contracts, and if the articulation joints are absent or badly placed the wall relieves the stress by cracking where it can.

Diagonal cracking that widens as it rises usually means the ground beneath one part of the footing has dropped. Diagonal cracking that widens as it descends often means the opposite, a portion has lifted. Stair-stepping through the mortar joints is a classic footing movement signature.

Horizontal cracking is the one I want to see quickly. In a cavity wall it can indicate the ties have corroded and the outer leaf is separating, which is a stability question rather than a cosmetic one.

Location narrows it further

A crack at the corner of a window is a stress concentration and often benign. A crack running through the middle of a wall, continuing through the slab edge, and reappearing internally is telling you the whole element moved together.

Cracking concentrated on one elevation, particularly the one nearest a large tree or a drainage line, points at moisture change in the soil rather than at the building.

Reactive soil is the usual culprit in Sydney

Much of Sydney sits on clay that changes volume with moisture. Wet season the clay swells, dry season it shrinks, and the footing rides that movement. A footing designed for the site classification handles it. A footing designed for a more benign site than the one it is on does not.

That is why a geotechnical assessment is often the useful next step rather than more structural inspection. The building is usually behaving reasonably. The ground is the variable.

Leaking stormwater, a broken sewer line, or a downpipe discharging at the footing will produce the same symptoms and are far cheaper to fix.

Whether it has stopped matters more than how bad it looks

An ugly crack that has not moved in five years is a different problem to a fine crack that appeared last month and has widened since.

Monitoring resolves it. Tell-tales across the crack, or simply a pencil mark at each end with a date, will answer the question within a season. I would rather monitor for three months and design the right repair than underpin a building that had already found its equilibrium.

When to call an engineer

Call when cracks exceed about 5mm, when they are horizontal, when doors and windows have started binding, when the crack passes through a structural element rather than just render, or when it is visibly growing.

Bring the assessment forward if you are about to buy the property, because the cost of the inspection is trivial against the cost of discovering it afterwards.

We deal with this constantly across structural engineering work in Sydney. Most reports end with monitoring and drainage correction. The ones that do not are the reason you check.