Cracks in brick walls: when to worry

What crack width tells you

Width is the first filter, and the thresholds are simple. Under 1 mm, roughly the thickness of a fingernail, a crack is classed as hairline and is almost always cosmetic: paint shrinkage, render movement, or the normal seasonal breathing of a brick house on clay soil. Between 1 mm and 5 mm you are in monitoring territory. That is not an emergency, but it is worth measuring monthly to see whether it is growing. Anything over 5 mm, wide enough to slide a house key into, warrants a professional assessment, especially if the crack is new or getting wider.

Width alone is not a verdict, though. A 3 mm crack that has looked identical for ten years is less worrying than a 2 mm crack that was not there last spring. Movement over time matters more than size on any given day, which is why the monitoring routine below is the most useful thing you can do this month.

Reading the pattern

The shape of a crack says more than its size. A stepped crack that follows the mortar joints in a diagonal staircase is the classic signature of footing movement: mortar is weaker than brick, so when part of the footing drops, the crack takes the path of least resistance. Stepped cracking is the most common wall symptom of subsidence, and it is worth reading alongside the other signs of subsidence before drawing conclusions, because one stepped crack on its own proves very little.

A vertical crack that runs straight through the bricks themselves is a different animal. Breaking a brick takes far more force than opening a mortar joint, so a clean break through brickwork points to concentrated, localised stress and deserves faster attention.

A horizontal crack running along a single mortar course is usually not a footing problem at all. The common culprits are corroded wall ties, which let the outer brick skin bow away from the frame, or a failing lintel above a window or door. It still needs fixing, but underpinning is rarely the answer, which is one reason a proper diagnosis matters before anyone quotes you.

Direction and taper

Cracks caused by ground movement are rarely a uniform width. They taper, and the taper points to the problem. A crack that is wider at the top than at the bottom usually means a section of wall is rotating or a corner is dropping away; the wall is hinging, and the wide end of the crack marks the part moving furthest. Corners are the most common casualty because they carry the load of two walls, sit closest to garden trees and downpipes, and dry out fastest in summer.

Check both sides of the wall while you are at it. A crack in the internal plaster that lines up with a crack in the external brickwork at the same spot has gone through the structure, not just through a surface finish. Matching internal and external cracks move a wall from "keep an eye on it" towards "get it assessed".

Fresh crack or old news

Age matters as much as size. A fresh crack has clean, sharp faces: bare brick or plaster inside, with no paint, dirt or cobwebs. An old one has paint bridging into it, grime in the gap, or the tell-tale ridge of someone's previous filler. Old, stable cracks are often the fingerprint of initial settlement from decades ago, which is normal and finished. Whether your house needs anything done at all is a genuine question, and our guide to whether you actually need underpinning works through settlement versus subsidence properly.

A simple monitoring routine

You do not need gauges to monitor a crack, just a phone and a habit.

• Photograph the crack with a coin held at its widest point for scale.
• Pencil a short line across each end of the crack and write the date beside it.
• Repeat monthly, from the same angle and in similar light.

If the crack grows past your pencil marks or visibly widens over two or three months, that is progressive movement, and it is time for an independent engineer's report. At $600–$1,200 it is money well spent before anyone quotes you for underpinning work that typically runs $10,000–$40,000.