Reading a crack: which ones matter
Not every crack is structural, and the alarming-looking ones are often the least serious. What an engineer looks at before deciding.
Cracking is the most common reason a homeowner calls an engineer, and the most common finding is that the crack is not the problem. That is worth saying plainly, because the fear that brings people to the phone is usually disproportionate to what is happening — and occasionally it is not disproportionate at all, which is exactly why guessing is a poor strategy.
What gets looked at
- Direction. Vertical cracking in masonry is frequently shrinkage or thermal movement. Diagonal cracking, particularly running from the corner of an opening, is more often differential movement and is taken more seriously.
- Width, and whether it changes along its length. A crack that tapers is telling you something is rotating. One of constant width more often is not.
- Whether it passes through units or follows the joints. A crack that has split the blocks themselves has been driven by more force than one that took the easy path around them.
- Whether it is moving. A single visit establishes what a crack looks like today. Only monitoring over time establishes whether it is live, and that distinction changes the remedy completely.
Why photographs are rarely enough
Clients often send a photograph hoping for a verdict, and it is an entirely reasonable thing to try. The reason it seldom works is that the important information is not in the crack. It is in what is around it — the load path above, the ground conditions below, whether the same pattern repeats elsewhere in the building, whether a tree was removed, whether a neighbouring plot was excavated. A crack is a symptom presenting at the surface. The diagnosis is somewhere else.
A crack is a symptom presenting at the surface. What caused it is almost always somewhere else in the structure.
When to stop waiting
Most cracking can be watched rather than acted on. The signals that change that are worth knowing: cracking that widens measurably over weeks, doors and windows that begin to bind having previously worked, cracking that appears in more than one element at once, and any crack accompanied by visible deflection in a beam or slab. Those are not the moment to start monitoring. They are the moment to have someone look.
