What an engineer reads in a crack
Cracks are evidence. Width, direction, location, which materials they pass through and whether they taper all point at different causes, and the cause determines whether anything needs doing.
The distinctions that actually matter:
- Pattern - vertical, horizontal, diagonal or stepped through the mortar joints. Stepped diagonal cracking widening towards the top is a different animal from a straight vertical crack at a junction.
- Width, measured rather than eyeballed - and whether the crack tapers, which usually indicates rotation rather than simple opening.
- Location - at an opening, at a change of height or material, at a junction between old and new build, or running through the middle of a solid panel.
- Whether it passes through the bricks or follows the mortar, and whether it shows on both faces of the wall.
- Age - fresh, clean edges versus decorated-over, dirt-filled cracking that has plainly been there for decades.
- What the rest of the building is doing - floor levels, door and window fit, and whether anything else corroborates movement.
How severity is actually judged
There is a published scale for this, and it is the one engineers use: the BRE damage classification set out in BRE Digest 251, which sorts visible damage to walls into categories 0 to 5 - negligible, very slight, slight, moderate, severe and very severe - largely on crack width and ease of repair.
It is worth knowing that the great majority of domestic cracking sits in categories 0 to 2. Hairline and up to around 5mm, repairable during normal redecoration or by easing and filling, with no effect on the stability of the building. That is the honest position on most of what we are called to look at, and we would rather tell you that than dress it up.
What moves a crack up the scale is width beyond that, doors and windows that no longer fit, service pipes fracturing, walls out of plumb, and - above all - continued movement.
Crack monitoring: when it is the right answer
One visit tells you what a crack looks like today. It cannot always tell you whether it is still moving, and that is usually the question that decides everything. Monitoring answers it with measurement instead of opinion.
We fit gauges or studs across the cracking and take readings at intervals over a period long enough to cover the seasons - because ground movement, thermal movement and moisture-related movement all vary through the year, and a summer-only record can flatter or frighten. A monitoring period that spans an autumn and a spring is what turns a guess into evidence.
The outcome is one of three findings, and all three are useful: the crack is stable and historic, and needs nothing beyond making good; the crack is moving seasonally and reversing, which points at thermal or moisture behaviour rather than foundation failure; or the crack is progressively opening, which justifies investigating the cause properly and designing a repair.
What causes cracking, in rough order of how often we find it
Very little of the cracking we investigate turns out to be subsidence, despite subsidence being the first word most people reach for.
- Thermal and moisture movement - materials expanding and contracting, especially at junctions and in long unrelieved runs of brickwork
- Shrinkage in new plaster, screeds and render, and settled making-good around previous work
- Differential settlement between an extension and the original building, which usually stabilises
- Lintel deflection or failure over openings, showing as cracking above windows and doors
- Wall tie corrosion in cavity walls, producing characteristic regular horizontal cracking
- Roof spread pushing out the tops of walls, and chimney or gable movement
- Overloading from an alteration that removed support without replacing it properly
- Genuine foundation movement - subsidence, heave or landslip - which is real, but a much smaller share of cases than the internet suggests
Why an engineer rather than a contractor
Search for cracking advice and most of what comes back is published by firms that sell the cure. That does not make them dishonest, but it does mean the diagnosis and the sale sit in the same hands.
We do not carry out repairs and we take no referral fee from anyone who does. That leaves us free to conclude that a crack is old, stable and cosmetic - which is genuinely the commonest finding - and to put it in writing so a buyer, a seller, a lender or an insurer can rely on it.
What you get
A written report describing the cracking, its likely cause, its classification, and the recommendation - which may be monitoring, may be repair design, and is often neither. Photographs and, where monitoring has been carried out, the readings and what they show.
Where repair is needed, we can design it: lintel replacement, crack stitching, re-tying, tie-rods, underpinning design where it is genuinely warranted, or a structural repair detail Building Control will accept. Designing the fix is a separate piece of work from telling you whether you need one, and we keep it that way deliberately.
Frequently asked questions
How wide does a crack have to be before I should worry?
Width alone is not the test, but it is the starting point. Under the BRE Digest 251 classification, damage up to about 5mm is generally categorised as negligible to slight - repairable with normal decoration and not affecting stability. Above that, and particularly where a crack is widening, passing through bricks rather than mortar, or accompanied by doors and windows sticking and floors sloping, it warrants an engineer's inspection.
How long does crack monitoring take?
Long enough to cover the seasons. Ground and building movement varies through the year, so a monitoring period that spans autumn and spring gives a meaningful record where a few weeks in summer does not. We agree the interval and the duration with you at the start and tell you what each reading is showing as we go.
Are stepped cracks in mortar joints always subsidence?
No. Stepped cracking through mortar joints is simply the path of least resistance in brickwork, and it appears in thermal movement, lintel deflection and settlement as readily as in subsidence. What matters is the direction it widens, what else in the building corroborates it, and whether it is still moving.
Will my insurer accept your report?
Insurers normally want an engineering assessment of cause, and that is what the report gives them. If you have already notified a claim, tell us at the enquiry stage - the insurer may have appointed their own engineer, and it is worth knowing where the report sits in that process before we start.
My cracks appeared after a hot summer. Is that relevant?
Very. Seasonal cracking that appears in a dry spell and closes again in winter behaves quite differently from progressive movement, and that difference is exactly what monitoring is designed to capture. It is one of the clearest cases where measurement over time beats a single opinion.