The single most useful question about a crack is whether it’s still moving. You can start answering that yourself with a pencil, a ruler and a phone camera, or with a cheap tell-tale crack monitor fixed across the crack. Record it regularly, through the seasons, and the pattern tells you whether the crack is stable, opening and closing with the weather, or steadily getting worse.
Here’s how to monitor a crack properly, how long to keep going, and when to stop measuring and call someone.
Why monitor a crack at all?
A crack’s width tells you how much a wall has moved in total. It doesn’t tell you when. A 3mm crack could have appeared in the 1970s and not changed since, or it could have opened last summer and still be growing. Those two cracks need very different responses.
Monitoring separates them. It’s also what insurers, engineers and surveyors rely on when investigating possible subsidence, because a cause can’t be confirmed, or a repair designed, until it’s clear whether the building is still moving.
Before you start: record the crack as it is now
- Photograph it square-on, with a ruler or tape measure held alongside so the scale is visible. Take a close-up and a wider shot showing where it sits on the wall.
- Measure the width at its widest point and at each end. A tapering crack, wider at one end than the other, is an important clue.
- Note the length and where it starts and finishes, for example “from the top right corner of the kitchen window to the ceiling”.
- Check the other side of the wall. A crack showing both inside and outside at the same point is more significant than one in the plaster only.
- Write the date on everything.
Our cracks in walls checker walks you through the shape, direction and width and shows where the crack sits on the standard damage scale. Our guide to horizontal, vertical and diagonal cracks explains what the direction usually means.
Method 1: pencil marks and dated lines
The simplest method costs nothing.
- Mark each end of the crack with a short pencil line across it, and write the date next to it.
- Draw two small, fine marks either side of the crack at its widest point, and measure the distance between them with a steel rule.
- Re-check every few weeks. If the crack has grown past your end marks, mark the new end and date it.
It’s crude, but it’s a good way to show whether a crack is getting longer, which is easy to miss.
Method 2: a crack-width gauge or card
A crack-width gauge is a clear plastic card printed with lines of known thickness, from hairline to several millimetres. You hold it against the crack and find the line that matches. They’re inexpensive and more consistent than eyeballing a ruler.
Measure at the same marked points each time, and photograph the card in place, so you can compare like with like.
Method 3: tell-tale crack monitors
A tell-tale is the device most people picture when they think of crack monitoring. The common type is made of two overlapping plastic plates. One plate carries a printed grid, the other a cross-hair. You fix one plate on each side of the crack (usually with screws and plugs, or adhesive) with the cross-hair set at zero. Any movement shows as the cross-hair shifting across the grid, both opening or closing and sliding sideways or up and down.
Tips for fitting one:
- Fit it across the widest part of the crack, on a sound surface rather than loose plaster or flaking paint.
- Record the date and the starting position on the plate or in your notes.
- Don’t knock it or move it. A disturbed tell-tale gives a meaningless result.
- Outside, fit it out of direct sun if you can, as some plastics expand slightly in heat.
Read by eye, a tell-tale isn’t as precise as engineers’ instruments, but it is enough to show real movement on most domestic cracks. They’re also easy for someone else (an insurer or surveyor) to read later.
How engineers monitor cracks: Demec studs
Where more precision is needed, for example during a subsidence claim, monitoring is often done with Demec studs. These are small metal discs, each with a tiny central dimple, glued to the wall either side of a crack. A calibrated gauge or digital caliper measures the distance between the dimples at each visit. With good equipment the readings are accurate to fractions of a millimetre, so small seasonal movements show up clearly.
For whole-building movement, professionals may also use level monitoring, measuring the height of fixed points around the house over time. BRE’s Digest 343 covers simple measuring and monitoring of crack movement in low-rise buildings.
How often and for how long should you monitor a crack?
- Frequency. Every four weeks or so is a sensible rhythm for home monitoring. Measure at a similar time of day and note the weather.
- Duration. Ideally at least a full year, so that the readings cover a dry summer and a wet winter.
The reason is clay shrinkage. Clay soils shrink as they dry in summer, especially near trees, and swell again as they rewet in winter. A house on shrinkable clay can show cracks that open in late summer and autumn and close again over winter and spring. Insurers’ guidance reflects this: monitoring cracks for at least twelve months is common, so that all the seasons are covered.
What do crack monitoring results mean?
Once you have several months of readings, the pattern usually falls into one of three types:
| Pattern | What you’ll see | What it usually suggests |
|---|---|---|
| Stable | No change beyond the accuracy of your method | Historic movement that has stopped. Often repairable cosmetically |
| Cyclical | Opens in summer and autumn, closes in winter and spring | Seasonal movement, often clay soil affected by trees or vegetation |
| Progressive | Gets steadily wider or longer, season after season | Ongoing movement, for example from a leaking drain or other ground loss. Needs investigating |
A cyclical pattern that also gets slightly wider each year is effectively progressive. And remember that some small movement is normal: plaster and masonry move slightly with temperature and moisture, which is why the very finest hairline cracks tend to come and go.
The BRE crack categories
Surveyors, engineers and insurers describe crack damage using the categories in BRE Digest 251, based mainly on how easy the damage is to repair, with a typical crack width for each:
| Category | Description | Typical crack width |
|---|---|---|
| 0 | Negligible | Hairline, less than 0.1mm |
| 1 | Very slight | Up to 1mm |
| 2 | Slight | Up to 5mm |
| 3 | Moderate | 5–15mm (or several cracks up to 3mm) |
| 4 | Severe | 15–25mm |
| 5 | Very severe | Over 25mm |
Categories 0–2 are generally regarded as aesthetic, 3–4 as affecting serviceability, and 5 as potentially affecting stability. Width alone isn’t the whole picture, but monitoring tells you whether a crack is moving between categories.
When should you stop monitoring and call someone?
Don’t wait for a full year of readings if you see any of these:
- the crack is getting wider or longer from one reading to the next;
- it’s wider than about 3–5mm, or tapered, or visible inside and out;
- doors or windows start sticking or gaps appear around frames;
- there’s a new crack where an extension, bay or porch meets the main house;
- a wall is bulging or leaning, or you can see daylight through a crack.
If you own the house, tell your buildings insurer early if you suspect subsidence. Most policies require you to report it promptly, and the insurer will usually arrange its own monitoring. Our guides to what subsidence is and how subsidence claims actually work explain the process.
Crack monitoring in Yorkshire homes
Much of West Yorkshire is built on sandstones, gritstones and mudstones rather than the highly shrinkable clays common in parts of the South East. There are clay deposits in places, so seasonal movement does happen here, but it’s not the default explanation. Cracks in our area are more often linked to leaking drains, old mine workings, made ground, wall tie corrosion or long-stable historic movement in stone terraces. Those causes don’t always follow a seasonal rhythm, which is one more reason to keep careful, dated records.
Getting a professional view
If you’re buying, you usually won’t have time to monitor a crack for a year, so the question becomes how likely it is to be active and what to ask the seller. A Level 2 or Level 3 survey will note cracking and advise where further investigation is needed. If you own the house, a subsidence assessment looks at the crack pattern, the ground, trees and drainage, and can set up a monitoring regime that will stand up to scrutiny. Get in touch with the address and a photo of the crack with a ruler alongside.
Sources and further reading
- Assessing cracks in houses — BRE
- BRE Digest 251: Assessment of damage in low-rise buildings — BRE
- BRE Digest 343: Simple measuring and monitoring of movement in low-rise buildings, part 1: cracks — BRE
- Subsidence: dealing with the problem — Association of British Insurers
- Subsidence and other types of ground movement — Financial Ombudsman Service