A crack width gauge (also called a crack monitor or tell-tale) is a simple, inexpensive way to answer the question every Wagga homeowner with a wall crack eventually asks: is this thing still moving? A basic plaster tell-tale costs next to nothing to set up yourself, while a proper adhesive crack monitor with a printed scale runs a few dollars each, and either can turn “the crack looks a bit different” into an actual measurement you can act on. This guide covers how to use one properly, what the readings mean on Wagga’s reactive clay, and when it’s time to stop monitoring and call in a professional.
What is a crack width gauge and how does it work?
A crack width gauge is a small device fixed across a crack that shows, at a glance or via measurement, whether the crack has changed width or shifted vertically since it was installed. The simplest version, a plaster or mortar tell-tale, is just a thin strip of plaster or dental-grade gypsum applied directly over the crack; if the crack is still moving, the tell-tale itself cracks along the same line within weeks or months. A step up from that is an adhesive crack monitor: two overlapping clear plastic plates, one fixed each side of the crack, printed with a grid or vernier scale so you can read the exact horizontal and vertical movement in millimetres each time you check it.
Both work on the same principle. They don’t stop or fix anything; they simply give the crack behind them a witness that can’t lie about whether it’s opening, closing or staying put. That distinction, between a device that measures and one that repairs, matters because a tell-tale or gauge is a diagnostic tool, not a substitute for the foundation inspection that tells you why a crack is moving in the first place.
DIY methods: tell-tales, gauges and phone apps
For a homeowner wanting to keep an eye on a crack before deciding whether to repair or call a professional, three DIY approaches are common:
Plaster or mortar tell-tales. Cheap, easy to apply with basic filler or plaster, and give a clear pass/fail result: if the tell-tale itself cracks, the underlying crack is still active. The downside is they don’t give you a precise millimetre reading, only a yes/no on movement.
Adhesive crack monitors. Small plastic plates with a printed grid, sold cheaply through hardware and building suppliers, fixed either side of the crack with strong adhesive. These give an actual measurement in millimetres for both horizontal width change and vertical displacement (whether one side of the crack has stepped relative to the other), which is more diagnostically useful than a simple tell-tale.
Photo-and-ruler monitoring. The lowest-cost option of all: photograph the crack against a ruler or coin for scale, dated, every few months, from the same angle and distance each time. Less precise than a physical gauge but still useful for tracking obvious changes, and it doubles as documentation if you later need to show a contractor, insurer or buyer how the crack has behaved.
Whichever method you choose, the same rules apply: install it on a clean, dry section of the crack, note the date, and check it on a consistent schedule rather than randomly.
How often should you check it, and what counts as “still moving”?
On Wagga’s reactive clay, seasonal movement is normal, so the useful comparison isn’t month to month, it’s across a full wet-dry cycle. A reasonable schedule is to check monthly through the transition seasons (when clay moisture is changing fastest) and every couple of months through the depths of summer and winter, for at least twelve months before drawing a conclusion.
What the readings tell you:
- No change in width across a full year, including through at least one dry summer and one wet winter, suggests the crack is stable, from historic movement that has since settled.
- Width that expands in summer and partially closes in winter, within a similar range each year, is consistent with ordinary seasonal articulation, common and usually not urgent on its own.
- Width that keeps growing year over year, without fully closing back up each winter, indicates progressive movement that isn’t just seasonal cycling, and is worth an inspection.
- Any vertical step appearing or growing (one side of the crack sitting higher than the other) points to differential movement between two parts of the footing, a more structural signal than width alone.
Our guide to foundation crack types: serious vs cosmetic goes through crack width, orientation and location in more detail if you want the fuller diagnostic picture alongside your gauge readings.
When does DIY monitoring stop being enough?
A tell-tale or crack monitor is a genuinely useful first step, but it has real limits. It tells you whether a crack is moving; it doesn’t tell you why, how serious the underlying cause is, or whether nearby cracks and other symptoms are part of the same problem. Move from DIY monitoring to a professional inspection when:
- The crack is already wider than about 5 mm, regardless of what the gauge shows next
- Your monitor shows the crack widening progressively over more than one season, without fully closing back up
- A vertical step appears or grows at the crack line
- The crack is joined by other symptoms: sticking doors, sloping floors, or gaps opening at skirting boards
- You’re about to sell, buy, renovate or need documentation for an insurance claim, where a homeowner’s own tell-tale readings aren’t a substitute for a qualified assessment
Our guide to cracks in walls and when to worry is a good next stop if you’re weighing up whether your specific crack has crossed that line.
What does professional crack monitoring cost?
Where a DIY tell-tale isn’t giving a clear enough picture, or where a crack needs documented, dated evidence (for a pre-purchase decision or insurance claim, for example), a licensed specialist can install proper crack gauges as part of an inspection and return for follow-up readings.
| Service (indicative only) | Indicative cost* |
|---|---|
| DIY plaster tell-tale or adhesive crack monitor | A few dollars in materials |
| Initial foundation inspection, including crack mapping | $300-$800 |
| Follow-up monitoring visit (crack gauges and level re-check) | $150-$400 per visit |
*Indicative budgeting ranges only, drawn from our published foundation inspections service. Actual pricing follows a site assessment.
Crack monitoring gauge FAQs
How long should I monitor a crack before deciding it’s stable?
At least twelve months, covering both a dry summer and a wet winter. Wagga’s reactive clay means a single season’s reading, especially if it’s an unusually dry or wet one, can be misleading either way.
Is a plaster tell-tale as good as a proper crack monitor?
For a simple yes/no on whether a crack is still moving, a tell-tale works fine. For a precise millimetre reading, or to detect a vertical step as well as width change, an adhesive crack monitor with a printed scale gives more useful information.
My tell-tale cracked. Does that mean I need underpinning?
Not necessarily. It means the crack is still active, which is the trigger for a proper inspection, not an automatic verdict. Plenty of active seasonal cracks resolve with drainage correction and crack repair rather than structural work; an inspection is what tells the two apart.
Can I use crack monitoring readings to support an insurance claim?
Dated, documented readings are useful supporting evidence, particularly if they show a sudden change coinciding with a specific event like a burst pipe. Whether they support a successful claim depends on your policy and the cause, which is worth checking with your insurer directly.
Where should I place the gauge on the crack?
Across the widest visible point of the crack, on a clean, stable section of wall away from the very edge, where it’s least likely to be knocked or disturbed. If a crack runs a long distance, a second gauge partway along can show whether movement is uniform or concentrated at one point.
Not sure what your readings mean?
If you’ve been monitoring a crack and aren’t sure whether the numbers add up to “wait and see” or “get it looked at,” get a free quote and a licensed local specialist can review your readings alongside a proper inspection, and tell you honestly what’s going on.