Crack Monitors: How to Prove a Hairline Crack Is Stable Or Progressing
What an Avongard tell-tale is, how to fit and read one, and why dated photographs of monitor readings settle defects-period disputes. Buy from £15.
A homeowner takes possession of a new kitchen extension. Six weeks later, they spot a hairline crack at the junction of the old and new brickwork. The crack is barely visible. They call the builder, who says the same thing every builder says: extensions move slightly as they settle, the crack will probably stop, paint over it. Six months later, the crack has widened. The homeowner takes it to the warranty body. The warranty body asks for evidence: was the crack monitored, what was its width on what date, has it progressed by how much. The homeowner has a vague memory and one phone photo from when they first spotted it.
A crack monitor (also called a tell-tale) is a 15 pound device that turns "the crack has got worse" into a dated record of its width over time. Without monitoring, you have a story. With monitoring, you have evidence. For homeowners on extensions tying into existing buildings, fitting one or two monitors at the junction during the defects period is one of the cheapest insurance policies available.
What a crack monitor is
A crack monitor (the trade name is usually "tell-tale" or specifically "Avongard tell-tale" after the brand that dominates the UK market) is a two-part plastic device fitted across a crack. One part has a clear plastic plate with a printed grid; the other part has a coloured arrow or crosshair that points to a position on the grid. The two parts are bonded to the wall on opposite sides of the crack with epoxy or screws.
If the crack widens, narrows, or moves laterally, the two parts shift relative to each other. The arrow on one part now points to a different position on the grid printed on the other. By recording the arrow position over time, you build up a record of crack movement.
Avongard tell-tales come in several patterns:
| Tell-tale type | What it measures | Where used |
|---|---|---|
| Standard plus-pattern | Width and lateral movement | Most domestic cracks at building junctions |
| Corner pattern | Movement at internal or external corners | Cracks that wrap around a corner |
| Displacement pattern | Larger movements (5mm+ scale) | Subsidence cases or known-active cracks |
| Calibrated tell-tale | Precision metric scale | Surveyor or engineer use |
For homeowner extension monitoring, the standard plus-pattern tell-tale is the right buy. Around 15 pounds for a single monitor; packs of five are typically 50 to 60 pounds. Avongard is the dominant brand; alternatives from cheaper suppliers exist but the Avongard plastic and printing are noticeably better quality.
Why monitor a crack at all
The point of monitoring is producing evidence that the crack is or isn't progressing. Three audiences care about that evidence.
Warranty bodies (NHBC, LABC Warranty, Premier Guarantee): A warranty claim during the defects liability period (typically 2 years from completion) needs evidence that a defect exists and is structural. A monitored crack that has widened from 0.5mm to 2.0mm over six months is clear evidence; an unmonitored crack has nothing measurable behind it.
Builders during defects period: Most builders will fix demonstrable defects. A monitored crack changes the conversation from "the homeowner says the crack is worse" (which builders dispute) to "the monitor reads 0.5mm wider than at handover" (which is harder to dispute).
Party wall surveyors: If your extension shares a party wall with a neighbour, the Party Wall etc. Act 1996 schedule of condition documents pre-existing cracks before work starts. Tell-tales after work completes show whether your work has caused movement in the neighbour's wall.
For homeowners on a kitchen extension that ties into the existing house wall, the value is mostly in scenario one: warranty evidence during the defects period. The 15 to 75 pound spend on monitors is small compared to the cost of a warranty claim being denied because the evidence is anecdotal.
Where to fit monitors
Crack monitors only earn their cost where cracks are likely to appear. Three positions are typical on a new extension.
At the junction between old and new brickwork. Where the new extension wall meets the existing house wall, differential movement is common. The new wall settles slightly as the foundations consolidate; the old wall is already stable. Even a well-built junction will sometimes show a hairline crack at the cement joint between old and new bricks. Fit a monitor here at handover or as soon as a crack appears.
At the corner where the extension roof meets the original roof. Movement at the wall plate level can show as cracks in the plaster around the junction. Internal monitor here catches movement before it shows as visible damage.
On any visible crack you spotted at handover. Don't wait to see if it progresses; monitor from day one. If the crack stays stable, you have a record proving stability. If it widens, you have a record proving progression. Either outcome is useful.
For most extensions, two or three monitors cover the realistic risk areas. Spend 30 to 45 pounds on the monitors at handover and you have evidence-grade defects-period protection.
How to fit a tell-tale
The fitting is the simplest part of the process.
Position the monitor across the crack
The monitor's two halves should sit on opposite sides of the crack, with the centre of the device aligned over the crack itself. The grid and arrow should both face outwards (visible from the room or street).
Mark the wall through the screw holes
Each half has two screw fixing holes. Mark through the holes with a pencil onto the wall.
Drill and plug the four holes
Drill the four marked holes with a 6mm masonry bit (for brick or block walls) and insert wall plugs. For plaster walls, choose hollow-wall plugs that grip the back of the plaster.
Screw the monitor halves to the wall
Use the supplied screws to attach the halves. Tighten enough that the monitor is firm but not so tight that the plastic cracks.
Note the starting reading
With the monitor fitted across the crack, the arrow will be in some position on the grid. Note the X-Y coordinates (the grids are typically labelled). Photograph the monitor with a date-stamped image.
Record monthly readings
Once a month for the first six months of the defects period, photograph the monitor and record the new reading. After six months, drop to once every three months. The cumulative record over the two-year defects period gives you a complete picture of crack behaviour.
For monitors fitted to external walls, ensure the camera position for monthly photos is consistent (same angle, same lighting) so the photos are comparable. A small tripod and a reference mark on the ground (a chalk cross) make this trivial to repeat.
Tip
Use epoxy adhesive instead of screws if the wall is delicate render or finished plaster you don't want to puncture. Two-part epoxy bonds the monitor to the surface without drilling. Removable later only with destruction; this is the trade-off.
What the readings tell you
Crack movement falls into a small number of patterns.
Stable crack (no movement over six months): The crack formed during settlement and has reached its final position. No further action needed. The crack can be filled and decorated; if it reopens later, the monitor will show it.
Progressive widening (0.5mm or more over a defects period): The crack is structurally active. Defects claim or builder remedial work is justified. The monitor record is the evidence.
Seasonal movement (widens in summer, narrows in winter, or vice versa): The crack is responding to thermal expansion in the masonry or to ground movement from clay shrinkage. This is a different category of issue; movement is not a defect but the wall may need expansion joints retro-fitted to manage it.
Sudden large jump (1mm+ in a single month): Something has moved significantly, possibly subsidence or foundation issue. Engage a structural engineer immediately; the monitor is now insufficient on its own.
The plus-pattern monitor reads in millimetres. Anything below 0.5mm of cumulative movement over the defects period is generally considered acceptable for a domestic build. Above 1mm, the warranty body will usually engage. Above 5mm, the situation is structural rather than a finishes defect.
What to buy
The market is small and Avongard dominates the UK.
| Item | Approx price | Use |
|---|---|---|
| Avongard standard plus-pattern tell-tale | £15-18 each | Single monitor for a single crack |
| Avongard tell-tale 5-pack | £60-75 | Multiple monitors for multiple cracks; better unit price |
| Avongard corner-pattern tell-tale | £18-22 | Cracks at internal or external corners |
| Avongard displacement-pattern tell-tale | £25-30 | Larger expected movements (5mm+ scale) |
| Cheap eBay generic tell-tale | £5-8 | Functional but printing fades and plastic ages quickly; not recommended for two-year monitoring |
| Calibrated tell-tale (Avongard CTT-MK2) | £40-60 | Surveyor / engineer use; precision scale |
For a homeowner extension with two or three monitoring points, the Avongard 5-pack at around 65 pounds from buildings-monitoring suppliers (or via Amazon UK) is the right buy. The unit price is better than buying singly, and the spare monitors can be deployed to other cracks that appear during the defects period.
For a single crack, a single Avongard plus-pattern monitor at 15 pounds is fine. Skip the cheap generic alternatives; the printed grid fades within months on the cheaper plastics, which defeats the purpose of recording readings over two years.
Where you'll use it
Crack monitors are a defects-period and post-completion tool:
- Snagging checklist for any visible cracks at the handover walk-round
- Defects liability period for the entire two-year monitoring window after handover
- Party wall agreement compliance work, where pre-existing and post-construction cracks need documentary evidence
- Building control final inspection doesn't typically test cracks, but BCO sign-off doesn't preclude defects appearing later; monitors are the homeowner's defence
The monitors stay on the wall through the entire defects period, then are removed when the period closes (or earlier if cracks demonstrate stability).
When monitoring is not enough
Crack monitors document movement. They do not diagnose causes or remedy structural issues. Three situations move beyond what a monitor can address.
Active subsidence. Sudden movement of 5mm+ over a short period, or progressive movement that accelerates rather than stabilises, suggests subsidence rather than thermal or settlement movement. Engage a structural engineer with experience in domestic subsidence.
Multiple cracks appearing at multiple points simultaneously. A single isolated crack is a localised defect. Multiple cracks across different walls suggest a building-wide cause: foundation issue, ground movement, or a major load path failure. Engineer territory.
Cracks accompanied by other symptoms. Cracks that appear with floor unevenness, doors that suddenly stop closing properly, or visible distortion in window frames are part of a larger failure. The cracks are symptoms, not the issue.
In all three cases, the monitor record is still useful evidence for the engineer or surveyor, but the issue has moved beyond what monitoring alone can address.
Common mistakes
Fitting the monitor without recording the starting reading. A monitor with no initial reading is useless. The whole point is comparison over time. Photograph and record at fitting.
Inconsistent reading dates. Once a month becomes every six weeks becomes every three months becomes "I forgot to check it for six months." Set a calendar reminder. Consistency is what makes the record useful.
Removing the monitor before the defects period ends. The two-year defects period is when warranty claims can be made. Monitors removed at six months leave 18 months of un-recorded time. Keep the monitors fitted through the full defects window.
Painting over the monitor. Decorators sometimes paint over wall fittings without thinking. A painted-over monitor cannot be read. Mask off the monitor before any decoration work.
Using a monitor for a crack that's clearly not structural. Hairline cracks in fresh plaster are common and almost never structural; they're shrinkage cracks during cure. Save the monitors for masonry junctions and any cracks that go through wallpaper, paint, or finished surfaces.
Throwing the monitor away when removing it. Used monitors that show stable readings are useful evidence in property sales (the next homeowner asks if there's been any movement; the monitor record answers definitively). Keep the monitor and the photographic record at least until you sell the property.