Crack Monitoring with Telltales and Photos: A Homeowner's Method

“Keep an eye on it” is common advice for a crack that doesn’t look urgent. It’s also vague. Glancing at a crack every few weeks and deciding it looks about the same isn’t monitoring. Memory isn’t a measuring tool, and a crack you walk past every day is hard to judge.

Monitoring means a record someone else can read: where the crack is, how wide it is at fixed points, whether the two sides are moving, and when each reading was taken. This post shows a simple way to build that record with a telltale gauge, a crack card, a sketch, and photos you can repeat. It works for cracks in foundation concrete and in the walls above. For what different cracks are associated with, start with our foundation cracks guide and the companion post on horizontal vs vertical foundation cracks. For what to do once a cause is known, see foundation crack repair.

The short version: map each crack, mark fixed measuring points, install a telltale where movement matters, take photos from the same spot every time, and log every reading with the date and weather. Then hand the record to whoever evaluates the house.

What monitoring can and can’t do

The company that makes the crack monitor pictured in the American Concrete Institute’s crack report is clear about the limits: “Monitoring in isolation to any other investigation will rarely provide sufficient information to fully diagnose the cause of cracks. But it will indicate how the building is deforming and the rate at which movement is occurring” (Avongard).

That is the point. A good record can show that a crack is static, that it opens and closes in a cycle, or that it keeps getting wider. It can’t tell you why. ACI’s guidance is that “the cause of the cracking should be established before repairs are specified.” Monitoring feeds that work. It doesn’t replace it.

Before you start

Don’t fill or paint over the crack yet. HUD’s Residential Rehabilitation Inspection Guide notes that “a clean crack indicates recent movement; a dirty or previously filled crack may be inactive.” Patching erases that clue, and it gives a gauge nothing to measure.

Pick the cracks that matter. You don’t need to monitor every hairline. Start with cracks in foundation concrete, cracks that have changed, cracks with one side higher than the other, and any crack you’re unsure about. For crack vocabulary, our hairline vs structural foundation cracks post covers the terms.

Stay safe. Don’t crawl into a crawl space that is wet, tight, or has exposed wiring or pests. Photograph what you can reach and leave the rest for an evaluation.

Start now. “Since crack monitoring takes time, it is essential it starts at the earliest opportunity,” the gauge maker advises. A record that begins today is worth more than a perfect one that begins next year.

Step 1: Make a crack map

ACI’s evaluation guidance starts on paper: “The locations and widths of cracks should be noted on a sketch of the structure. A grid marked on the surface of the structure can be useful to accurately locate cracks on the sketch.”

For a house, that can be simple:

• Sketch each wall or slab area with cracks. A rough floor plan or elevation is fine.
• Give each crack an ID, such as G1 for the first garage crack or N2 for the second on the north stem wall.
• Measure from a fixed reference, like a corner or a door edge, to each end of the crack.
• Write the ID on a small piece of painter’s tape near the crack so it shows up in every photo.
• Note anything else you see at the crack. ACI lists “spalling, exposed reinforcement, surface deterioration, and rust staining.”

Step 2: Measure width and offset at fixed points

Make two or three small pencil marks along each crack and always measure at those marks. Measuring a different spot each time makes readings impossible to compare.

ACI describes two width tools. A crack comparator is “a small, hand-held microscope with a scale on the lens.” The more practical option for most people is “a clear card having lines of specified width marked on the card.” You hold the card over the crack and find the line that matches.

Then check the step. “Any displacement of the surface (change in elevation) across the crack should also be documented,” ACI says. Run a finger or a straightedge across the crack. If one side is higher or sits farther out, measure that offset at the same marks and write it down.

Mark the ends of each crack with a small tick and the date. If the crack grows past the tick, you’ll see it.

Step 3: Install a telltale where movement matters

How a telltale works

A telltale is a two-piece gauge that sits across a crack. In the common design, one plate is “calibrated in millimetres and the top plate is transparent and marked with a hairline cursor in the form of a cross,” and the two plates “overlap for a part of their length” (Avongard). One end is fixed on each side of the crack. “As the crack opens, or closes, the cursor moves relative to the calibrated scale.”

Because the scale is a grid and the cursor is a cross, you can read movement in two directions across the wall’s surface. ACI’s report describes this type of monitor as one that “gives a direct reading of crack displacement and rotation.” Mechanical gauges have another advantage: “they do not require moisture protection,” which helps in crawl spaces and on exterior walls.

Variations exist for awkward spots. A hinged corner version works on inside and outside corners and can be used “singly to monitor two dimensional movement” or “in pairs to monitor three dimensional movement.” It can also be set up “to monitor cracks between ceilings and walls or floors and walls.”

Fixing it

Follow the instructions that come with the gauge. The manufacturer’s guidance includes a few points worth knowing before you buy:

• It is “fixed across the crack using screws and plugs and/or adhesive.” Ideally both, since “there is the risk of tampering if fixed with screws alone.”
• Small pegs hold the two plates in place until the gauge is fixed. “The adhesive must fully cure before the pegs are removed.”
• Center the gauge over the crack at one of your marked points, and add it to your sketch with the crack ID.

Take a photo and a first reading the day you remove the pegs. That is your baseline.

Reading it

Each time, note where the center of the cross sits on the grid, side to side and up and down, and write both numbers in your log with the date. The manufacturer supplies a record sheet with each gauge.

Know the tool’s limits. The manufacturer says its standard telltales “are calibrated to 1.00mm and can be read by interpolation to 0.5mm.” Very small changes are below what you can read reliably. Finer readings with calipers are possible, but at that resolution “extraneous information such as thermal, shrinkage and moisture content changes may be detected.” A crack “will behave in a similar way to a movement joint” as temperatures change, so note the weather, or better, the temperature, with every reading.

The good news: “The owner of the building can take the readings, complete the crack monitoring sheet at each reading and forward it to the professional for analysis.” Reading the gauge is your job. Interpreting it isn’t.

Step 4: Take photos you can repeat

ACI notes that “sketches can be supplemented by photographs documenting the condition of the structure.” Photos only help if you can compare them. A few habits make that possible:

• Fix the camera position. Put a tape mark on the floor or a small stake in the ground where you stand. Shoot from the same mark, at the same height, every time.
• Take two shots. One wide shot that shows where the crack is on the wall, and one close-up of the gauge or a marked point.
• Put a scale and a label in the frame. The crack card or a ruler, plus the tape with the crack ID. Writing the date on the tape helps too.
• Match the light. Shoot at about the same time of day, or use a flashlight held at the same angle each time.
• Keep the originals. Don’t crop or filter. Put the photos in one folder per crack, named by date.

Our post on footing damage homeowners can spot has a short photo checklist for footings specifically.

Step 5: Keep a log, and keep it long enough

A simple table works. One row per reading:

How long? There’s no official homeowner schedule, but HUD gives a sense of scale: “Cyclical movements may take six months or more to measure, but diurnal movements can be recorded over a few days.” In San Diego, that means seeing at least one wet season and one dry season before reading much into a cycle. Our post on seasonal vs progressive cracks explains why.

A rhythm that’s easy to keep: a reading every few weeks, plus one after a heavy rain, after any plumbing or irrigation change, and after any earthquake you felt. Consistency matters more than frequency.

HUD also notes that hand measurements “will be less precise and require repeated field visits” compared with a gauge. If you only have a crack card, that’s fine. Take more readings, and take them the same way.

Reading your own record (carefully)

The gauge maker describes three broad patterns. A crack “may not increase in width and therefore would be static.” It may “progressively increase in width over time,” which “may suggest there is a serious defect in the foundations and / or the superstructure.” Or it may “open and then partially close, and then open again,” following “a cyclic pattern of movement,” where “correlating the crack movements with external factors will assist with identifying the cause.”

That’s why your log has weather, irrigation, and repair columns. A crack that opens every time the sprinklers run near that wall tells a different story than one that widens steadily no matter what.

Don’t set your own pass/fail number. We don’t publish crack-width or movement thresholds, because what a given change means depends on the material, the location, the age of the house, and the cause.

When to stop monitoring and call

Monitoring is for cracks that aren’t urgent. Stop waiting and get an evaluation if:

• A wall that holds back soil has a horizontal crack, or starts to bow or lean
• The two sides of a crack develop a step, or an existing step grows
• A gauge reading moves in one direction across several readings
• New cracks appear near the one you’re watching, or doors and windows nearby start to bind
• Water, soil, or rust staining shows up at the crack

If you are watching a retaining wall, our retaining wall monitoring checklist covers what to check on walls that hold back a slope.

What to hand an evaluator

Bring the record to the visit, or send it ahead:

• The crack map with IDs and reference measurements
• The log table
• Photo folders by crack ID
• The date each gauge went in, and its baseline photo
• Notes on anything that changed: plumbing leaks, irrigation, grading, new patios, additions

A record like this lets a foundation inspection spend less time reconstructing history and more time on cause. HUD points out that when problems appear to involve differential settlement, “a soils engineer also may be required,” and a good record helps that conversation too.

Cost factors, not prices

We don’t publish prices. Monitoring itself is mostly your time and a few simple tools. What shapes the scope of an evaluation or repair afterward: how many cracks and where, access to the foundation (crawl space, finished walls, hardscape), whether movement is ongoing, whether drainage or plumbing is involved, whether a wall holds back soil, whether engineering or a soils report is needed, and permit requirements.

Where we fit

We review homeowner crack records as part of on-site evaluations across La Jolla, and we’ll tell you if what you’ve recorded supports continued monitoring or points to a cause worth addressing. When a crack needs engineered design or soils input, we coordinate with licensed engineers as needed. This post is general education, not an engineering opinion about any specific house.

Next step

If your record shows a crack that keeps widening, a growing step, or a horizontal crack in a wall that holds back soil, request a Free Quote for an on-site evaluation anywhere in La Jolla, or call (650) 739-6537.

Sources (accessed October 18, 2026)

• American Concrete Institute Committee 224, ACI 224.1R-07, Causes, Evaluation, and Repair of Cracks in Concrete Structures (2007), Sections 2.1 and 2.2.1
• U.S. Department of Housing and Urban Development, Residential Rehabilitation Inspection Guide (2000), Section 4.3
• Avongard, Standard Tell-Tale and Corner Tell-Tale product documentation, and FAQs