Method 1: Plaster of Paris Telltale
A telltale is a small blob of plaster of Paris applied directly over a crack. If the crack moves in any direction, the plaster cracks first, because dried plaster has almost no flexibility. It's a go/no-go indicator: either the plaster is intact (no movement) or it isn't (some movement occurred).
How to do it: Mix a small amount of plaster of Paris to a thick paste. Apply a blob about two inches long and one inch wide directly over the crack, spanning from one side to the other. Label it with the date using a marker while it's still wet. Let it cure fully before evaluating — typically 24 hours in a basement environment.
What it tells you: If the plaster cracks, movement happened. It doesn't tell you how much movement, just that some occurred. If the crack in the plaster runs perpendicular to the foundation crack, the wall sections are pulling apart. If the plaster shears — one side riding up over the other — there's both horizontal and vertical movement happening.
What it doesn't tell you: Amount of movement, precise direction, or rate of change. It's a trigger indicator, not a measurement tool. Check it monthly and replace it after any movement event.
Cost: About $5 for a small bag of plaster of Paris from any hardware store. You can make dozens of telltales from one bag.
Method 2: Witness Marks with a Ruler
This is the method my neighbor Rick — a retired building inspector with 30 years of field experience — taught me when I first started monitoring my own basement. It's the simplest measurement approach and requires nothing more than a marker and a ruler.
How to do it: Draw two small reference marks on either side of the crack, spaced about four to six inches apart and centered on the widest point. Measure and record the distance between the marks on Day 1. Photograph the setup alongside a ruler. Come back monthly and measure again, recording each measurement with the date.
What it tells you: Exact width changes over time. If the marks start at 4.2 inches and three months later they're at 4.3 inches, you have 1/10 inch of lateral movement. That's meaningful data — it tells you both that movement is occurring and approximately how fast.
What it doesn't tell you: Vertical displacement, where one side of the crack rides up or drops relative to the other. For that, add a second pair of marks — one on each side of the crack at the same height — and measure vertical alignment separately. The FEMA home inspection guidance recommends this type of baseline documentation for any structural monitoring program.
Cost: Nothing beyond a marker and ruler you probably already own.
Method 3: Clear Packing Tape
This one sounds too simple to work, but it's a decent quick indicator for narrow hairline cracks where other methods are harder to apply. Place a strip of clear packing tape directly over the crack, pressing it flat. Date it with a marker. If the crack moves, the tape will either lift, tear, or deform visibly.
What it tells you: Whether movement occurred — similar to a plaster telltale in that respect. Binary feedback. You can sometimes see the crack through the tape, which lets you assess whether the opening has widened. If the tape tears, the direction of the tear gives you information about which way the wall moved.
Limitations: Tape doesn't bond well to dusty or slightly damp concrete, which describes most basement walls. It can also lift on its own from moisture changes without any foundation movement. Use this method only after cleaning and drying the concrete surface thoroughly. It's better suited for interior drywall cracks or garage floor cracks than for basement walls.
Don't rely on this as your only method. Treat it as a quick check while you're setting up a more reliable monitoring system.
Cost: Essentially free if you have packing tape on hand.
Method 4: Commercial Crack Monitors
If you want precision and durability, a calibrated plastic crack monitor is worth the investment. The most common types use a two-piece plastic gauge attached on either side of the crack. As the crack moves, the two pieces slide relative to each other across a printed grid, and you can read horizontal and vertical displacement at a glance.
How to do it: Clean and dry the concrete surface. Attach one half of the monitor on each side of the crack using epoxy or masonry screws — most monitors come with mounting hardware. Photograph the initial position. Check monthly and record both horizontal and vertical readings from the grid.
What it tells you: Precise displacement in two axes. Commercial monitors are accurate to within fractions of a millimeter and stay readable for years. This is the same type of system structural engineers use for professional monitoring programs. The American Society of Civil Engineers includes calibrated crack monitoring as a standard component of structural assessment programs.
When to use it: When you have a crack that's already showing activity, when you're tracking a crack before and after a nearby construction project, or when a structural engineer has recommended monitoring as part of an evaluation. If you're watching a crack closely over a 12-plus month period, spend the $30 to $50 on a proper monitor. It gives you data you can actually show to a professional.
Brands to look for: Avongard (widely available online), Mastercrete, or generic vernier-scale monitors sold through masonry supply companies. Avoid flimsy plastic versions without calibrated grids.
Choosing the Right Method for Your Situation
For most homeowners doing basic monitoring, the witness mark method is the best starting point. It costs nothing, gives you real numbers, and creates a documentation record you can share with a contractor or engineer later.
Add a plaster telltale if you want a visual alert between monthly check-ins. It'll tell you if something moved significantly without requiring you to remember to measure every single month.
Upgrade to a commercial crack monitor if you're tracking a crack that's already showing movement, or if you're planning to use the data in any professional or legal context. Thirty to fifty dollars is a reasonable investment when you're dealing with a potentially serious structural situation.
Whatever method you use, document everything. Take photos with dates. Write down your measurements in a notebook or a simple spreadsheet. A year of consistent documentation is worth far more than a single snapshot when you're trying to explain your foundation's history to an engineer or a buyer's inspector.