Foundation Crack Width and Monitoring: What to Record

Quick answer

Foundation crack width and monitoring is the practice of measuring how wide a crack is at a marked spot, and re-measuring the same way over time, so change can be seen and reported, instead of judging a crack by one look.

Width alone does not say whether a crack is safe. No universal width separates harmless from serious. Repeated, dated readings, photos and notes about weather and other signs give a licensed engineer something to interpret.

Quick facts

The key facts about Foundation Crack Width and Monitoring, with sources:

Purpose: Show whether a crack is stable, seasonal or still changing ¹
Single readings: A one-time observation cannot show behavior; repeated comparable readings can ¹
Plaster tell-tale: Shows that movement happened, not how much, which way or when ¹
Graduated gauge: Two plates read against a scale give a value that can be compared over time ¹
Length of record: At least one full climatic cycle is suggested to separate seasonal from progressive movement ¹
Width threshold: None here: NDSU gives no numeric size that decides when to call a contractor ³
Safety: Monitor from the surface; never dig, pry or force the crack ²

Key takeaways

Change over time tells you more than width on one day. Record the same crack, the same way, on a schedule.
There is no universal width that makes a crack safe or unsafe, and this page gives none.
A plaster tell-tale only shows that movement happened. A graduated gauge gives a number you can compare.
Seasonal movement can open and close a crack. Watching through a full climate cycle helps separate that from steady drift.
Call a licensed structural engineer for significant, widening or changing movement, a height step across a crack, or any safety doubt.

What crack width monitoring is

A crack has several properties: where it is, which way it runs, how long it is, how wide it is, whether one side is higher than the other, and whether any of that is changing. Width is the easiest to measure, so it gets the most attention. But width is only one clue.

General information, not a diagnosis of your house. A trade article on movement monitoring makes the key point: a single observation cannot show behavior, and what you need is a series of repeated readings that can be compared with one another ¹. Monitoring is the habit that produces that series.

Concrete also cracks for ordinary reasons. FEMA’s structural collapse training notes that most normal concrete develops narrow, or hairline, cracks from shrinkage, temperature change and predictable structural behavior ².

That is why a first look at a crack can mislead in both directions: a narrow crack can still be moving, and a wide-looking one can be old and stable. Only a record over time helps.

For the broader picture of tracking movement, see foundation movement monitoring. This page focuses on the crack itself.

A width is not a diagnosis

Never judge safety from one measurement. Movement history, displacement, water, construction details and site conditions all affect what a crack means. If you are worried about safety, call a licensed structural engineer now instead of monitoring.

How crack monitoring works, step by step

The method is the same for a hairline crack in a wall or a crack in a slab. Everything stays on the surface:

Pick a spot: Choose a place along the crack where the edges are fairly straight and parallel. Mark it with a pencil tick on each side or a small tape tick.
Choose a method: A plaster tell-tale shows only that movement happened, giving neither magnitude, direction nor date. A graduated gauge, two plates read against a scale, produces a value that can be compared ¹.
Take a baseline: Photograph the crack with a ruler and a date card, then record the reading and the temperature and weather.
Repeat on a schedule: Read the same way at the same marks, monthly or after big weather changes, and write the date each time ¹.
Look for the pattern: Does the crack open and close with seasons, or does it drift in one direction? The trade article describes progressive drift that never returns to its starting point as the thing to separate from seasonal movement ¹.
Share the record: Give the series to a licensed engineer, who reads it with the structure and the soil.

Cycle The monitoring loop

Mark the spotSame place every time
Measure and photographSame method, with scale and date
Note weather and signsTemperature, rain, doors, floors
Compare with the last readingIs it the same, seasonal or drifting?
DecideKeep watching or call an engineer
Back to step 1

A general loop built from the repeated-reading approach in the cited monitoring article. It is not a repair plan. Source: [1]

Diagram Where to mark on common crack shapes

Illustrative sketches of crack shapes. A shape is a clue, never a diagnosis, and the drawing is not to scale or a design for any house.

Ways to track a crack: what each shows and what it misses

Resolution is from one trade article that carries a gauge maker’s sponsorship line, and is given only as an example of what graduated gauges can do. No product is endorsed.
Method	What it shows	What it misses
Pencil marks and a ruler	Rough change in width at a marked point	Small movement and direction; depends on how you hold the ruler
Photo with scale and date	The crack’s look and length at that date	Exact width; angle and light change what you see
Plaster tell-tale	That movement happened	Magnitude, direction and date ¹
Graduated crack gauge	A value for movement that can be compared over time; trade article says vernier models resolve to 0.1 mm ¹	Out-of-plane movement unless the gauge is made for it
Logged sensors or alarms	Frequent readings and alerts; used where safety is at stake ¹²	Cost and setup; usually engineer-directed

Alarms and continuous reading are for hazards

FEMA’s training materials say crack-measuring devices used to monitor damaged structures must be read continually and paired with an alarm and evacuation plan to be effective ². That is a rescue setting. If you think a building is in danger, leave and call for help instead of monitoring.

Example: reading a monitoring log

An example log for one invented crack, read with the idea of separating seasonal from progressive movement ¹. The numbers are made up and are not thresholds.

Reading	Season	Width at the mark	Pattern so far
1	Late summer, dry	2 mm	Baseline
2	Autumn, rainy	1 mm	Narrower after rain
3	Winter	1 mm	Stable
4	Late summer, dry again	2 mm	Back to the baseline: looks seasonal
5	Next autumn	3 mm	Wider than any earlier reading: ask an engineer

The example shows how a series answers a question one reading cannot. Reading 5 is a change from the earlier cycle, which is the kind of change an engineer wants to know about. Your numbers and your engineer’s judgment replace this illustration.

Where and what to monitor around a house

Monitor the cracks that matter to you or that look different from the rest.

Common places are a crack in a basement or crawl space wall, a crack in a slab floor, a crack at the corner of a door or window, a crack in a brick or block wall above grade, and a gap where an addition or porch meets the house.

Pair the crack with other signs. NDSU extension lists sticking doors or windows and cracks in walls or ceilings as possible signs that a building is shifting, and says the degree of alignment distortion is one of the main indicators of whether a contractor is needed ³.

Note doors, windows, floor levels and baseboard gaps in the same log.

Fresh concrete cracks differently from old. FEMA’s materials say shrinkage cracks usually form within about 60 days of the pour as the concrete dries ². A new house with early hairline cracks is a different case from a thirty-year-old wall with a new crack. See cracks in slab foundations for slab patterns.

What monitoring gives you, and what it cannot

What a good record gives you

Shows whether a crack is stable, seasonal or drifting, which a single look cannot ¹.
Gives an engineer dated evidence, and a dated curve carries more weight than judgment alone, as the trade article puts it ¹.
Costs little when done with marks, photos and a log.
Helps you decide when to stop waiting and call.

What monitoring cannot do

Does not say why a crack is moving. That needs a review of soil, water and structure.
Does not make a crack safe. Watching is not repair.
Needs time: the trade article suggests at least one full climatic cycle ¹.
Surface readings can miss out-of-plane movement such as one side stepping higher.

Why there is no safe width

People want a number. A width that means one thing in a basement wall of poured concrete means something else in a brick wall and something else in a slab.

Concrete type, wall height, loads, soil, reinforcing and how the crack formed all change the meaning.

A fresh, tapered surface crack from plastic shrinkage looks different from a through-wall crack, and FHWA’s petrographic manual describes these types by cause and appearance ⁴.

NDSU’s extension article says crack size in basement walls generally indicates whether a contractor is needed, yet it gives no numeric threshold, and the same is true of alignment distortion ³.

Penn State’s concrete guidance likewise treats some cracking as expected in most concrete construction ⁵. Neither source sets a universal safe or unsafe width, and neither does this page.

Hairline cracks deserve a note. FHWA describes microcracks as visible only with magnification and crazing as a fine, shallow surface pattern ⁴. Tiny cracks are common and need context. Never say any specific crack is harmless, and never say every crack needs repair.

What drives the cost of crack monitoring

We print no prices, because fair numbers need dated local quotes. A home log costs only your time and a ruler or gauge. Professional monitoring costs depend on the number of points, the type of instruments, how often readings are taken, how long the program runs, and who reads and reports the data.

An engineer’s visit adds a cost, and so does a level survey. Both can keep a repair from being aimed at the wrong cause. See the foundation cost guide for how quotes are built.

How to set up a crack log in an afternoon

Gather a pencil, a clear ruler, a tape measure, a notebook or phone, and a light.

List the cracks: Walk the inside and outside, and number each crack you want to track.
Mark each spot: Make small tick marks on both sides, where the edges are straight.
Take the first photos: Wide view, then close view with a ruler and a card with the date and crack number.
Record the baseline: Width at the mark, length, any height step felt by hand, and the weather.
Note the signs: Doors, windows, floors, baseboard gaps and any musty smell or damp.
Set reminders: Pick a schedule, such as monthly and after big storms or dry spells.
Keep the log together: One folder with photos, dates and notes, ready for an engineer.

Use the log tools on this site

The elevation log and the crack identification tool are built for this kind of record. A gauge is optional, and it does not replace an engineer.

How to keep a monitoring log trustworthy

Measure from the same marks, with the same tool, at the same angle.
Write the date and time, and the weather, every time.
Do not touch up, fill or paint the crack while you are monitoring it, since that changes what you see.
Re-mark if a tick wears off, and note that you did.
Wait to repair wall and ceiling cracks until soil moisture returns to normal, as NDSU advises, and let an engineer decide when repair makes sense ³.
Back up the photos so a phone change does not erase the record.

Warning signs and when to call a structural engineer

Do not wait for a monitoring period to end if you see any of these. Call a licensed structural engineer when you see:

Significant, widening or changing movement in a crack.
A height step across a crack, or a wall or floor that moves out of line.
A wall that bows or leans, or a horizontal crack in a basement wall.
Sticking doors or windows together with new cracks.
Separation at corners or where an addition meets the house.
Any safety doubt, including sounds, sudden changes or water entering.

The structural engineer guide and the engineer or contractor tool explain who to call first.

What codes and standards say about crack width

The residential code does not set a crack width that decides whether an existing foundation is safe.

Chapter 4 of the model IRC covers how new footings, walls and drainage are built and when soil tests are required, not a width for cracks in a house that already stands ⁷.

Concrete design guides, such as the ACI documents NRMCA cites for slabs, cover design and construction, not homeowner triage ⁶.

That is why the source material here refuses to name a number. NDSU extension, FEMA’s collapse training and FHWA’s manual all describe how cracks look and form without giving a universal safe or unsafe width ³²⁴.

If a lender, buyer or insurer cites a width, ask which document sets it and for whom. Codes and licensing rules differ by jurisdiction, so check with your building department.

Insurance and sale paperwork

Insurance coverage depends on policy wording, exclusions, endorsements, cause and state law, and a crack log is evidence, not a promise of coverage. See the insurance guide.

Crack width vs. other clues an engineer weighs

Clue	What it adds	Limit
Width at one reading	A snapshot at one date	Cannot show behavior ¹
Width over a full year	Seasonal or progressive pattern	Needs time ¹
Height step across the crack	Parts of the structure moved relative to each other	Needs a felt or measured offset
Crack pattern and location	Possible contributors; see diagonal and stair-step cracks	A clue, not a diagnosis
Doors, windows and floors	Alignment distortion across the structure ³	Humidity can also stick doors
Soil and water history	Moisture, drainage, leaks, trees	Needs site review

Common myths about crack width

Myth A crack under a certain width is always harmless.: Reality No universal width exists. Change, offset and other signs matter, and extension guidance gives no numeric line ³.
Myth A crack over a certain width needs underpinning.: Reality Repair is chosen by an engineer for a specific house from loads, soil and site, never from width alone.
Myth If it did not change this month, it is stable.: Reality A full climatic cycle is suggested to separate seasonal from progressive movement ¹.
Myth Patching a crack stops it moving.: Reality A patch hides it from your record, and it can reopen if the cause continues.
Myth A crack monitor makes a house safe.: Reality A gauge records movement. It does not stop or explain it.

Where season and soil change what you will see

Monitoring results depend on climate and soil. On clay with shrink-swell cycles, the trade article says separating reversible from progressive movement is often the central question ¹.

In dry northern areas, soil shrinking away from basement walls in drought can open cracks, and NDSU says to wait on repairs until moisture returns to normal ³. Frost depth, groundwater and local fill practices vary too.

Ask your building department or a local engineer what typical seasonal movement looks like in your area.

When to keep monitoring, and when to call

Keep monitoring when a crack is small, stable, and not tied to other signs. Call when it widens, steps, or comes with sticking doors, sloping floors or gaps. Call right away for a bowing wall, a horizontal crack in a basement wall or any safety doubt.

Bring your log to the visit. Try the inspection checklist to prepare, and the foundation inspection page to see what a visit covers.

Decision guide What to do with a monitored crack

If Stable through a full season, no other signs: Then Keep logging at a lower pace and keep water away
If Wider, longer or stepped from the last cycle: Then Call a licensed structural engineer with your log
If A bowing wall, sudden change or safety doubt: Then Do not wait; call an engineer now

Questions about Foundation Crack Width and Monitoring

How wide is too wide for a foundation crack?

There is no universal width that separates safe from unsafe, and this site does not give one. NDSU extension says crack size helps show whether a contractor is needed but names no number. Change over time, height steps and other signs weigh more than width alone.

How do I measure a foundation crack?

Pick a spot where the edges are straight, mark both sides with pencil ticks, and measure the gap at the same marks with the same tool each time.

Photograph it with a ruler and a date card. A graduated crack gauge gives a value you can compare, while a plaster tell-tale only shows that movement happened.

How long should I monitor a crack?

A trade article on movement monitoring suggests at least one full climatic cycle, so seasonal movement can be separated from steady drift. Read on a schedule, such as monthly and after big weather changes, and write the date each time.

What is a crack gauge or tell-tale?

A tell-tale is a simple gauge fixed across a crack. A plaster version only shows that movement happened. A graduated gauge, two plates read against a scale, gives a measured value that can be compared over time. Fixing one is a surface task, not repair.

Do hairline cracks matter?

Narrow cracks are common. FEMA training materials say most normal concrete develops hairline cracks from shrinkage and temperature change. A hairline crack alone cannot be called harmless, so mark it and watch. If it grows or the floor steps across it, call an engineer.

Should I fill a crack before I monitor it?

No. Filling or painting changes what you see and can hide movement. NDSU advises waiting to repair wall and ceiling cracks until soil moisture returns to normal. Let a licensed engineer decide when and how to repair.

Can a crack open and close with the seasons?

Yes. Concrete and soil respond to temperature and moisture. A trade article describes separating reversible movement from progressive drift that never returns to its starting point. Overlaying readings with weather notes helps. An engineer interprets the pattern.

When should I call a structural engineer instead of monitoring?

Call right away for significant, widening or changing movement, a bowing or leaning wall, a horizontal crack in a basement wall, separation at corners, sticking doors with new cracks, or any safety doubt. Monitoring is for stable-looking cracks, not for danger.

Does insurance care about my crack log?

A dated log is useful evidence of when you noticed a problem, but coverage depends on policy wording, exclusions, endorsements, cause and state law. A log does not promise a payout. Read your policy and see the insurance guide on this site.

Sources

Specification Online trade article, Monitoring structural movement: from visual observation to measured record (September 3, 2026; carries a gauge maker’s sponsorship line, used only for general monitoring practice), retrieved October 14, 2026.
FEMA National US&R Response System, Structural Collapse Technician, Module 1C Part 3: Hazard Identification and Building Monitoring, retrieved October 14, 2026.
NDSU Extension, Ken Hellevang, Dry Soil May Cause Building Problems (June 30, 2017), retrieved October 14, 2026.
FHWA, Petrographic Methods of Examining Hardened Concrete, Chapter 4 Cracks, FHWA-HRT-04-150 (July 2006), retrieved October 14, 2026.
Penn State, Singh and Scanlon, Concrete in Residential Construction, 1st Residential Building Design and Construction Conference (2013), retrieved October 14, 2026.
NRMCA, CIP 19 Curling of Concrete Slabs (2018 edition), retrieved October 14, 2026.
City of Seattle, 2021 Seattle Residential Code, Chapter 4 Foundations (2021 IRC text with Seattle amendments), retrieved October 14, 2026.

Expert review

Written by the HyreFoundation Research team. Not yet reviewed by an outside expert. We say so rather than imply a review that has not happened; see our editorial policy.

How the numbers were checked: Monitoring practice (repeated dated readings, tell-tale vs. graduated gauge, one full climatic cycle) comes from one trade article that carries a gauge maker’s sponsorship line and is used only for general practice.

Crack facts come from FEMA structural collapse training, FHWA’s petrographic manual and a Penn State paper. Alignment and no-threshold statements come from NDSU extension.

Code scope comes from Seattle’s 2021 residential code. No crack width threshold is given anywhere, by design, and the example log numbers are invented. No prices.

Suggest a correction. We fix errors and say what changed.