You found it with a flashlight, probably in the garage or down under the house, and you've been thinking about it ever since. Two questions brought you here. Is your house in trouble, and is the crack going to keep opening?
Most cracks in residential concrete are cracks the concrete made by itself while it cured, and they never do another thing. A smaller number are telling you the ground under one stretch of footing gave way. The two don't look alike, and you can sort yours in about ten minutes with a flashlight, a quarter and anything with a straight edge.
Any crack in a concrete foundation matters when it's wider than an eighth of an inch, when one face sits proud of the other rather than flush, or when it's still growing. A hairline crack that stays flush across both of its faces is usually just shrinkage from curing, and shrinkage stops on its own.
Not all the concrete under your house is holding it up
A house sits on two very different kinds of concrete, and only one of them is carrying any weight.
The floor slab you park on
Your garage floor and your basement floor were poured flat on the dirt after the walls were already standing. A floor slab carries you, your car and whatever else is sitting on it, and nothing more. The walls of the house don't rest on it. Crack a floor slab clean across and the brick above never feels a thing.
The footing and the wall that stands on it
Around the perimeter of the house, down in a trench, sits a strip of concrete called the footing, and the foundation wall stands on top of it. Every pound of your house travels down through that wall into that footing and into the dirt. Scott Anglin describes a common builder version this way: "the house is built on a separate foundation, 12 inches by 18, then they just build backfill, and then they pour a little footer over there." A crack in that concrete is a different kind of news than a crack in the floor.
If you can't tell which one you're looking at, the test is whether the concrete has anything standing on it. Concrete with wall or brick above it is structural. Concrete you walk on, with air above it, generally isn't.
Five things to check on the crack itself
None of these needs a tool you don't already own.
Which way the crack runs
Vertical cracks, and cracks leaning a few degrees off vertical, are the ordinary ones in poured concrete. Diagonal cracks that start at a corner of an opening and run down at an angle are following a load path and deserve a closer look. A crack running dead level across a below-grade wall is a different subject altogether, covered in why basement walls bow. Direction is the first thing the office asks about on the phone. In Scott's words: "basically there's a crack in my wall. And then we'll ask, is it diagonal? Then we automatically know."
How wide the crack is
Use a quarter. A US quarter is 1.75 millimeters thick, which is right about a sixteenth of an inch. If the crack won't take the edge of a quarter, it's hairline. If it takes two quarters stacked, you're at roughly an eighth of an inch, and an eighth is the width where most people in this trade stop calling a crack cosmetic. Measure at the widest point, not the average.
Whether the two sides are still flush
Lay a ruler, a level or a credit card flat across the crack and feel where it bridges. Faces that sit in the same plane mean the concrete pulled apart in place, which is exactly what shrinkage does. One side standing proud of the other, or one side sitting lower, means the two halves are no longer where they were poured. Offset is movement. Flush is shrinkage. This one check sorts more cracks correctly than width does.
Where the crack sits along the wall
Concrete cracks where its cross section is thinnest, so cracks starting at a crawl space vent, a basement window or the end of a garage opening are common and often mean nothing more than geometry. A slab crack that runs down a tooled groove is sitting in a joint the finisher cut on purpose, and the joint did its job. A crack out in the middle of a long unbroken run of foundation wall, with no opening anywhere near it, has fewer easy explanations.
Whether anything else in the house agrees with it
A footing crack that matters usually has company directly above it on the same elevation: mortar joints opening in the brick, a door on that side that quit latching, a floor that runs downhill toward it. Concrete that cracked while curing has nothing above it that agrees. For the rest of the house, signs of foundation problems walks through what to look at.
Why so much residential concrete cracks on its own
Concrete shrinks while it cures. It keeps giving up water for months after the finishers go home, and as it shrinks it pulls against the dirt, the rebar and itself until something lets go. Houses here are typically poured somewhere around 3,000 to 3,500 PSI concrete, which is plenty strong in compression and not especially strong in tension, and cracking in tension is how it relieves the stress.
Shrinkage cracks tend to show up in the first year or two, run fairly straight, stay flush across both faces, stay thin end to end, and then stop. A new house getting its first crack is usually the concrete settling its own account, not a defect.
When the footing was wrong on the day it was poured
The other quiet explanation has nothing to do with soil and nothing to do with you. Some footings never had a fair chance. Scott has watched it happen: "the footing, when it was poured, they actually, from the beginning, poured it too thick, too thin, and changed it after the inspector left, and it wasn't code compliant from the beginning."
You'll also read everywhere that Georgia red clay swells and shrinks and works foundations loose. USDA soil survey data for the map units under metro Atlanta rates this clay low for shrink-swell, so that explanation doesn't hold up here. What actually undermines a footing around Marietta is water, slope, fill and erosion, and what Georgia clay really does lays out the soil numbers.
How to find out whether the crack is still moving
Nobody can tell you that from a single look, including anyone who comes out to your house. Scott says it plainly: "we don't know if it's still moving or not, nobody knows that. The only way to know that is if you watch this crack get bigger and bigger the last few months or years."
You can watch it yourself for the price of a roll of masking tape. Stick a strip straight across the crack, press it down hard, and write the date on it in pen. Put a second strip a foot away along the same crack. Tape splits when the crack opens under it, and two strips tell you whether the whole crack is working or only one end of it. Check them after the next long wet stretch.
Most cracks watched this way never move again, and finding that out yourself beats guessing at it for another year.
The crack that belongs to a different trade
Sometimes the cracked concrete isn't part of the house at all. Concrete leveling, where a sunken driveway, patio or pool deck gets floated back up by pumping material underneath it, is a different trade, and Anglin's doesn't do it. What Anglin's does is stabilize and lift the foundation itself on steel piers driven down to load-bearing strata, and then repair the brick and mortar the movement opened up.
Plenty of these calls end with nobody selling anybody anything, which is how one homeowner's went:
"I called Anglin's because there's a crack on my concrete retaining wall and I was worried about the crack would impact anything structurally. Anglin's came out and took a look for free, and gave me their honest opinions. They seem like very honest people/business"
Amy Hou, 5 stars
What a free look actually settles
Someone who does this daily can stand at the crack, read its direction, width and offset, check whether the elevation above it agrees, and tell you which of the three it is: concrete that cured, concrete that was never right, or a footing that moved. Anglin's has been answering that question in metro Atlanta since 1930, and answering it costs you nothing.
If the answer turns out to be the third one, foundation repair is the page that covers the piers, the price and the warranty.
Frequently asked questions about cracks in concrete
How wide is too wide for a crack in poured concrete?
An eighth of an inch is the working threshold, roughly two US quarters stacked edge to edge. Below that, a crack that stays flush across its faces and hasn't grown is almost always shrinkage. Width alone never settles it though, because a narrow crack with an offset between its faces says more than a wide one that's perfectly level.
Should I worry about a crack in my garage floor slab?
Rarely. A garage floor was poured on the dirt after the walls went up and carries nothing but what's parked on it, so it can crack without saying anything about the house. The version worth a call is a slab crack with a lip you can catch a shoe on, or one where the floor has visibly dropped away from the wall it meets.
Do new houses get foundation cracks too?
Yes, and more often than old ones. Concrete keeps shrinking for months after it's poured, so the first hairline cracks in a new foundation usually appear in the first year or two, run fairly straight and then stop. A crack that first appears in year eight on a house that never had one is the more interesting case.
What does an offset crack mean?
Offset means the two faces of the crack no longer line up, so one side stands proud of the other or has dropped below it. Concrete that simply shrank stays in plane, because nothing pushed it. Offset says the two halves have physically changed position, which points at the ground under one of them rather than at the concrete.
Can a cracked concrete foundation be repaired, or does it have to be replaced?
Repaired, nearly always. Where the footing has dropped, steel piers are driven to load-bearing strata, the weight transfers onto them, and the foundation comes back toward where it started, which closes much of what opened. Replacing a foundation is a rare and far larger job, and it isn't the answer to an ordinary settled corner.
How long can I leave a foundation crack alone?
A crack that's flush, thin and not growing can be left alone indefinitely, and taping and dating it is a reasonable way to keep an eye on it. A crack with an offset, or one that's widened since you first noticed it, is worth having looked at soon rather than eventually, because a footing caught early usually needs fewer piers than the same footing two years later.
Have someone read the crack for you
You shouldn't have to decide this from a flashlight and a forum thread, and you shouldn't have to pay to find out. Someone can look at the crack, check the elevation above it, and tell you straight whether the concrete cured, the builder cut a corner, or the ground gave way.
Call 770-422-2924 for a free look.