You went down for a box of something and noticed one of your basement walls is bowing, a slow curve in the block, or a crack running sideways through the mortar that you're fairly sure wasn't there before. Now you want two answers: what's doing that, and how worried you should be tonight.
Start here. Nothing inside your house is pulling that wall in. Something outside is pushing it, and in nearly every case the thing pushing is soil that water made heavy.
A basement wall bows because the soil outside is pushing sideways on it. Water runs down behind the wall faster than any drain can clear it, the water table comes up, and the soaked soil gets heavy enough to shove block. Block walls bow because of how they were built. Poured walls rarely bow at all.
First question: is your wall block or poured concrete?
Before anything else, figure out which kind of wall you have, because the two fail in completely different ways and the answer changes what you're looking at.
Put your hand on it. A block wall is built from concrete masonry units, so you can see rectangles and mortar joints in a grid. A poured wall is one continuous face, often with faint horizontal lines where the forms met and small round plugs where the form ties came through.
That distinction decides almost everything. In the owner's words, after describing the two wall types: "Those [poured] walls don't... I haven't seen one bowed. I've never seen one bow. Block wall usually is the one bowing."
If it's block, bowing is the expected failure
A block wall is a stack of hollow units held together with mortar. It's strong straight down and weak sideways, so when the soil leans on it, it gives at a mortar joint somewhere around the middle of its height. That's the belly you're looking at.
If it's poured, look for settlement instead
A poured wall is one piece, and one piece doesn't take on a curve. It cracks and drops. As the owner describes it, a poured wall splits "because underneath it wasn't able to hold up, so it split the whole wall." That's a footing problem rather than a soil pressure problem, and it points at the signs of foundation problems conversation instead of this one.
Where the push comes from, the way an engineer explained it
This is the part almost nobody explains properly, and it's worth reading slowly. The account below is the one the owner got from an engineer and has been repeating to homeowners ever since.
Water goes down the back of the wall faster than it can leave
Rain doesn't fall on your basement wall. It falls on the yard above it and runs down the backfill, which is looser than the undisturbed ground around it. The owner: "It is so much water, we're talking 2 or 3 days period, as it goes down behind that wall, that drain isn't capable enough to get that amount of water out. No drain is, just about."
Read that again, because it's the key. This isn't a story about a failed drain. Give it two or three days of steady rain and the volume beats the drain whether the drain works or not.
The water table comes up, and the soil gets heavy
Once the water can't get out through the wall or through the drain, it stacks up. In his words: "It starts building up. The water table raises. That water builds up, that soil gets so dense and heavy, you get that lateral push."
Dense, saturated soil is far heavier than the same soil dry, and it behaves less like dirt and more like a liquid leaning on whatever is holding it. That sideways load is what the trade calls hydrostatic pressure, and it's the single reason block basement walls bow.
It comes and goes, which is why the wall moves in steps
The owner again: "Your water table, when it goes down and it goes up, your water actually goes up and it kicks the wall." The load isn't constant. It arrives with the wet season and backs off in the dry one, and each cycle takes a little more out of the mortar joints. That's why homeowners so often say the crack seemed to appear overnight after years of nothing.
What it isn't
You'll read everywhere that Georgia red clay swells up and shoves basement walls in. The USDA rates the soil under most of metro Atlanta as low for shrinking and swelling, so that explanation doesn't hold here. It's dense ground, not ground that expands. Water, slope, fill and where the water ends up are the honest story, and they explain what you're seeing better anyway.
What was supposed to be inside that wall
Here's the part that usually lands as a relief: a bowing block wall is generally a construction story, not a maintenance story. You didn't cause it, and there was nothing you could have done differently.
A block basement wall holding back earth is supposed to be built with three things working together, and the owner lists them plainly: "Anything over 4 or 5 feet, you gotta have that width to retain that pressure. If you don't have it 2 foot on center with your rebar, you don't have that strength. If you don't have that DuraWire every other course, it's like a little ladder inside your block."
Block width
Block comes in 4, 6, 8 and 12 inch widths. A taller wall retaining more soil needs a wider block, and plenty of basement walls were built with less than the height called for.
Vertical rebar in filled cells
Steel bar set down through the hollow cells at intervals, with those cells filled with concrete. That's what gives the stack something to resist bending.
Horizontal reinforcement in the mortar
The wire reinforcement laid into the mortar joints as the wall goes up, tying the block together sideways. The owner's description of it is the clearest one you'll hear: "It's like a little ladder inside your block."
Miss any of those and the wall stands fine for decades carrying the house straight down, then gives the first time the soil behind it gets heavy enough. "So it wasn't built right," as he puts it. That's usually the whole story.
Reading the crack
When homeowners call, they describe the crack before anything else. The owner's list of what he hears: "I've got a crack, a horizontal crack, I have a stair-step crack, I have a lateral crack, I have a vertical crack."
The shape is worth paying attention to, because it tells you which direction the wall moved.
A crack running sideways through a mortar joint
This is the signature of pressure from outside. It usually shows up around the middle of the wall's height, because that's the point furthest from the floor slab below and the joists above, and it often runs a long way along the wall. If you also see the wall curving inward above that line, the two are the same event.
Cracks climbing diagonally in steps
A crack that steps up through the mortar following the block pattern generally means one part of the wall dropped relative to another. It's a settlement pattern rather than a pressure pattern, so the question becomes what happened under the footing.
A straight up-and-down split
A vertical crack running through block or through a poured wall is the least specific of the three. It can be shrinkage while the wall cured, and those hold still forever. It can also be a wall pulling apart at a weak point. Marking it and watching it is how you tell.
Cracks in more than one direction at once
When a wall has a sideways crack in the middle and steps at the corners, more than one thing is happening, and that's the case where a look from somebody who does this every day is genuinely worth having.
A bowing wall often isn't a wet wall
A lot of people assume that if water is the cause, they'd have water on the floor. Frequently they don't, and the reason is a good one.
The owner explains it this way: "A lot of basement walls don't even leak. Believe it or not, they do. They compress on the back side, and what you see is the cracks on the inside."
As the wall bends inward, the outside face of the block goes into compression. The crack you're looking at on the inside is wide, but on the soil side the same joint is being squeezed shut, which keeps the water out. So a dry basement floor tells you nothing about whether the wall is under pressure. Don't let it talk you out of looking.
If water is coming in, that's worth knowing, and it's the first thing anyone will ask you. It also points you to a second hire. Keeping water out of a basement is its own trade with its own contractors, and it isn't the trade Anglin's is in. Anglin's is a masonry and structural company. Holding a wall is their work; water control is somebody else's, and the two are handled as separate jobs by separate people.
If a home inspection is what started this
A large share of these calls begin the same way. The owner, describing a typical one: "My house is for sale, and the inspector noticed that I have a crack in my block wall, or I have a crack in my basement wall, and it's bowing, and it's cracking."
If that's you, one distinction will save you some anxiety. A home inspector's job is to see and report, not to diagnose. As the owner puts it: "An inspector looks at everything on your house and just gives recommendations, and then defer back to a foundation specialist, so they are off the hook." A line in an inspection report is a flag, not a verdict, and it usually says nothing about how far the wall has moved or whether it's still moving.
What closes a sale is a straight answer in writing, from someone who has stood in front of the wall. Sometimes that answer is that the wall is within normal tolerance. Either way, you and the buyer stop negotiating against a question mark.
What a good outcome feels like
The whole point of getting a real look is walking away knowing what you're dealing with. One customer, on that specifically:
"Anglin's was so responsive and knowledgeable about the best way to fix the problem in our basement!! It was a big relief to know we had somebody honest and competent working with us!!!"
Ashley U F, 5 stars
How to measure the bow yourself
Four things, and none of them cost anything.
- Measure the bow. Run a string line tight from one top corner of the wall to the other, a few inches off the block, then measure the gap between the string and the wall at the widest point. Write down the number and the date. That single measurement tells you more than any photo.
- Mark the ends of the crack with a pencil and write the date on the block. In a month you'll know whether it's still opening or holding where it is.
- Note whether any water shows up on the floor or the wall after a hard rain, and where. That answer changes who you need.
- Walk the ground outside that wall. Look for grade falling back toward the house, a downspout dumping at that corner, or a low spot that holds water for days. That's the side of the wall where the story starts.
Then give it a month and measure the string line again. If the gap hasn't changed, the wall is holding, and you've learned something useful for free.
Frequently asked questions about bowing basement walls
Is a horizontal crack in a block basement wall serious?
It's the one crack shape that should always get looked at, because it's the signature of soil pressing on the wall from outside rather than ordinary curing or shrinkage. That doesn't make it an emergency this week. It does mean the wall is carrying a load it wasn't built for, and that load comes back every wet season.
Do poured concrete basement walls bow?
Almost never, in this company's field experience across metro Atlanta. A poured wall is one continuous piece, so instead of taking on a curve it cracks and settles when the ground under the footing gives way. If your wall is poured and it's cracked, you're looking at a footing question rather than a soil pressure question.
Why is my wall bowing when the basement has always been dry?
Because a bowing wall closes itself up on the outside. The soil side of the block goes into compression as the wall bends inward, which squeezes those joints shut even as they open on the face you can see. A dry floor is common with a bowed wall and is not evidence that nothing is pushing on it.
How can I tell whether the wall is still moving?
Measure it and wait. Pull a string line across the wall a few inches off the block, measure the biggest gap, write it down with the date, then mark both ends of the crack in pencil. Check both again in a month or two. A number that grows means the wall is active; a number that holds means you have time to think.
Is a bowing basement wall about to collapse?
Very unlikely to be tonight's problem. These walls move slowly, over wet seasons and years, which is why the crack seems to appear suddenly when really it's been coming for a while. What waiting costs you is options, because a wall that keeps moving eventually passes the point where it can be held and has to come down instead.
Does a bowing basement wall mean my foundation is failing?
Usually not. Bowing is a retaining problem in that one wall, caused by soil pressure and missing reinforcement, while foundation settlement is the ground under the footings giving way. They're separate problems with separate causes. A bowed basement wall on a house with square doors and level floors is generally telling you about the wall alone.
Find out what's pushing on your wall
You shouldn't have to guess at this from a flashlight and a crack. Someone can stand at the wall, measure the bow, look at which way the cracks run and what the ground outside is doing, and tell you plainly what moved and whether it's still moving.
Anglin's has been doing masonry and structural work in metro Atlanta since 1930. If the wall needs steel, basement wall repair covers the method, the warranty and what it costs.
Call 770-422-2924 for a free look.