Foundation Crack Size: How Big Is Too Big?

Not every crack in your foundation means disaster, but some are a big red flag. Think of it like this: almost every house will have a few tiny cracks somewhere, especially after a couple seasons of rain, heat, or a little shifting ground. Those hairline cracks? Usually nothing to panic about. It’s when they get bigger, wider than a pencil tip, or start to run jagged through multiple bricks that you need to pay closer attention.

If you notice a crack that’s more than a quarter of an inch wide—the thickness of a standard pencil—it’s time to be cautious. Cracks that keep growing or show up suddenly after a storm or flood are the ones that shouldn't be ignored. Those are signs your foundation might be shifting for real, and the problem could get a lot worse fast.

The right move starts with grabbing a tape measure or even a coin for a quick check. Don’t guess—width matters way more than just the length or shape. Track any changes over a few weeks. If things aren’t getting worse, you might just need to keep an eye on it. But if your crack is growing or letting in water, that’s your cue to dig deeper or call in the experts.

• Types of Foundation Cracks
• When a Crack Size Becomes Serious
• How to Measure Crack Width
• What Causes Foundation Cracks to Worsen
• DIY Monitoring and Quick Fixes
• When to Call a Professional

Types of Foundation Cracks

Not all cracks are the same, and figuring out which kind you’re seeing makes a big difference. Here’s a quick breakdown so you don’t confuse a harmless line for a sign of real trouble.

• Vertical cracks: These usually run up and down or just a little off vertical. They’re the most common and tend to show up as concrete shrinks while it cures. If you spot a vertical crack under a quarter inch wide and it isn’t growing, it’s rarely an emergency.
• Diagonal cracks: These swing at an angle—often from window corners or along basement walls. Diagonal cracks can mean uneven settling. If they’re wider at one end, pay closer attention. It could mean your home is dropping on one side.
• Horizontal cracks: Now we’re in serious territory. Horizontal cracks along basement walls or blocks can mean major pressure from the outside—like swelling soil or water. The wider these get, the more urgent the problem. The U.S. Department of Housing and Urban Development puts it simply:

  "Horizontal foundation cracks are typically more serious than vertical cracks, especially when found in block or brick foundations."

• Stair-step cracks: These look exactly like they sound—following the mortar joints in a zig-zag pattern across block or brick. Sometimes they just mean a little settling, but if they’re getting bigger or letting water in, don’t wait to get help.
• Hairline cracks: Super thin, barely there, these are often just from the concrete curing process. Usually, nothing to worry about unless they start to widen or show up with other problems.

If you’re not sure what you’re looking at, try to match your crack to one of these types first. And don’t just go by the shape—how wide it is matters most for foundation repair decisions later on.

When a Crack Size Becomes Serious

It’s easy to spot a little hairline crack and shrug it off, but knowing when a crack is actually trouble can save you big money. The main thing to look at is width. If it’s under 1/8 inch (about the thickness of a nickel), you’re usually fine. Between 1/8 inch and 1/4 inch, keep an eye out—these cracks can start small and slowly grow. Once it hits 1/4 inch or bigger, you’ve crossed into "serious" territory. That’s when most pros recommend getting a foundation specialist to check things out.

Here’s a quick reference:

But width isn’t the only thing that matters. If you spot horizontal cracks, or ones that step through bricks or blocks, that’s a bigger deal than a thin vertical crack. Cracks that widen at the top or bottom, or those that leak during heavy rain, can point to more serious shifting. If doors and windows nearby start sticking, that’s another warning sign your foundation is moving.

• Horizontal cracks: These can mean the outside pressure is pushing your wall inward—never ignore them.
• Cracks wider than a quarter inch: Almost always need a pro’s look.
• Cracks with movement or displacement: If one side is higher than the other, things are shifting for sure.

Bottom line: Don’t just guess or hope. Measure, watch for changes, and know that bigger or changing cracks usually mean bigger problems hiding underneath.

How to Measure Crack Width

Measuring a foundation crack isn’t tricky, but doing it right can help you avoid costly mistakes. Don’t just eyeball it—tools matter. You want to know if your crack is a normal shrinkage line or something you can’t ignore. Grab a flashlight, a standard ruler or tape measure, and something to take notes with. A feeler gauge is even better, but most folks just use a dime or a credit card as a basic comparison.

• First step: Clean the area. Dirt or loose concrete can make cracks look smaller or larger than they really are.
• Next, place your ruler or tape measure right over the widest part of the crack. Don’t just measure at the edges—go for the thickest spot.
• Check the width. If it’s less than 1/16 inch (the thickness of a dime), you’re usually in the safe zone. If it’s hitting or going past 1/4 inch (about the width of a pencil), take it seriously.
• Jot down the measurement and mark the ends of the crack with a pencil or a piece of tape. This helps you track if it gets bigger over time.
• Snap a photo for your records. Visual proof beats memory every time, especially if you need to show a pro later.

If you want to keep it simple, here’s a quick table comparing common household objects to crack sizes:

Keep in mind: the foundation crack width is what matters most. Even a short, wide crack usually signals more trouble than a long, thin one. If you see cracks getting wider every month, don’t just patch them up—find out why it’s happening before things spiral.

What Causes Foundation Cracks to Worsen

The thing most people don’t realize is, foundation crack problems usually get worse because of what’s happening outside the house, not inside. Water is the biggest player here. If your gutters are clogged and water pools around the base of your house, you’re actually helping create bigger cracks. Wet soil expands and pushes on the concrete; then it dries out and shrinks, pulling things apart even more. This back-and-forth movement forces cracks to spread or multiply.

Trees are another sneaky culprit. Roots searching for water can shift soil under your foundation, making things uneven and stressing the concrete until cracks just have to happen. If you’ve got big old trees close to your home, their roots might be doing some damage underground that you can’t see until those cracks start to grow.

Soil type matters, too. Clay-heavy soils swell up a lot when wet and shrink a ton when dry. Even a dry spell followed by a rainstorm can make your foundation move enough to widen or lengthen cracks. This is why some homeowners notice cracks get worse after a big weather change.

Don’t forget about poor construction. If the original foundation wasn’t poured deep enough or didn’t use the right concrete mix, it’s more likely to crack under regular stress. And if there’s seismic movement—like a small earthquake or even blasting at a nearby construction site—that’s another way a small crack can turn into a serious problem.

• Water pooling near the house is the most common cause of worsening cracks.
• Tree roots can shift soil and force foundations to bend or crack further.
• Clay soil expands and contracts a lot with moisture changes, making cracks worse.
• Cheap or rushed foundation jobs are usually more vulnerable over time.
• Vibrations from road traffic or construction can open up existing cracks.

Knowing what’s working against your foundation lets you spot problems earlier and, sometimes, prevent those small cracks from turning into a huge headache.

DIY Monitoring and Quick Fixes

Spotting a foundation crack is just step one. The next move is keeping tabs on it without overcomplicating things or spending a ton. Here’s the simple way to stay on top of changes before they become expensive problems.

First, snap a photo of the crack with your phone. Toss a coin or a ruler next to the crack in the shot so you can compare later. Every month or so, take another picture in the same spot. If it’s getting wider or longer, you’ll see it clearly in the side-by-side shots.

• Mark the ends of the crack lightly with a pencil. This helps you spot any growth right away.
• Measure the width with a measuring tape or slide a credit card in — if it fits, the crack is bigger than 1/16 inch.
• If the crack is straight and thin, it’s probably normal settling. If it’s jagged or wider at one end, monitor it extra closely.
• Keep an eye out for moisture. If you see water or dampness, that means the crack is going all the way through.

For quick, easy fixes on small cracks (less than 1/8 inch wide), you can grab some concrete caulk or epoxy from the hardware store. Here’s how:

1. Clean out loose debris and dust from the crack with a stiff brush or vacuum.
2. Squeeze in concrete caulk or epoxy sealant. Fill the crack slowly, making sure it seeps in.
3. Smooth it over with a putty knife and wipe any excess away.
4. Let it dry as recommended on the product you use—usually a day is enough.

Don’t ignore even these small fixes. Filling them blocks out water and bugs, slows crack growth, and helps you spot any real changes down the road. But for cracks bigger than a pencil eraser, growing fast, or letting in water, it’s smart to get a pro’s opinion before rolling up your sleeves.

When to Call a Professional

Not all foundation cracks mean you need to call in the cavalry, but some do need expert eyes. If you spot a crack wider than 1/4 inch, anything that keeps getting longer, or one that’s letting in water even during a small rain, it’s time to get a pro out for a look. The same goes if you see a crack going across several bricks or blocks, or if walls on the inside are starting to bow inward. A strong musty smell, doors that stick, or windows that suddenly won’t shut properly—all these can connect back to serious foundation problems.

Don’t just take a wild guess. Professionals have tools like crack monitors and laser levels that can show if your home is shifting while you sleep. They’ll check for things you might miss—like hidden moisture, the soil around your foundation, and even what’s happening in your crawlspace. And they can tell the difference between a normal shrinkage crack and something much riskier that might cost thousands if ignored.

Here's a cheat-sheet for when to make that call:

• Cracks wider than 1/4 inch
• Cracks that keep getting longer, wider, or change shape
• Water coming through any crack
• Interior doors or windows won’t close right or stick suddenly
• Cracks that run diagonally, like a staircase, or go across several bricks
• Bulging walls or musty smells in basements or crawlspaces

Contractors have seen it all, and they can usually do an inspection in about an hour, sometimes free or for less than $200. If you catch it early, small fixes might cost a few hundred bucks and a Saturday. Wait too long, and you could be looking at five figures or more for major repairs.

Ignoring the problem doesn’t make it go away—in fact, foundation issues are one of the few home headaches that always get worse. If you recognize any of these red flags, don’t wait. Peace of mind and a solid house are worth the call.