Foundation Crack Size: When Does It Become a Real Problem?

• Gavin Whitaker
• |
• June 6
• |
• foundation repair

Spotting a crack in your foundation sends a jolt of worry through any homeowner, right? But not every crack means your house is falling apart. Some are just the concrete settling in. The real question: How big does a foundation crack need to be before you should get concerned?

Here’s a quick rule of thumb: if a crack is less than 1/8 inch wide (about the width of a standard pencil tip), it’s usually pretty harmless. These tiny cracks often pop up within a year or two of building, thanks to the natural settling of your home. Just keep an eye on them—they likely won’t cause problems.

But once cracks get wider than 1/4 inch—picture the thickness of two stacked nickels—you're entering the danger zone. Cracks that wide can let in water, bring pests, and signal deeper foundation issues brewing below the surface. That’s when things can get expensive if ignored.

Cracks that are wide enough to fit a dime or bigger, or ones that grow quickly over a few months? Time to take action. Foundation problems don’t fix themselves and waiting only leads to bigger bills and headaches.

• Crack Sizes: What’s Normal, What’s Not
• Why Cracks Happen and When to React
• Signs a Crack Is More Than Just Cosmetic
• What to Do If You Spot a Serious Crack

Crack Sizes: What’s Normal, What’s Not

So what counts as a serious foundation crack? Let’s start by clearing up a common fear: not every crack is a sign your house is sinking. Small, thin cracks are often just the concrete’s way of dealing with temperature swings, minor settling, or shrinkage as it dries. Builders even expect most homes to develop a few hairlines early on.

Here’s a quick breakdown to keep things straight:

• Hairline cracks (less than 1/16 inch wide): These are super thin and usually harmless. Think of the width of a sheet of paper. Just check on them every couple of months to make sure they aren’t spreading.
• Small cracks (1/16 to 1/8 inch): Still pretty normal, especially within a year or two of construction. Keep them dry and watch for changes, but no need to panic.
• Moderate cracks (1/8 to 1/4 inch): These deserve attention. If they stop growing, it’s probably still safe as long as there’s no water coming in. If they get bigger, start to connect, or keep widening, call in a pro for peace of mind.
• Wide cracks (wider than 1/4 inch): This is when you start worrying. These cracks can let in water, let heat or cool air escape, or hint at a bigger structural issue underneath. Don’t ignore these—get them checked out soon.

Not sure how to measure? Stick a ruler or even a coin in the crack and see what fits. A dime is just under 1/16 inch thick, a nickel is about 1/16 inch, and a quarter is about 1/16 inch too. If you can fit a quarter, that’s not a tiny crack anymore.

Here’s a table to make it easy:

Keep in mind, it’s not just width. If a crack is getting longer, changing direction, or branching out, that’s another red flag. Even thin cracks can be bad news if water or bugs show up inside. Size is a starting point, but changes over time matter even more.

Why Cracks Happen and When to React

So, why do cracks show up in your foundation in the first place? Most of the time, it boils down to the ground beneath your house not staying put. If the soil shifts, gets soaked with water, or dries out and shrinks, it can move around enough to mess with your foundation.

• Foundation cracks often start showing up after heavy rain, flooding, or even a really dry season. Clay-rich soil is a big culprit—it swells up when wet and shrinks when dry, playing tug of war with your foundation.
• Tree roots searching for water sometimes wiggle their way under foundations, moving dirt and making things uneven.
• Poor drainage adds to the trouble. If water pools around your house, the extra pressure can force even healthy concrete to give in to stress.
• Temperature swings, like freeze-thaw cycles, also speed up crack growth. Water in the ground freezes, expands, and puts extra stress on your foundation.

Knowing what’s normal and what’s not makes all the difference. Hairline cracks—less than 1/16 inch—usually pop up in the first year and rarely get worse. Wider, longer, or crooked cracks (especially those wider than 1/4 inch) mean something bigger is going on and call for more attention. Here’s some real-world numbers for context:

If a crack is suddenly growing, you notice multiple cracks in one area, or you see doors and windows sticking, don’t wait. Homes move a little all the time, but fast or unpredictable changes are never a good sign. When in doubt, grab a ruler, snap a photo, and make a note of the date. Keeping track helps experts know what’s really happening if you call them in.

Signs a Crack Is More Than Just Cosmetic

Let’s face it—most homes get cracks somewhere, and sometimes they just look bad. But some cracks are troublemakers in disguise, and figuring out which ones cross the line isn’t just guesswork. There are clear warning signs you shouldn’t ignore.

• Width and Growth: Cracks larger than 1/4 inch or cracks that keep getting wider or longer over weeks or months usually mean your home’s foundation is moving, not just settling.
• Direction Matters: Vertical cracks that are narrow and stay that way are usually less serious. But cracks running horizontally or at sharp angles can mean a lot more stress. Horizontal cracks especially are known trouble—they can be from soil pushing hard against your foundation.
• Offset or Displacement: If you rub your hand over the crack and one side is higher than the other or there’s a gap you can stick something into, that’s a red flag. This is called “displacement,” and it usually means real movement and damage.
• Water Issues: Moisture, stains, or even a musty smell near cracks can mean water is seeping in. Water and concrete are a bad combo—the longer it leaks, the worse the damage gets.

Here’s a quick rundown of what to watch for:

Keep an eye out for patterns, too. More than one crack in different directions, or doors and windows that suddenly won’t close, are often connected to bigger foundation cracks. If you spot any of these, it’s best to get your home checked by a pro before things get ugly and expensive.

What to Do If You Spot a Serious Crack

Don’t just ignore a large crack and hope it goes away. First step? Grab a ruler or tape measure and check the width. If it’s bigger than 1/4 inch, or growing fast, it’s time to act. Cracks this size aren’t just ugly—water, bugs, and even cold air can sneak through. The most important thing here: resist the urge to patch it yourself unless you’re just sealing tiny, unmoving cracks. Big or expanding cracks usually mean there’s a real problem underneath.

Here’s a simple plan to follow if you find a serious foundation crack:

1. Document It: Take clear photos with a ruler for scale. Write down when you noticed it and if it’s gotten bigger.
2. Keep Watch: Check it every few weeks. If it’s growing, that’s a big red flag.
3. Call the Pros: Get a foundation repair specialist—not just a general contractor. These folks know how to diagnose if you have just a cosmetic issue or something major, like settlement or soil movement beneath your house.
4. Ask the Right Questions: When the repair expert arrives, don’t be shy. Ask them for a full breakdown—what’s causing it, what repairs they recommend, and what happens if you wait.

An engineer or foundation specialist may suggest solutions ranging from epoxy injections for stationary cracks to underpinning with piers if your house is shifting. According to the National Association of Home Builders, “Cracks wider than 1/4 inch typically require evaluation by a structural engineer, especially if accompanied by other signs such as sticking doors or sloping floors.”

"A small, stable foundation crack in an older home that hasn’t changed for years is usually not a concern, but new or expanding ones should always be checked out by a professional." — American Society of Home Inspectors

Costs can vary a lot. Filling small cracks might run $300‒$700, while major structural repairs can cost $4,000‒$10,000 or more, depending on what’s going on underneath your home. Here’s a quick comparison:

Bottom line—if you see a crack that makes you pause, it’s always safer (and cheaper, in the long run) to get it checked. Your house isn’t going to heal itself, and foundation professionals have the gear to see what’s really going on.