Foundation Crack Size – Simple Ways to Measure and Decide What to Do

Ever notice a line in your concrete floor or walls and wonder if it’s just a hairline or a big problem? The size of a foundation crack tells you a lot about what’s happening beneath your home. A tiny crack might be harmless, while a wider one could mean the structure is moving. Below you’ll get quick steps to measure any crack and clear guidance on what the numbers mean for you.

How to Measure a Foundation Crack

First, grab a ruler or a tape measure. Put the end at one end of the crack and note the length. If the crack is deeper than it is long, use a depth gauge or a small screwdriver to feel how far it goes. Write down two numbers: length (in inches or centimeters) and width (in millimetres). For most homeowners, a width under 1 mm is called a hairline – it usually doesn’t need a pro right away.

Next, check the pattern. Is the crack straight, V‑shaped, or a series of small gaps? Straight, horizontal cracks often point to settling or soil movement, while diagonal cracks can show shear stress. Take a photo with a ruler next to the crack – this helps when you call a tradesperson.

Finally, track changes. Mark the ends with a pencil and revisit the crack every month. If it gets longer or wider, write down the new measurements. A crack that keeps growing is a red flag and should be inspected ASAP.

What Crack Sizes Mean for Your Home

Here’s a quick cheat‑sheet:

  • Hairline (≤1 mm width, any length): Usually just surface shrinkage. Keep an eye on it, but most homes live with these forever.
  • Small (1‑3 mm width, up to 6 inches long): Still often harmless, but watch for water infiltration. Seal with a concrete patch if you see moisture.
  • Medium (3‑6 mm width, 6‑12 inches long): Could signal minor settlement. A professional should assess whether a carbon‑fibre strap or epoxy injection is needed.
  • Large (>6 mm width, longer than 12 inches): Likely structural. Expect a thorough inspection and possibly a foundation underpinning or wall anchors.

Horizontal cracks larger than 1/4 inch (≈6 mm) are especially worrisome because they can indicate soil pressure pushing the walls inward. These often need a specialist’s solution – think pier installation or wall bracing.

Cost-wise, repairing a hairline crack might be under £100 – a simple epoxy fill. Medium cracks can run £300‑£800 depending on access and the method used. Large, structural cracks can push the bill into the thousands, especially if underpinning is required.

Bottom line: measure, record, and watch. If a crack stays tiny and steady, you’re probably fine. If it grows or is already wide, call a qualified foundation repair company right away.

Got a crack you’re not sure about? Jot down the size, snap a photo, and share the details with a local expert. They’ll know whether a quick fix or a bigger repair is needed, and you’ll avoid costly surprises later.