Foundation Crack Size: Measure, Understand, Act
Found a crack in your floor or wall and wondering if it’s serious? The first thing to check is the crack’s size. A tiny hair‑line split usually isn’t a red flag, but a wider gap can signal a deeper problem. Below you’ll find a simple way to measure, what the numbers mean, and what to do next.
How to Measure a Crack Correctly
Grab a ruler or a cheap digital caliper – both work fine. Place the tool directly over the crack and note the widest point. Record the measurement in millimetres (mm) for accuracy. If the crack is longer than a few centimetres, measure a few spots and use the widest reading.
Don’t rely on visual guesses. Even a crack that looks narrow can be 0.3 mm wide, which is enough to let moisture in. Write down the size and take a photo for reference. You’ll need this info if you call a structural engineer later.
What Different Sizes Mean
0 – 0.3 mm (hair‑line): These are usually cosmetic. They happen as concrete cures or due to minor movement. Keep an eye on them, but most of the time they won’t grow.
0.3 – 0.6 mm: The crack is getting noticeable. It could be from settlement or slight soil movement. Check if the surrounding area feels damp. If moisture is present, seal the crack to stop water damage.
0.6 mm +: This is a warning sign. Cracks wider than 0.6 mm often indicate shifting foundations, poor soil compaction, or structural stress. You should get a professional assessment ASAP.
Size isn’t the only factor. Look at the pattern: a single straight line is less worrisome than a network of cracks spreading out. Horizontal cracks in load‑bearing walls are especially concerning.
Next Steps Based on Size
For hair‑line cracks, clean the area and apply a flexible crack filler. This keeps water out and stops the crack from widening.
If the crack is between 0.3 mm and 0.6 mm, monitor it monthly. Use a simple marker to see if the length or width changes. Seal it with a polyurethane sealant if you notice any moisture.
When you hit 0.6 mm or more, call a structural engineer. They’ll check foundations, soil conditions, and recommend repairs such as underpinning, resin injection, or steel reinforcement. Don’t try DIY fixes for large cracks – they can make the problem worse.
Knowing the exact size gives you a clear picture of the risk and helps you talk confidently with contractors. Keep a small notebook of all cracks you find, their measurements, and any changes. Over time you’ll see if a particular area is stable or worsening.
Bottom line: a quick measurement can save you time, money, and stress. Use a ruler, write down the number, and act accordingly. Your home’s foundation is the base of everything – treat it with the attention it deserves.
