Crack Width Chart: How to Read and Use It
If you’ve spotted a line in your foundation, the first thing to ask is how big it is. A crack width chart gives you a visual scale, so you can tell whether the crack is a harmless hairline or a sign of serious trouble. It’s like a ruler built into a picture – just line up the crack and see where it lands.
Why does size matter? Small cracks (under 0.1 mm) usually come from normal shrinkage as concrete cures. They don’t move much and rarely need a repair. Once you get to 0.3 mm, the crack can start letting moisture in, which might lead to damp or steel corrosion. Anything over 1 mm often means the structure is shifting and you should get a professional opinion.
Why Crack Width Matters
In the UK, building regs use crack width to decide if a defect is structural. A narrow, static crack often passes inspection, but a wide or growing crack can fail a survey. Homeowners who ignore the signs may end up with costly repairs later on.
Besides safety, the crack width influences insurance claims. Many policies only cover damage if the crack exceeds a certain width, typically 0.5 mm. Knowing the exact measurement helps you talk to insurers with confidence.
Steps to Measure and Interpret
1. Grab a tool. A fine steel ruler, a digital caliper, or even a crack gauge works. For hairline cracks, a magnifying glass with a built‑in scale is handy.
2. Clean the area. Dust and debris can make a crack look wider. Gently brush away loose particles with a soft brush.
3. Place the tool. Align the zero end of the ruler at one edge of the crack and read the opposite edge. If you’re using a caliper, close the jaws until they touch each side of the crack.
4. Match to the chart. Look at the crack width chart on your phone or printed sheet. Find the band that matches your measurement – hairline, minor, moderate, or severe.
5. Check for movement. Mark the ends of the crack with a pencil and note the date. Re‑measure after a week or after heavy rain. If it’s growing, even a small crack can become a big problem.
6. Decide what to do. For hairline cracks, you can seal them with a flexible epoxy. Minor cracks may need a waterproofing membrane. Moderate and severe cracks usually require a structural engineer to design a repair, such as underpinning or carbon fibre stitching.
Remember, the chart is a guide, not a legal verdict. If you’re ever unsure, call a qualified surveyor. A quick inspection can save you months of worry and thousands of pounds.
Finally, keep a photo log of any cracks you measure. A visual record helps you spot trends and provides solid evidence if you need to claim insurance or negotiate a sale.
Using a crack width chart puts the power of assessment in your hands. It’s simple, cheap, and gives you the confidence to know when to watch and when to act.
