Concrete Cracks: What They Mean and How to Deal With Them
If you’ve noticed a line creeping through your garage floor or a gap in a garden wall, you’re probably looking at a concrete crack. It’s easy to ignore, but those little fissures can signal bigger problems underneath. This guide breaks down why cracks appear, how you can quickly judge their seriousness, and what steps to take to fix or prevent them.
Why Concrete Cracks Appear
Most cracks come from three main sources: movement, shrinkage, and external pressure. When the soil beneath a slab shifts – because of seasonal moisture changes, tree roots, or nearby construction – the concrete bends and splits. Shrinkage happens as fresh concrete dries, especially if the mix was too wet or the curing process was rushed. Finally, heavy loads like a new hot tub or a piling of stored items can push enough force to split the surface.
In the UK, frost action is a big player. Water that seeps into tiny pores freezes, expands, and forces the concrete apart. Poor drainage or a high water table can make this a regular headache, especially in older homes with no damp‑proof course.
How to Size Up a Crack
First, grab a piece of paper and a ruler. Measure the width at its widest point – you’ll be surprised how fast you can spot a problem. A hair‑thin line (under 0.3 mm) is usually harmless shrinkage. Anything over 0.5 mm and you should start worrying, especially if it’s growing.
Next, look at the pattern. A single, straight crack that runs across the whole slab often points to settlement issues. A network of small, random cracks usually means the concrete dried too quickly. Vertical cracks that start at the edge and move inward can be a sign of pressure from expanding soil.
Don’t forget to check for movement. Place a piece of chalk on the crack, wait a day, then see if the mark has shifted. Any movement means the underlying soil is still active, and fixing the surface alone won’t stop new cracks from forming.
When in doubt, call a structural engineer. They’ll use a crack width chart and may recommend a load‑bearing test to see if the slab still meets safety standards. It’s better to spend a few hundred pounds on a professional opinion than to risk a costly foundation failure later.
DIY Fixes and When to Call the Pros
For hair‑thin, non‑moving cracks, a simple epoxy or polymer filler does the trick. Clean the crack, apply the filler with a putty knife, and smooth it out. The result looks like new concrete and stops moisture from seeping in.
If the crack is wider than 3 mm or shows signs of movement, you’ll need a more robust repair. This usually involves removing loose concrete, cleaning the gap, and injecting a hydraulic cement that expands as it sets. Hydraulic cement is a favourite in the UK because it bonds well even with damp surfaces.
But remember: if the crack is a symptom of shifting soil, a surface repair won’t last. You may need to improve drainage, install a French drain, or even underpin the foundation. Those jobs are best left to qualified contractors who know local ground conditions and building regulations.
Finally, keep an eye on the repaired area. Re‑cracking within a few months signals that the underlying issue wasn’t solved, and you’ll need to revisit the foundation strategy.
Concrete cracks don’t have to be a nightmare. By spotting them early, measuring correctly, and choosing the right repair method, you can keep your home safe and avoid expensive surprises down the line.
