You're carrying a box to the storage shelves, you glance at the basement wall, and there it is: a crack you're pretty sure wasn't there before. It probably was. Concrete cracks as it cures and keeps cracking a little as the house settles, and nearly every basement in London has a few. The useful skill isn't spotting cracks. It's telling the routine ones from the handful that are actually telling you something.
Do this tonight: mark it, measure it, walk away
Grab a pencil and a tape measure. Draw a short line across each end of the crack and write today's date beside it. Measure the width at the widest point, and take a photo with a coin held next to it for scale. That's the whole job. Come back in a month and check whether the crack has grown past your pencil marks or widened.
This one habit settles most foundation worries for free. A crack that hasn't moved in months of checking is old news, whatever it looks like. A crack that's visibly outrunning your marks is worth a professional look no matter how thin it is. When someone calls us about a crack, the first thing we ask is whether it's changing, and almost nobody knows. The pencil line is how you know.
Direction tells you more than size
A thin crack can matter and a wide one can be nothing, so start with the direction it runs.
Vertical cracks are the most common and the least worrying. Poured concrete shrinks as it cures, and it usually relieves that stress in a mostly vertical hairline, often starting from the corner of a basement window because openings concentrate stress. If it's roughly vertical, thinner than about 3 mm, and dry, it's on the cosmetic end of the scale.
Diagonal cracks running from a corner suggest one part of the footing has settled a bit more than the rest. Houses do this, especially in their first years and especially on clay. Worth marking and watching, not worth losing sleep over unless it keeps growing.
Stair-step cracks follow the mortar joints in block or brick foundations, climbing like a staircase. These track differential settlement and deserve more attention than a vertical hairline, because block walls handle uneven movement worse than poured concrete does.
Horizontal cracks running along the middle of a block wall are the ones we take seriously every time. That pattern usually means the soil outside is pushing the wall inward. London sits on clay-heavy soil that swells when it's soaked and shrinks when it's dry, so the pressure against a basement wall changes with the seasons, and a wall that's started to give shows it as a horizontal line, sometimes with a slight inward bulge. Put a straightedge or a level against the wall. If the wall isn't flat anymore, stop monitoring and get someone in.
The short list that shouldn't wait
A few signs move a crack from the watch list to the call list, calmly but promptly:
- It's horizontal, or the wall bulges inward when you check it with a level.
- It's wider than about 6 mm, roughly the width of a pencil.
- One side of the crack sits proud of the other, so your finger catches a ledge crossing it.
- It's actively leaking water, not just damp after a storm.
- It keeps growing past your pencil marks month after month.
- Doors or windows directly above it have started sticking or showing gaps at the frame.
One honest note on the scary-looking ones: a wide, ragged vertical crack that hasn't moved in years is often less of a problem than a tidy horizontal one that's just getting started. This is why the monitoring matters more than the first impression.
What fixing actually looks like, and where DIY stops
For a stable, non-moving crack in poured concrete that only lets water in, injection is the standard repair, done from inside. Hardware stores sell polyurethane injection kits, and on a clean, accessible, genuinely stable crack a careful homeowner can get a decent result. We're not going to pretend otherwise.
Where the kits fail is on cracks that are still moving, cracks that have been painted or patched before so the material can't bond, and anything in a block wall, where the crack you see is usually not the path the water takes. Sealing a moving crack accomplishes nothing except hiding your reference marks. And if the wall itself is deflecting, no injection fixes that, because the crack is the symptom and the soil pressure is the cause. That class of problem needs bracing or excavation work, which is foundation repair in the real sense, and it gets priced after someone has actually looked at the wall, not from a photo.
While you're down there, look at the outside too. The cheapest foundation repair we know is moving a downspout. A surprising share of the leaking cracks we see in London are directly below a downspout dumping roof water beside the wall, and extending it two metres does more than any sealant. If you're seeing other symptoms alongside the crack, damp patches, musty smell, a floor that's drifted out of level, our guide on spotting foundation damage from your basement walks through the full checklist.
Frequently asked questions
Are hairline cracks in a foundation normal?
Yes. Poured concrete shrinks as it cures and almost always produces a few thin vertical cracks, commonly at window corners. Mark them, measure them, and check back in a month. Stable and dry means cosmetic.
When should I actually worry about a foundation crack?
When it's horizontal, wider than about 6 mm, offset so one side sticks out past the other, leaking, or growing past dated reference marks. Any one of those is enough to justify a professional look.
Do foundation cracks mean the house is unsafe?
Almost never in the sense people fear. The large majority of cracks are shrinkage or minor settlement and pose no structural risk. The exceptions, mainly bulging or horizontal cracking in block walls, develop over time, which is exactly what monitoring catches.
Will a crack injection kit from the hardware store work?
On a clean, stable, non-moving crack in poured concrete, it can. On a moving crack, a previously patched one, or a block wall, it won't hold, and the money is better put toward finding out why the crack is moving.