Leaking Concrete Crack? Which Cracks Need Repair and How It’s Done

Concrete cracks. It’s one of the few things in construction you can count on. Most of those cracks are harmless: a fine line in a basement wall or a parkade slab that never changes and never lets in a drop. They’re not worth losing sleep over.

The ones that leak are a different story. A leaking concrete crack lets water into your basement, parkade or pit, and over time that water can damage finishes, belongings and the concrete itself. This post covers the common types of concrete cracks, how to tell which need repair (and which need an engineer), why cracks leak, and how a leaking crack is actually fixed. For the service side, see our concrete crack repair and waterproofing page.

Types of concrete cracks

Not all cracks mean the same thing. Here’s a quick guide to what you’re likely looking at.

Shrinkage and hairline cracks

Concrete loses water and shrinks as it cures, and that leaves fine, hairline cracks in most walls and slabs. They’re usually thin, often vertical or random, and they don’t get worse. On their own they’re cosmetic. They only become a problem when they’re on a wall or slab with water behind it, which is more common than you might think in a basement or parkade.

Settlement cracks

As a building settles into the ground, the concrete takes some stress and can crack. Settlement cracks are often a bit wider than hairline cracks and may run diagonally from a corner, window or opening. Most stabilise once the settling slows, but they’re a common leak path because they run right through the wall.

Structural cracks

These are the ones to take seriously. A structural crack is wide, one side is offset from the other (displaced), or it keeps getting longer or wider. Horizontal cracks in a foundation wall and walls that bow inward also fall in this group. These need an engineer before anyone talks about sealing them.

Cold joints

A cold joint isn’t technically a crack, but it acts like one. It forms where one pour of concrete meets another that had already started to set, so the two never fully bond. Wall-to-floor joints, and the seams between pours in parkades and elevator pits, are classic examples. They’re one of the most common leak points we see.

Tie holes

When walls are poured, metal ties hold the formwork together. They leave small holes through the wall, usually in a regular grid. If they weren’t sealed properly, each one is a little channel for water. They look minor, but in a wet wall they leak just like a crack.

When a crack needs repair, and when it needs an engineer

Here’s a simple way to sort it out.

A crack needs repair when it shows:

• Water. Drips, a wet edge or a line of water after rain.
• Staining. White, crusty efflorescence (minerals left behind by water moving through the concrete), or dark tide marks.
• Rust. Orange or brown streaks that mean water is reaching the steel inside.

A crack needs an engineer first when it shows:

• Widening. It’s getting wider or longer over weeks or months. Mark the ends with a pencil and date them to track it.
• Displacement. One side sits out from the other.
• Signs of structural stress. Horizontal cracking, bowing, doors or windows that stick, or concrete breaking away and exposing rusted steel.

We’re leak and crack repair specialists, not structural engineers. If we see signs of structural movement, we’ll recommend you have an engineer look before we seal anything. Once the structural question is answered, we can deal with the water.

Why cracks leak

A crack only leaks if three things line up.

Water pressure. There has to be water on the other side, pushing. In the Lower Mainland and Fraser Valley, our long wet season and high groundwater in low-lying areas mean many below-grade walls and slabs have water against them for months at a time. In places like Richmond, built on the low, flat Fraser River delta, the groundwater sits close to the surface. After heavy rain it rises further, and the pressure goes up. Water will always travel along the path of least resistance.

A path through the concrete. The crack, joint or tie hole has to run far enough through the concrete to connect the wet side to the dry side. Many hairline cracks don’t. Settlement cracks, cold joints and tie holes usually do.

Corrosion making it worse. Once water reaches the reinforcing steel, the steel rusts. Rust takes up more space than steel, so it pushes the concrete apart, widening the crack and letting in more water. That’s why a small leak left alone tends to grow.

How a leaking concrete crack is repaired

A leaking crack has to be treated through the concrete, not covered on the surface. Here’s the general process.

Routing and packing with crystalline materials

The crack is cut out along its length into a clean channel, called a chase. This removes loose material and gives the repair a solid surface to bond to. If water is running, it’s stopped with a rapid-setting hydraulic cement such as Krystol Plug, which sets in minutes and is designed to stop flowing water.

The chase is then packed with crystalline repair material, and crystalline treatment is applied to the concrete around it. Crystalline materials react with moisture to grow crystals in the pores and fine cracks, so the concrete itself resists water.

Injection where needed

Some cracks need crack injection: a material is pushed into the crack under pressure to fill it through the full depth of the wall. It’s typically used where a crack is wide, deep or still moving slightly. We use compatible injection where a crack needs it, and we’ll explain on site why a particular crack does or doesn’t.

Why surface sealers and caulk fail on active leaks

Caulk, paint-on sealers and patching compounds sit on the surface. Water pushing from behind gets under them and pops them off, or simply goes around them and comes out a few centimetres away. They can look fine in a dry spell and fail in the first big storm. Good concrete crack sealing on a leaking crack works inside the concrete, against the pressure, not on top of it.

Sealing a crack while water is running

Here’s something that surprises people: we’d rather seal a crack while water is pouring through it. In fact, we prefer it!

There are good reasons. A running leak shows us exactly where the water path is, along its full length, including any branches. A dry stain only tells us where water was at some point. With an active leak, we can see the repair work as we do it: when the water stops, we know we’ve got the path.

Crystalline materials also need moisture to do their job. A wet crack is the right environment for them, not a problem to wait out. So if your crack only leaks in the rain, the wet season is a good time to call us.

Walls, slabs, parkades, pits: where we see leaking cracks

Leaking cracks turn up anywhere concrete holds back water. The most common places we work:

• Basement and foundation walls. Shrinkage and settlement cracks, tie holes and the floor-wall joint. See our foundation leak repair page.
• Floor slabs. Cracks in basement and lower-level slabs, especially where groundwater pushes up from below.
• Parkades. Cracks in suspended slabs and walls, cold joints and penetrations. Our guide to parkade leaks covers causes and how to plan the repair in an occupied building.
• Elevator pits and sumps. The lowest points in a building, often sitting in groundwater. Read why elevator pit leaks happen and how they’re fixed from inside.
• Retaining walls and patios on strata properties.

Common questions

Are hairline cracks in concrete normal?

Yes. Concrete shrinks as it cures, and fine hairline cracks are normal in most walls and slabs. On their own they’re cosmetic. They’re worth watching, though. If a hairline crack starts to leak, stain, show rust or get wider, it’s time to have it looked at. Mark the ends with a pencil and the date so you can tell whether it’s growing.

Can a crack be sealed while it’s actively leaking?

Yes, and we prefer it. A running leak shows us exactly where the water path is, so we don’t have to guess from a dry stain. We cut out the crack, stop the flow with a rapid-setting hydraulic plug, then pack and treat it with crystalline materials, which need moisture to work. You’ll know right away that the flow has stopped.

Why did my DIY crack sealer fail?

Most hardware-store sealers and caulks sit on the surface of the concrete. Water pushing from the other side gets behind them and lifts them off, or finds its way around them. They often look fine in dry weather and fail in the first heavy rain. A leaking crack needs to be opened up and treated through the concrete, against the water pressure.

When should I call a structural engineer about a crack?

Call an engineer if a crack is wide, if one side is offset from the other, if it’s getting longer or wider, if it’s horizontal in a foundation wall, or if the wall is bowing. Concrete breaking away and exposing rusted steel is another reason. Once the engineer has dealt with the structural side, we can repair the leak.

Do repaired cracks come back?

It depends on whether the crack is still moving. Crystalline repairs are meant for rigid concrete, and a crack that’s stable usually stays sealed once it’s properly treated. If a crack keeps moving because of ongoing settlement or a structural issue, it can open again, which is why we look for signs of movement first and use injection or refer you to an engineer where that’s the case.

Book a free crack inspection

Got a crack that’s wet, stained or getting worse? We’ll look at it, tell you honestly whether it needs repair, an engineer or nothing at all, and quote from a site look, not a phone guess. We work across Richmond, Burnaby, Vancouver, Surrey and the Fraser Valley. Read more about our concrete crack repair and waterproofing, then call (778) 686-2221 (Mon–Sat, 8:30 AM–5:00 PM) or book your free inspection.