Reading a crack takes about thirty seconds once you know what you're looking at. Here's how to sort the ones that leak from the ones that lie.
The Method
Read Three Things: Width, Direction, and Whether It's Wet
You don't need a lab. You need to answer three questions, in order.
How Wide Is It?
- Hairline (thinner than a credit-card edge, up to ~1/16"). Extremely common. Usually shrinkage from the concrete curing, or minor settling. On its own, a hairline crack is a cosmetic and moisture note, not a structural alarm.
- Moderate (roughly 1/16"–1/4"). Worth attention. Could still be benign shrinkage, but this is the range where you start asking whether something is moving.
- Wide (1/4" and up), or any crack where one side sticks out past the other (offset), or that's clearly wider at one end than the other. This is a structural conversation, not a waterproofing one.
Which Way Does It Run?
- Vertical or near-vertical cracks are the most common and usually the least worrying as they are commonly attributed to shrinkage or minor settlement.
- Diagonal cracks, especially running from the corners of windows or other openings, point to settlement stress concentrating at a weak point.
- Horizontal cracks are the one to respect. A horizontal crack running along a wall, particularly in a block wall around mid-height, can mean the wall is being pushed in by soil or water pressure from outside. That's a structural flag, not a caulk-it problem.
- Stair-step cracks following the mortar joints in a block wall usually mean differential settlement, which is when one side of the structure settles or sinks at a quicker pace than the other sides, potentially leading to serious structural damages.
Is It Wet?
This is the one that decides your waterproofing move. Is the crack bone dry, or is it damp, stained, or actively weeping? A dry crack and a running crack get completely different treatments even if they're the same width.
The Repair Question
Dead Crack or Active Crack?
Put width and direction aside for a second because, for the repair, the biggest question is simply: is this crack done moving and leaking, or is it alive? A dead (static) crack isn't growing and isn't passing water, making it a great candidate for a straightforward fill. An active crack is doing one of two things: still moving with seasons and loads, still passing water, or both. Two quick field checks:
- Is it moving? Bridge the crack with a dab of a dated marker or a strip of tape and check it over a few weeks; or just note whether an old patch has re-cracked. A crack that keeps re-opening needs a flexible repair, not a rigid one.
- Is it wet? If it's weeping now, you can't treat it like a dry crack as most rigid fillers won't bond to a wet, running surface.
Match the Fix
Match the Fix to the Crack
Here's where reading the crack pays off, because the wrong repair on the right crack still fails.
Clean it out, widen it slightly if needed, and pack it with hydraulic cement. Hydraulic cement is rigid and sets fast, which is perfect for a crack that isn't moving and isn't wet. It's the wrong choice for anything actively running because it won't adhere to a wet surface.
This is where polyurethane injection earns its keep. Hydrophilic PU is injected into the crack, reacts with the moisture that's already there, and expands to fill the full depth of the crack from a single port. Unlike cement, it wants a wet crack. It seals the flow path all the way through the wall rather than just capping the surface. When water is coming through the crack right now, injection is usually the move.
A rigid fill will just crack again along the same line. This is where a flexible, elastomeric approach matters: bridge the crack and let the repair stretch with the movement instead of fighting it.
Get a structural opinion before you waterproof anything. Sealing a crack that's caused by a wall failing under pressure just hides the symptom while the real problem gets worse. Waterproofing keeps water out of a sound wall; it is not a substitute for structural repair. Know where that line is, and be the contractor who calls it honestly.
How It Works in a Full System
For the crack that keeps moving, a rigid patch is a losing fight — that's the logic behind detailing cracks with a flexible product like Enviroguard's Bridging & Joint Compound and embedding reinforcing mesh at the crack before the Foundation Waterproofer membrane goes over the top.
The membrane then bridges the dynamic movement instead of tearing at it. The detail work stretches with the wall — so the crack you already read correctly stays sealed through the next season of movement.
The Field Version
The Thirty-Second Version
Walk up to a crack and run the checklist:
- Width — hairline, moderate, or wide/offset? Wide or offset means structural, not waterproofing.
- Direction — vertical (usually fine), diagonal (settlement stress), horizontal (respect it).
- Wet or dry? — dry and stable takes a rigid fill; actively leaking takes injection; keeps-moving takes a flexible bridge.
Get those three answers and the repair picks itself. The skill isn't filling cracks (anyone can smear cement in a line); the skill is knowing which crack you're looking at before you decide what it needs, so you're not patching a structural problem or spending injection money on a crack that was never going to leak
The Products
What Handles the Moving Crack
For the crack that keeps moving, the detail work goes on in order: bridge the crack, tie it to the wall, then cover the field.
Bridging & Joint Compound
A flexible, elastomeric compound that bridges cracks and joints and stretches with seasonal movement instead of cracking again along the same line like a rigid fill.
View Product →Foundation Base Coat
The tie-coat that locks the crack detailing and mesh to the wall and gives the membrane a uniform, well-bonded surface to grip.
View Product →Foundation Waterproofer
The waterproofing membrane that goes over the top, bridging dynamic movement across the whole wall so the detailed crack stays sealed through the next season.
View Product →Seal the Crack the Right Way
See the full system: crack detailing, flexible bridging, base coats, and membranes built to move with the wall instead of tearing at it.
Explore Our Foundation Waterproofing Area →