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Not every crack leaks, and not every leak comes from a crack. Telling the difference matters because the repairs are different, the costs are an order of magnitude apart, and the wrong one solves nothing.
Cracks that are normal
Poured concrete shrinks as it cures. That shrinkage produces hairline vertical cracks, most often near the middle of a long wall or beside a window or door opening. They are extremely common, they appear within the first year or two, and by themselves they are not a structural problem.
Block walls crack differently — usually along mortar joints, in a stair-step pattern following the courses. Again, common, and not automatically serious.
These become leaks only when there is standing water outside to push through them. The crack is the opportunity; the saturated soil is the cause.
Cracks that need attention
- Horizontal cracks in a block or poured wall, particularly near mid-height. These can indicate lateral soil pressure exceeding what the wall was designed for.
- Wide or widening cracks — anything you can fit a coin into, or that has visibly grown.
- Cracks with displacement, where one side sits proud of the other.
- Bowing or bulging walls, with or without visible cracking.
- Stair-step cracking accompanied by inward movement.
These are structural questions rather than water questions, and they belong with a structural engineer rather than a waterproofing contractor. The distinction matters because the incentives differ.
The cove joint is not a crack
The most common seepage point in a basement is the cove joint — the junction where the wall meets the floor slab. The wall and the slab are poured separately and are not bonded, so there is a seam by design.
Under hydrostatic pressure, water comes through that seam. It presents as water appearing along the base of a wall, often across a long stretch rather than at a single point, and it is frequently mistaken for a wall crack.
This matters a great deal for repair selection. Injecting a crack does nothing about a cove joint. Cove joint seepage is a drainage problem, addressed by reducing the water reaching the foundation or by giving it somewhere to go — interior drain tile to a sump, for example.
How repairs differ
Crack injection — polyurethane or epoxy injected into a specific crack from the interior. Appropriate for an isolated, non-structural crack in a poured wall. Relatively inexpensive.
Exterior excavation and membrane — digging to footing depth and waterproofing the outside. Definitive and expensive, and warranted when the wall itself is the problem.
Interior drain tile with a sump — does not stop water reaching the wall, but collects it at the footing and pumps it out. The usual answer to cove joint seepage and general hydrostatic pressure.
Exterior drainage correction — grading, downspouts, window well drains. Cheapest, and it is the correct starting point because it reduces the water load driving everything else.
Sequence that avoids wasted money
Fix the water delivery before waterproofing the wall. A large proportion of seepage stops when roof runoff is moved away from the foundation and settled grading is restored. Spending on excavation before trying that is common and frequently unnecessary.
Related pages
For the pressure mechanism, see heavy rain and saturated soil. For drainage correction, see downspouts, gutters, and grading. For repair costs, see waterproofing vs mitigation.