Quick answer
If water seeps through a thin crack in a poured concrete basement wall, inject it from inside with the RadonSeal Complete DIY Foundation Crack Repair Kit, 10 Ft, rated 4.6 stars across 1,182 Amazon ratings; per its listing, the two-component urethane goes in with a standard caulking gun and no drilling. For a floor or driveway crack up to 1/2 in wide, a pour-in product such as DAP Concrete Crack Filler is the simpler fix. A horizontal crack, a bowing wall or a stair-step crack in block is a structural question, so have a structural engineer look before you seal anything.
Buyer's Guide
What to know before you buy.
Match the Product to the Crack
Start with where the crack is and which way it runs. A thin vertical crack in a poured concrete basement wall that seeps after rain is an injection job: the RadonSeal kit in most cases, or the Hydra Stop kit if water is running through the crack when you work. A narrow, dry crack in a wall, a step or a mortar joint takes a gun-applied product such as the DAP Concrete and Mortar Filler and Sealant. A crack in a flat surface such as a garage floor, patio or driveway takes a pour-in filler, and DAP Concrete Crack Filler states a limit of 1/2 in wide by 1/2 in deep per pass. A crack or joint that opens in winter and closes in summer needs a flexible sealant rated for movement, such as Sikaflex Concrete Fix at ±35%. Only one of these listings states a width limit, so read the label of the others before you buy.
Injection, Surface Sealant or Epoxy: How They Differ
A surface sealant closes the face of a crack. An injection fills the crack through the thickness of the wall, so water has to get past the whole depth rather than one bead. Both injection kits here use urethane or polyurethane, which stays flexible after it cures, so a wall that shifts slightly with the seasons does not tear the seal. Epoxy is the other injection material. It cures hard and bonds the two sides of a crack together, which is why it is used for structural repairs, but if the wall is still moving, a rigid repair tends to crack again beside the old line. That makes epoxy a material for an engineer's repair plan rather than a first guess. In a stable wall with a leaking crack, flexible polyurethane is the usual homeowner choice.
What a Sealer Cannot Stop: Water Pushing From the Soil
A sealed crack stops water coming through that crack. It does not lower the pressure of wet soil and groundwater against the wall, so if the ground outside stays saturated, the water looks for the next path: another crack, the joint where the wall meets the floor, or the slab itself. That is why basements that leak after every storm are usually fixed outside first. EPA's moisture advice is to clean and repair roof gutters and to make sure the ground slopes away from the foundation. The arithmetic shows why: every inch of rain on 1,000 sq ft of roof is about 620 gallons, and a downspout that empties at the wall drops that water right beside it. Extend downspouts, regrade low spots, and if water rises through the floor, plan for a drain and sump pump rather than more sealant.
Cracks That Need a Structural Engineer, Not a Sealer
Some cracks are a sign that the wall is moving, and no product on this page is the right first step for them. Call a structural engineer before you seal if you see a horizontal crack in a basement wall, a wall that bows or leans inward, a stair-step crack running through the mortar joints of a block wall, a crack where one side sits farther out than the other, or a crack that is wider at one end or keeps getting wider. Cracks wider than a product's stated limit belong in the same group. Signs upstairs count too, such as doors and windows that start to stick or new gaps at trim. Sealing these hides the evidence an engineer needs to see. To watch a crack you are unsure about, mark both ends in pencil with the date and measure its width each month.
Who Should Not Buy a Crack Sealer
A crack sealer is the wrong purchase in several common situations. If the basement wall is concrete block or fieldstone, injection kits are a poor fit, because block has hollow cores that swallow the material and fieldstone rarely leaks through one clean crack. If water enters where the wall meets the floor, that seam is a joint between two separate pours, and the fix is drainage rather than a bead along the edge. If the floor is wet across a wide area, or water rises through it after rain, you need a drain and a sump pump. After a flood with standing water, dry the space before any repair; EPA advises drying water-damaged areas within 24 to 48 hours to prevent mold. And if you rent, report the crack to the landlord, since a foundation repair is the owner's decision.
Prep, Cure Time and Checking Your Work
Most failed crack repairs fail at the prep. Clear loose concrete, dust and old filler out of the crack with a wire brush and a vacuum, and follow each label on whether the crack should be dry or damp. Cure times appear on only two of these listings: the DAP Concrete and Mortar Filler and Sealant returns to service in 24 hours, and the DAP Concrete Crack Filler takes light foot traffic in 4 hours and vehicles in 48. The RadonSeal, Sikaflex and Hydra Stop listings give no cure time, so read the cartridge or kit instructions before you paint, cover the area or expect rain. Sika's listing asks for storage between 40°F and 95°F. Then wait for the next heavy rain and look at the crack. A dry line means the seal held; a damp one usually means water is finding another way in.
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