Basement crack guide
How to Seal a Leaking Foundation Crack From the Inside
Many leaking basement wall cracks can be sealed in an afternoon with a polyurethane injection kit — and some should not be sealed at all until someone has looked at them. Here is how to tell which you have, which material works, and the injection method step by step.
Quick answer
Sealing a leaking crack
- Confirm it is a vertical or diagonal crack in poured concrete, 1/4 inch or less.
- Use polyurethane injection, not caulk or hydraulic cement alone.
- Space ports about as far apart as the wall is thick.
- Inject from the lowest port upward, then let it cure 24–48 hours.
Horizontal, stair-step, widening or offset cracks: get an assessment first.
Short answer
Most leaking cracks are sealed by injection, not by patching the surface
For a leaking vertical or diagonal crack in a poured concrete wall, inject it with a polyurethane crack-repair kit: seal the surface, set ports about as far apart as the wall is thick, and inject from the bottom port up. Hydraulic cement plugs a drip but rigidly re-cracks. Horizontal, stair-step, widening or block-wall cracks need assessment first.
The reason surface patches fail is that water does not leak along the face of the wall; it comes through the whole thickness of it. A patch over the inside face leaves the crack full of water behind it, and the pressure that pushed water in the first place pushes it around the patch. Injection fills the crack itself, from the inside face back toward the soil.
Go or no-go
Can you seal this crack yourself?
Classify the crack before buying anything. The shape and the wall type decide whether this is a sealing job or a structural question.
| What you have | What it usually is | What to do |
|---|---|---|
| Poured concrete, vertical or slightly diagonal, hairline to 1/4 inch, not changing | Shrinkage or minor settling crack | Good DIY candidate — polyurethane injection. |
| Poured concrete, leaking around a pipe or tie-rod hole | A penetration, not a crack | DIY — hydraulic cement plug or polyurethane. |
| Water at the joint where the wall meets the floor | Not a crack; water under pressure at the footing | Not a crack repair — see wall-floor joint leaks. |
| Crack wider than about 1/4 inch, or one side offset | Movement beyond shrinkage | Get it assessed before sealing. |
| Horizontal crack, or the wall bows inward | Lateral soil or water pressure on the wall | Do not seal it and move on — structural assessment first. |
| Stair-step crack in a concrete block wall | Pressure or settlement through mortar joints | Injection does not work in hollow block; assessment and a drain or reinforcement. |
| A crack that was sealed before and leaks again, or is widening | Active movement or persistent water pressure | Assessment, and look at drainage outside. |
Materials
What each repair material actually does
| Material | Best for | Strength | Limit |
|---|---|---|---|
| Polyurethane injection | Leaking non-structural cracks in poured concrete | Reacts with water and expands to fill the crack through the wall; stays flexible | Does not add structural strength; needs a poured wall |
| Epoxy injection | Dry cracks needing a structural bond | Very strong; bonds the two sides back together | Poor on actively wet cracks; rigid; not a DIY fix for moving walls |
| Hydraulic cement | Plugging an active drip, tie-rod holes, port holes | Sets in minutes, even against flowing water | Rigid and surface-deep; commonly re-cracks |
| Caulk, patching compound, paint-on sealer | Cosmetic hairlines that do not leak | Cheap and quick | Surface only; water pressure pushes behind it |
| Exterior excavation and membrane | Persistent leaks, block walls, several cracks | Stops water on the soil side | Professional, disruptive and expensive |
Polyurethane is the default for leaks because it is water-activated: many DIY kits use a resin that foams on contact with moisture and expands inside the crack, which is exactly the condition a leaking crack is in. Epoxy is the choice when the goal is to glue a dry crack back together structurally, and it is a job where the assessment matters more than the product.
Before you start
What you need, and the conditions it needs
- A polyurethane crack injection kit sized for the crack length — they are sold by linear feet and include ports, surface seal paste and resin cartridges.
- A caulk gun (many kits use one) or the dispenser the kit specifies.
- A wire brush or grinder to clean 1 to 2 inches either side of the crack back to sound concrete.
- Gloves, eye protection and ventilation — uncured resin and paste irritate skin.
- Temperature within the kit's range, commonly about 40 to 100°F for both the wall and the materials.
- Access to the whole crack, floor to top. Framing, insulation or panelling over it has to come off.
Step by step
How to inject a leaking crack
- Clean the crack line. Remove paint, old patching, efflorescence and loose concrete along the crack so the paste bonds to sound concrete. Old caulk or cement patches in the crack must come out.
- Mark the port positions. A common rule is to space ports about as far apart as the wall is thick — every 8 inches on an 8-inch wall — with one port within a few inches of each end of the crack.
- Set the ports and seal the surface. Bond each port over the crack, then trowel the surface seal paste over the ports' bases and along the entire crack between them, so resin can only leave through the next port. Let it cure per the kit — often 20 to 60 minutes.
- Inject from the lowest port. Inject slowly and steadily until resin appears at the port above. That means the crack is filled to that height.
- Cap and move up. Cap the port you just filled, move the nozzle to the port where resin appeared, and repeat, working to the top of the crack.
- Let it cure. Leave it 24 to 48 hours at room temperature before disturbing it. Then, if the kit allows, cut or knock off the ports and scrape back the paste.
- Test it. Run a garden hose against the outside of the wall over the crack for 15 to 30 minutes, or wait for the next heavy rain, and check the inside face.
Water running now
If it is actively gushing: plug first
A crack with water running out of it may need a plug before injection. Chisel the leaking spot into a small undercut groove, press hydraulic cement into it, and hold it until it sets, usually a few minutes. Then inject the rest of the crack around it. Treat the hydraulic cement as a stopper, not the repair: on its own it is rigid and commonly re-cracks.
Afterwards
Check it through a few storms
A crack repair is proved by weather, not by the afternoon you did it. Check the wall after the next several heavy rains before finishing over it. Keep the photo and the date. If the crack leaks again along the repair, or a new crack appears nearby, that points to water pressure or movement the injection could not address, and it is time for a professional assessment.
Do both sides
Take the water away from the outside of the wall
Cracks leak because water is sitting against the other side of them. Before or after sealing, look at the outside of the house directly above the crack: a downspout emptying at the foundation, soil sloping toward the wall, a window well that fills, or a flower bed edged so it holds water. Removing that water supply makes the repair last longer and sometimes stops the leak on its own. If water enters along the base of whole walls rather than one crack, the answer is drainage, not sealing — see water at the wall-floor joint and interior vs. exterior waterproofing.
Get help
When not to do this yourself
- The crack is horizontal, or the wall bows, leans or has shifted.
- The crack is wider than about 1/4 inch, offset, or getting wider over months.
- It is a concrete block wall.
- You have sealed it before and it leaks again.
- There are doors or windows sticking upstairs, or new cracks in drywall, alongside the basement crack.
None of these mean something is necessarily wrong, but each one means sealing the crack could hide a problem rather than fix it. This site does not inspect or diagnose; a qualified waterproofing contractor or structural engineer can.
Related pages
Where to go next
Basement Wall Cracks Leaking
What the crack's shape and timing tell you.
Read nextBasement Crack Repair
Professional injection, exterior repair and when each fits.
Read nextWater at the Wall-Floor Joint
When the leak is not a crack at all.
Read nextFind the Source of Basement Water
Before you seal, confirm where it comes from.
Read nextFAQs
Sealing foundation crack FAQs
What do I use to seal a leaking foundation crack?
For a leaking, non-structural crack in a poured concrete wall, a polyurethane injection kit is the standard DIY repair: the resin expands on contact with water and stays flexible, so it tolerates small seasonal movement. Epoxy injection is for dry cracks that need structural bonding. Hydraulic cement stops an active drip quickly but is rigid and often re-cracks. Caulk and paint-on sealers only cover the surface.
How big is too big for a foundation crack?
As a practical rule, cracks wider than about 1/4 inch, cracks that are getting wider, cracks where one side is offset from the other, horizontal cracks, and stair-step cracks in block walls are beyond a simple DIY seal and should be assessed first. Width alone is not the whole story: a hairline horizontal crack in a bowing wall matters more than a wider vertical shrinkage crack.
How do I stop water from seeping through my foundation?
Deal with both ends of the path. Inside, seal the specific crack with polyurethane injection, or collect seepage with an interior drain where water enters along whole walls. Outside, reduce the water against the wall: downspout extensions, soil sloping away from the house, and clear window wells. A sealed crack with a saturated wall behind it can simply move the leak somewhere else.
Should foundation cracks be fixed?
Leaking cracks should be sealed, because water through a wall damages finishes and raises humidity. Dry hairline shrinkage cracks in poured concrete are common and often only need monitoring. Any crack that is horizontal, widening, offset, or accompanied by a bowing wall should be evaluated before it is sealed, because sealing hides the evidence of movement without addressing it.
Does hydraulic cement permanently stop foundation leaks?
Usually not. Hydraulic cement sets in minutes, even against running water, which makes it excellent for plugging an active drip or a tie-rod hole. But it is rigid and bonds mostly at the surface, so as the wall moves with temperature and moisture a fine crack often reopens beside or through the patch. Treat it as a temporary stop, or as the cap over a proper injection.
Can you inject a crack while it is leaking?
With polyurethane, yes — water-activated polyurethane is designed to react with water in the crack and expand to fill it. Epoxy is the opposite: it needs a dry or only damp crack to bond, so it is not suited to a crack with water running through it. If water is flowing hard, some kits have you plug the worst spot first and inject around it.
Can I seal a crack in a concrete block wall the same way?
Not with injection. Block walls are hollow, so injected resin runs into the cores instead of filling the crack, and block cracks usually follow mortar joints in a stair-step pattern that often reflects soil pressure or settlement. Leaking block walls are normally handled with an interior drain system, exterior waterproofing, or structural reinforcement after assessment.
Sources
Sources
- Concrete Network — epoxy vs polyurethane crack injection
- Emecole — DIY polyurethane crack injection kit specifications
- SealBoss — epoxy versus polyurethane crack injection
- My Foundation Repairs — why crack repair kits don't always work
Procedure figures reflect common DIY kit instructions and industry guidance; the instructions for the kit you buy take priority. This guide is general information, not a structural assessment.