Last Updated on August 24, 2026 by Umar Farooq
Sealing a leaking crack yourself is realistic for some cracks and a waste of a weekend for others. The difference is not the product. It is what kind of crack you have and which side of the wall you are working on.
Every page currently ranking for this is published by a contractor or a company that sells the sealant. None of them carries a single citation. So this one works from the engineering literature instead, which is free, public, and says some things the product pages do not.
First question: is the crack active or dormant?
This is the question the repair standards ask before anything else, and it appears nowhere in the consumer advice on this topic. It decides the method, so getting it wrong wastes whatever you buy.
| Dormant | Active | |
|---|---|---|
| What it means | The thing that caused it has finished happening | The cause is still at work |
| Typical cause | Drying shrinkage in the first year or two. Poor construction practice. | Ongoing settlement. Seasonal soil movement. Corroding reinforcement. |
| Behaviour | Width stays the same over months | Width changes with season, weather or load |
| Suitable repair | Rigid fills: epoxy, hydraulic cement, drypack | Flexible sealant that lets the crack keep moving, or fix the cause first |
Measuring it is easy. Mark the crack ends with pencil, note the width against a card with printed lines, and check again after three months and after a season change. If you want a number, a crack monitor is inexpensive and gives you a direct reading of movement.
Also look for displacement, meaning one side sitting proud of the other. A crack with offset across it is a different conversation from a crack that is simply open.
Second question: which crack do you have?
| Pattern | Usually means | Structural? |
|---|---|---|
| Vertical or near-vertical in a poured wall, fairly uniform width | Drying shrinkage. Concrete shrinks and the footing restrains the bottom edge, so it cracks. This is the most common basement crack there is. | Usually not. Below about 1/8 inch it is generally not considered structurally significant. |
| Diagonal, often from a corner of a window or door | Differential settlement, or shrinkage concentrating at a weak point | Depends. Monitor it. |
| Stair-step along mortar joints in a block wall | Settlement in that area | Often yes. Get it looked at. |
| Horizontal, especially mid-height, sometimes with the wall bowing inward | Lateral soil pressure | Yes. Stop and call a structural engineer. Do not seal it and consider it dealt with. |
| Cracks along a rust stain | Reinforcement corroding and expanding | Sealing will not help. The steel keeps corroding and cracks the concrete again. |
Concrete cracks are normal, and the literature is unembarrassed about it: all concrete has a tendency to crack, and most random early-age cracks rarely affect structural integrity or service life. That is not a reason to ignore a horizontal crack, but it is a reason not to panic about a hairline vertical one.
One note on the numbers you will see quoted. The widely repeated quarter-inch threshold comes from a home builders’ warranty performance guideline. It is a warranty tolerance, not an engineering threshold, and it is not a safe substitute for looking at the pattern.
The thing nobody selling sealant will tell you
Water pushing against a basement wall is pushing from the outside. A repair you apply on the inside is on the opposite face from the pressure.
The US Army Corps of Engineers guidance on sealing cracks is explicit that the method will not be effective on cracks under pronounced hydrostatic pressure except in sealing the pressure face, and even then you get some reduction in flow rather than a fix.
For a basement wall, the pressure face is the soil side. Almost every DIY method sold for this job is applied to the wrong side of the wall.
Even the epoxy manufacturers say so in their own limitations: not for injection of cracks under hydrostatic pressure at the time of application.
Building code follows the same logic. Where a high water table or severe soil-water conditions exist, waterproofing is required on the exterior of the foundation wall. Nothing in the code contemplates resisting water pressure from inside.
This does not make interior repair pointless. It makes it a way to stop nuisance seepage, not a way to hold back a water table. State guidance on interior sealants puts it plainly: the measure is not intended to prevent high level flooding, only to help prevent low-level leakage.
A risk specific to injection
If you inject a crack that runs right through the wall, the material has to go somewhere. Repair guidance warns that where a crack cannot be seen on all surfaces, uncontrolled injection can cause damage such as plugging a drainage system, and advises extreme caution.
A basement wall crack is by definition not visible on all surfaces. If there is footing drain outside, you can fill part of it with grout and never know until the next storm.
Matching method to crack
| Method | What it actually does | Use it when |
|---|---|---|
| Epoxy injection | Structural. Bonds the two faces and restores strength. Works on cracks from about 0.002 inches upward. For load-bearing repair the material should meet ASTM C881 Type IV, so check the grade on the box. | The crack is dormant and you need strength back. |
| Polyurethane injection | Water-stopping only. Repair guidance states bond strengths are typically low, so structural repairs are not made with chemical grouts. | The crack leaks, is not structural, and may move slightly. |
| Flexible urethane sealant | Seals against water while letting the crack keep moving. Restores no strength at all. | The crack is active and you want it to behave like a joint. |
| Hydraulic cement | Sets in minutes, even against running water. Rigid. | Stopping an active flow, in a dormant crack. Guidance is explicit that drypack is not advisable for active cracks. |
| Nothing | Genuinely a listed option in the engineering literature, under the name “judicious neglect”. | The crack is dormant and there is no moisture problem. |
Preparation, where most DIY repairs fail
Two materials, two opposite instructions. Generic advice gets one of them wrong.
| Epoxy / sealant | Hydraulic cement | |
|---|---|---|
| Groove shape | V-groove is acceptable, and is used where high injection pressure needs a stronger cap seal | Square or undercut. Avoid V-shaped cuts. A V-groove packed with hydraulic cement is shaped to push itself back out. |
| Cleaning | Wire brush about half an inch each side. Mechanical grinders are discouraged because they pack the crack with dust. | Same. Contaminants prevent bonding. |
| Working time | Cap seal must cure before injection pressure is applied | 2 to 3 minutes. Mix golf-ball-sized batches only. |
Does the crack have to be dry?
It costs you strength if it is not. One moisture-tolerant epoxy’s own data sheet gives bond strength to concrete as 3,000 psi after a dry cure against 2,200 psi after a moist cure. That is roughly a quarter of the bond given away, from the manufacturer’s own testing.
Standard epoxies are worse than that. If the crack is actively leaking and cannot be dried, use a moisture-tolerant product and accept the penalty, or use polyurethane, which is designed to work wet.
Temperature
Most structural epoxies want a minimum substrate and air temperature around 40 °F. Hydraulic cement sets faster when hot and slower when cold, and the manufacturer suggests warm mixing water below about 50 °F.
Two mistakes that make things worse
Turning up the injection pressure
Repair guidance is blunt: increased pressure often does little to accelerate injection, and excessive pressure can propagate the existing cracks, causing additional damage. Inject vertical cracks from the bottom up and continue until the material refuses to take more.
Choosing a flexible epoxy so the crack can still move
This sounds sensible and does not work. A low-modulus adhesive inside a crack will not allow significant movement, because the layer is thin and laterally restrained by the concrete either side. If the crack moves, you need a sealant in a properly shaped groove, not a softer glue.
Before you seal anything
The guidance is consistent on this and it is the point the whole page turns on: unless the cause has been corrected, new cracks will probably form near the original one, and repairs that ignore the cause may only be temporary.
- Work out where the water is coming from. A crack that only leaks after rain is a drainage problem presenting as a crack.
- Fix the outside first. Gutters, downspout extensions, grading. Building science guidance on basements reduces to draining the site and draining the ground.
- Then decide whether the crack still needs sealing. Often the seepage stops when the water stops arriving.
- Seal what is left, matched to whether the crack is active or dormant.
When to stop and call someone
- A horizontal crack, particularly with any inward bowing of the wall.
- Displacement across the crack, one side proud of the other.
- A crack that is measurably widening over months.
- Stair-step cracking in a block wall with any bulge.
- Cracks tracking a rust stain. The steel keeps expanding.
- Anything you are about to seal on a wall retaining several feet of saturated soil. Federal flood guidance is that dry floodproofing is not recommended for homes with basements, because saturated soil pressing on the walls can damage them.
FAQ
The crack leaks only when it rains hard. Do I still seal it?
Deal with the water first. That pattern says surface water is reaching the wall, and the crack is the route rather than the cause. Sealing it usually just moves the leak to the next weakest point.
Can I use hydraulic cement on a crack that is still moving?
No. It is rigid and it will crack again along the same line. Use a flexible sealant so the crack can function as a joint.
How do I know if my crack is widening?
Mark the ends in pencil, write the date, and measure the width against a printed crack card. Re-check after three months and after a full season change.
Is a hairline vertical crack a problem?
Usually not structurally. In poured walls, cracks under about an eighth of an inch are generally not considered structurally significant. If it leaks, seal it. If it does not, you can reasonably leave it.
Should I seal from the outside instead?
That is where the pressure is, so yes in principle. In practice it means excavating to the footing, which is why most people work from inside and accept the limitation.
