Foundation Crack Repair: How to Choose the Right Fix

Which foundation crack repair does your crack need?

It depends on three things: the crack’s direction, its width and whether it’s still moving. Thin, stable vertical cracks in a poured wall get sealed. Sideways, wide, offset or growing cracks get an engineer first.

Work through the questions in order. The first “yes” is your answer.

If you’re not sure which row you’re in, the when to worry guide walks through width, direction and movement in more detail. The crack identifier on the homepage asks the same questions.

When is do it yourself foundation crack repair a good idea?

When the crack is in a poured concrete wall, runs vertically or diagonally, is 1/4 inch wide or less, and its two sides line up flat. That describes most basement cracks. Pillar To Post’s foundation guide says typical shrinkage cracks are under 1/8 inch and are not structurally significant (opens in a new tab).

Do it yourself foundation crack repair is a sealing job, not a structural one. It stops water, soil gas and insects coming through the crack. It doesn’t hold a wall in place, and no kit claims to.

Don’t try to fix foundation cracks yourself when:

• The crack is horizontal, or the wall bows or leans in.
• It’s a stair-step crack along the mortar joints of a block wall.
• One side of the crack sits farther in or out than the other.
• It’s wider than 1/4 inch, or it’s getting wider over time.
• A crack you already sealed has cracked again next to the repair.

Each of those means the wall moved or is still moving. A structural engineer can tell you why and what the wall needs, without selling you the repair.

What are the ways to repair a cracked foundation?

There are two DIY methods for a non-structural crack (polyurethane foam injection and epoxy injection) and several contractor methods for walls that moved. A surface patch alone is the one people try most and the one that fails most.

The American Concrete Institute is direct about the order: the cause of the cracking must be assessed and corrected (opens in a new tab) before any repair. A sealed crack in a wall that’s still being pushed will crack again.

The ACI’s guide to sealing leaking cracks sorts them by movement. For active cracks, whose width changes over time, it lists chemical grout pressure injection (opens in a new tab) such as polyurethane, routing and sealing, or a flexible membrane. Epoxy is only on its list for dormant cracks.

For bowed or displaced walls, ASHI’s guide to basement cracks says an interior steel beam is a common repair for minimal displacement (opens in a new tab), and serious displacement needs an engineer’s design. That’s not a homeowner product, and this site won’t sell you one.

Should you use polyurethane or epoxy?

Polyurethane foam, for almost every DIY repair. It reacts with water, bonds to a wet crack and stays flexible, so it seals cracks that leak or move a little with the seasons. Epoxy is the structural choice, and it needs a dry, dormant crack.

The ACI’s epoxy injection guide says that if a crack is subject to movement, epoxy may not be applicable (opens in a new tab), and that for cracks that aren’t structural, polyurethane may be more suitable. The foam kit’s product page says the same in plainer words: use foam when movement is expected or epoxy isn’t necessary.

That’s why the decision tree only reaches epoxy through an engineer. The full comparison is in epoxy vs polyurethane for foundation cracks.

How does foam injection work?

You glue small plastic ports over the crack, seal the crack’s face with a paste, then pump foam into the ports from the bottom up. The foam fills the crack through the wall’s thickness, so it seals from the inside out. Polyurethane foam injection is the most common DIY concrete foundation crack repair for poured walls.

The short version, from the 10 ft kit’s instructions:

1. Clean the crack

   Wire brush about 1 to 2 inches either side of the crack, removing paint, efflorescence and loose concrete. The surface has to be dry for the paste to stick.

2. Set the ports

   Space the ports about as far apart as the wall is thick (usually about 8 inches), centered on the crack, starting near the floor.

3. Seal the surface

   Spread the paste about 1/8 inch thick and 2 inches wide over the crack and around each port, without pushing it into the crack.

4. Let the paste harden

   Wait about 2 to 4 hours, until it’s hard to a fingernail. The instructions don’t recommend waiting overnight.

5. Flush and inject

   Pour 1 to 2 cups of water into the top port, then inject foam slowly at the bottom port until it shows at the next one up. Cap it and move up.

6. Remove the ports

   After about 3 or 4 hours, knock the ports off with a hammer. The paste can be painted.

The details that make it work, like how long to inject a hairline crack and what to do when foam leaks out the face, are in the foundation crack injection guide. If something already went wrong, see injection troubleshooting.

Why don’t hydraulic cement and caulk work for sealing foundation cracks?

Because they only cover the face of the crack. Water in the soil pushes through the crack inside the wall, around the patch, and finds the next weak spot. A rigid patch also can’t flex when the crack opens and closes with temperature.

Sealing foundation cracks means filling the crack through the wall, which is what injection does. Hydraulic cement still has a job: the foam data sheet says to plug a wide crack or one with heavy water flow with hydraulic cement or epoxy gel first, so the foam stays in long enough to cure.

The hydraulic cement guide covers when it’s the right product. A basement crack repair done with caulk alone leaves the rest of the crack open behind it.

Should you repair a foundation crack from the inside or the outside?

From the inside, for a single crack. Injection from inside the basement fills the crack all the way through the wall, so it seals the outside face too, without digging. The ACI notes cracks can be injected from one or both sides (opens in a new tab).

Outside repair means excavating down the wall to the footing, then waterproofing it from the soil side. The NRMCA says positive side waterproofing (exterior) is generally better (opens in a new tab) for keeping water out of a whole wall. That’s a contractor job, and it makes sense for a wall with many leaks or failed drain tile, not one crack.

The basement wall crack repair guide compares the two in detail, with a cutaway of each.

Which foam kit size do you need?

Measure every crack you plan to seal and add the lengths. One crack up to about 10 feet takes the 10 ft kit, and several cracks or one long one take the 30 ft kit. The 10 ft kit’s instructions say it repairs approximately 6 to 10 linear feet, so a crack near 10 feet with heavy foam use is the edge of one kit.

The kit product pages list cracks up to 3/8 inch wide. This guide still sends any wall crack over 1/4 inch to an engineer first, because width that size often means the wall moved. The kit sizing guide shows how to measure, and peel-off vs epoxy paste explains the surface seal choice for a wall that will stay exposed.

Do you need to fix the water outside first?

Yes, or at least at the same time. Basement cracks often leak because surface water collects against the wall, and sealing one crack doesn’t stop that water from finding another. The NRMCA says to check and correct drainage before repairing leaking cracks (opens in a new tab).

Its guidance is to slope the ground away from the house 1/2 to 1 inch per foot (opens in a new tab) for at least 8 to 10 feet. Extend downspouts the same way, and keep gutters clear.

ASHI’s guide makes the same point from the inspection side: seepage is often caused by surface water (opens in a new tab) reaching the outside of the wall. If leaks continue after the drainage is fixed, they need more investigation.

What if the crack is leaking right now?

You can still inject it. Polyurethane foam reacts with water, so it works in a damp or actively wet crack, though the paste needs a dry surface around the crack to stick. Dry the face with a hair dryer or heat gun, or wait for a dry spell.

A crack that’s pouring water needs a temporary plug before the foam goes in. The leaking crack guide covers that, plus why water pressure in the soil pushes it through.

When does a crack need an engineer instead of a repair?

When the wall itself moved. Horizontal cracks, bowing, stair-step cracks in block, offset edges and cracks over 1/4 inch all point to soil pressure or settlement, and sealing them hides the evidence. A structural engineer decides what the wall needs.

ASHI’s guide calls movement of more than 1/2 inch potentially serious (opens in a new tab), and says a horizontal crack wider than 1/8 inch with 1/2 inch of movement needs to be addressed. The same guide says maintenance or simple repairs often head off major expenses, which is the case for catching it early.

• Sideways crack: horizontal crack in a basement wall
• Wall leaning in: bowing basement walls
• Steps along block mortar joints: stair-step and diagonal cracks
• Vertical but tapered or offset: vertical cracks

How do you know the repair worked?

Watch it through the next heavy rain and the next season change. A good repair stays dry, and the wall beside it doesn’t crack again. Mark the ends of any other cracks with a pencil and the date so you’ll see if they grow.

New cracking right beside a repair means the wall is still moving. The crack monitoring guide shows how to track width over time, and the NRMCA advises seeking professional advice (opens in a new tab) for cracks that keep widening.

Every guide in this section is listed on the foundation cracks hub, and every product is on the crack repair products page.

Every repair guide in this section

• Basement wall crack repair
• Leaking basement wall crack
• Foundation crack injection guide
• Injection troubleshooting