How to Repair Concrete Cracks (Step by Step)

A concrete crack looks like a single problem with a single fix, so the instinct is to grab a tube of filler and squeeze. The trouble is that a hairline shrinkage line in a garage floor and a widening crack in a foundation wall are not the same job at all, and the filler that quietly protects the first can dangerously disguise the second. Most cracks in a slab, a driveway, or a patio are cosmetic and genuinely worth repairing yourself, but the repair only holds when you match the method to the crack: read what the crack is telling you, clean and open it so the filler has something to grip, choose a product sized to its width and its movement, and seal the surface afterward so water never gets a second chance at it.

This walkthrough repairs a concrete crack in plain steps, from the honest assessment that decides whether you fill it or phone a professional, through cleaning, routing, filling, curing, and sealing, with a worked example carried the whole way. It leans on the cleaning and weather discipline from our how to seal a concrete driveway manual and the mixing method from our how to mix concrete manual, and narrows both to the single job of fixing a crack. For the other half of the story, why the crack formed at all, our crack causes explainer works through the shrinkage, settlement, and structural mechanics behind every line this walkthrough repairs. Run your crack length and width through the crack repair estimator in about a minute, and the companion below sizes the filler and flags the structural cases against your own numbers as you read.

Before you start: tools, materials, time, and difficulty

Repairing a concrete crack is one of the more forgiving concrete jobs, because unlike a fresh pour there is no setting clock forcing your hand, but it still rewards a clean setup and an honest first look. Before anything is opened, four things need settling: what the crack actually is, the tools and filler suited to it, the time across a day or two of cure, and the safety gear for grinding and mixing. The single most common way a crack repair goes wrong is filling the wrong crack, so the first real task is not a tool at all, it is the assessment in Step 1.

What to settle and gather:

• The filler, matched to the crack. A thin masonry crack caulk or self-leveling sealant for hairline cracks, a sanded concrete patch or a flexible self-leveling polyurethane for medium cracks, and a two-part epoxy or heavy-bodied patch plus foam backer rod for wide, deep cracks. Buy the product after you have measured the crack in Step 2, not before.
• Cleaning and routing tools. A stiff wire brush, a cold chisel and hammer or an angle grinder with a masonry wheel to chase the crack open, a shop vacuum and compressed air or a blower to clear the dust, and a bottle of water or degreaser for any staining. A crack full of dust or oil is a crack the filler will not stick to.
• Application and finishing tools. A caulk gun for cartridges, a margin trowel or putty knife for patch material, a mixing bucket and paddle or a mixing tub for two-part or bagged products, backer rod for deep cracks, and painter’s tape to keep edges clean. A concrete sealer for the final step.
• Safety gear. Eye protection and a proper dust respirator for grinding, gloves for the filler, and good ventilation. Concrete dust and many two-part fillers are hazardous, so read every label before you start.

Time and difficulty: a single crack is often an hour or two of hands-on work, with the cure then running from a few hours to a day or more depending on the product and depth, so plan the sealing for after the fill has fully set. The active skill is patience and honesty: routing and vacuuming thoroughly, sizing the filler to the width, and being willing to stop and call a professional the moment the crack looks structural. It is a genuinely beginner-friendly repair for cosmetic cracks, and a job to hand off entirely for structural ones.

Step 1: Identify the crack type before anything else

Before you buy a thing, settle the one question that changes everything: is this a cosmetic crack you can fill, or a structural crack you should not touch until a professional has looked at it. Filling the wrong crack is worse than doing nothing, because a smooth patch over a moving foundation crack hides the very movement someone needs to see, and it buys false comfort while the real problem grows. So read the crack before you reach for a tool.

Cosmetic cracks are the common, fillable kind. Hairline and shallow surface cracks in a slab, driveway, patio, or garage floor usually come from concrete shrinking as it cured or from minor surface stress, they are narrow, they do not run deep, and the two sides sit flush at the same height. These are exactly the cracks this walkthrough is built to repair. Structural cracks are the warning kind: they are wider (a quarter inch is a commonly cited threshold), they may show vertical displacement where one side of the crack sits higher than the other, they run through foundation walls or footings, they keep widening over weeks, or they arrive alongside doors that suddenly stick, floors that slope, or stair-step cracks climbing through block or brick.

Watch out for treating a symptom as the whole story. A crack that keeps coming back after a proper repair, a crack that leaks water into a basement, or a crack paired with any of the movement signs above is telling you the cause is still active. In those cases the honest move is not a better filler, it is an assessment by a concrete or foundation professional, covered in Step 8. If the crack is narrow, flush, shallow, and not growing, you almost certainly have a repair you can finish this weekend, and the rest of these steps are for you.

Step 2: Measure the crack width and depth

With the crack judged cosmetic, measure it, because the width is the single number that picks your filler, and the depth decides whether you need backer rod. Guessing here is how people end up with caulk that sags into a deep crack or a rigid epoxy in a slab that still moves. A tape measure, a ruler, or even a coin held against the crack is enough to sort it into a band, and the bands are what the product aisle is really organized around.

Sort the width into three practical ranges. A hairline crack is roughly under 1/8 inch, about the width of a credit card edge, and needs only a thin, flexible filler. A medium crack runs from about 1/8 to 1/2 inch and wants a sanded patch or a self-leveling flexible product. A wide crack is over about 1/2 inch, deep enough that filler alone would sink in, so it needs foam backer rod pressed in first to support the fill and control how much product you use. Check the depth too by running a stiff wire or a screwdriver into the crack: a shallow surface crack fills directly, while a deep one benefits from backer rod regardless of width.

Watch out for measuring at the widest point only. A crack often varies along its length, wider in the middle and tapering at the ends, so note the widest section and buy for that, and measure the total running length while you are there, because that length is what sizes how many cartridges or how much patch you need. The companion below turns your length and width band straight into a filler pick and a cartridge count.

Step 3: Clean and rout out the crack

This is the step that decides whether the filler bonds or lifts back out, and the one most often skipped because it feels like it is not the real repair: clean the crack completely and, for anything past a fine hairline, rout it slightly open before a drop of filler goes in. Filler bonds to clean, sound concrete and cannot grip dust, loose crumbs, or oil, so any debris left in the crack becomes the spot where the repair fails first. Treat the cleaning as the main event, because in effect it is.

Open the crack so the filler has something to hold. A raw surface crack is often narrower at the top than deeper inside, an upside-down shape that gives filler almost nothing to grab, so chasing it open with an angle grinder and a masonry wheel or a cold chisel and hammer to cut a slightly wider, cleaner channel, commonly a shallow V or square groove, gives the filler real depth and surface to bond to. Very fine hairline cracks that will take only a thin flexible caulk can often skip routing, but any crack you want a lasting structural-feeling fill in benefits from it. Wear eye protection and a dust respirator, because cutting concrete throws hazardous silica dust.

Then get every trace of dust and loose material out. Brush the crack with a stiff wire brush, then vacuum it with a shop vacuum and blow it out with compressed air or a blower until nothing loose remains, and clean any oil or grease with a degreaser and let it dry. A crack that looks clean can still hold a film of fine dust that stops the filler bonding, which is why the vacuum-and-blow step is not optional. The same lesson our how to seal a concrete driveway manual makes about sealing applies here: the surface, not the product, decides the result.

Watch out for filling a damp crack unless the product is made for it. Many fillers need a dry crack to bond, so if you rinsed the crack or it rained, give it time to dry, and confirm the wet-or-dry requirement on the label before you fill.

Step 4: Choose the filler by crack size

With the crack measured and cleaned, choose the filler, because this single call shapes how the repair looks, how strong it is, and whether it survives the next season of the slab moving. There is no universally best product, only the right match for the width you measured in Step 2 and whether the slab still moves, and the common families behave very differently. Match the filler to the crack first and the finish second.

For hairline cracks under about 1/8 inch, a thin masonry crack caulk or a self-leveling sealant is usually all you need: it flows into the fine crack, flexes with small movement, and needs no routing. For medium cracks up to about 1/2 inch, a sanded concrete patch fills and blends to match the slab, while a flexible self-leveling polyurethane is a strong pick if the slab still shifts, because it stretches instead of cracking again. For wide cracks over about 1/2 inch, pack the depth with backer rod first, then fill with a two-part epoxy or a heavy-bodied patch, since a deep gap needs support and far more material than caulk alone. The epoxy-versus-polyurethane call comes down to movement: epoxy cures rigid and bonds the two sides with high strength for a dormant crack, while polyurethane stays flexible for a crack that still moves.

Watch out for buying on strength alone. A rigid epoxy sounds like the toughest choice, but in a slab that still expands and contracts it can simply crack again beside the repair, while a flexible polyurethane moves with the slab and holds. Treat every product name, width threshold, and cure figure here as illustrative, and confirm the specific filler against the manufacturer instructions for your crack, your slab, and your climate before you buy.

Step 5: Apply the filler into the crack

Now the visible part, and the rule that governs it: work the filler into the full depth of the crack, not just across the top, so there are no voids underneath to collapse later. A bead smeared over the surface looks done and fails fast, because the crack under it is hollow, while filler packed down into a clean, routed channel bonds along its whole depth and holds. Mask the edges with painter’s tape first if you want a clean line, and mix any two-part or bagged product exactly to the label, the same care our how to mix concrete manual brings to a batch.

Match the technique to the filler. For a caulk or self-leveling sealant, cut the nozzle small, run the tip along the bottom of the crack, and squeeze steadily so the filler rises from the depth up, then tool it flush with a putty knife or a gloved finger. For a patch or epoxy, press the first material hard into the base of the crack with a margin trowel to drive out air, then build up in layers to the surface, packing each one down. For a wide crack, seat the backer rod at the right depth first so the filler forms over support rather than sinking into the void. Slightly overfill, because most fillers shrink a little as they cure, then strike it level.

Watch out for tooling the surface before the depth is full. It is tempting to smooth a pretty top over a half-filled crack, but that traps a void that later cracks or sinks the repair, which is the single most common finish failure. Fill from the bottom up, pack out the air, overfill slightly, then strike it flush, and the repair is as sound below the surface as it looks above it.

Step 6: Let the repair cure fully

The filler is in, but the repair is not finished until it has cured, and rushing this is how a good fill gets pulled loose before it ever had strength: let the product cure fully before you seal it, drive on it, or put any load on it. Cure time varies widely by product, depth, and weather, so treat any figure as illustrative and take it from the label, but the pattern is consistent: a thin flexible caulk may skin over in about an hour and handle light foot traffic the same day, while a two-part epoxy or a deep patch can need a full day or more to reach real strength. Deeper fills and cooler, damper air both slow the clock.

Give a deep or high-strength repair the time it actually needs. A patch that feels firm on top can still be soft underneath, so follow the label’s separate times for surface skin, foot traffic, and full load rather than one number, and lean long when the crack was deep or the weather is cool. Keep water off the fresh repair during the cure unless the product is made to cure wet, because moisture at the wrong stage can weaken or discolor many fillers. Shade a repair curing in hot direct sun so it does not skin over too fast and crack.

Watch out for stressing the repair early. Driving a car onto a garage-floor or driveway crack repair, dragging furniture across a patio patch, or sealing over a fill that has not set can all break the bond before it forms. The cure is passive and unglamorous, but it is where the repair earns its strength, so plan the sealing step for the next day rather than the same hour.

Step 7: Seal the surface to protect the repair

The crack is filled and cured, but the job is not truly done until you keep water from starting the next one: seal the repaired area, and ideally the whole slab, so water, salt, and freeze-thaw stay out of both the repair and the surrounding concrete. Water is what turns a small crack into a big one, seeping in, freezing, expanding, and levering the crack wider each winter, so a sealer over a fresh repair protects the work you just did and slows new cracks from forming nearby. This is where a crack repair and a routine seal become the same maintenance habit.

Choose a sealer suited to the surface and let the repair guide the timing. A penetrating sealer soaks in and protects from within with no film, while an acrylic leaves a light protective sheen, and the choice follows the same logic our how to seal a concrete driveway manual lays out in full. Wait until the crack filler has fully cured before sealing over it, because sealing a soft repair can trap solvent or cloud, then clean the surface, pick a dry mild day, and apply the sealer thin and even across the repaired area. On a driveway or patio it is worth sealing the whole slab rather than a patch, so the color and protection are uniform.

Watch out for skipping the seal on an outdoor slab. An unsealed repair in a driveway or patio sits exposed to exactly the water and freeze-thaw that opened the crack in the first place, so it is the outdoor repairs, not the sheltered garage ones, where sealing earns its keep the fastest. A quick reseal on the slab’s normal schedule keeps the repair, and the concrete around it, protected for years.

Step 8: Know when to stop and call a pro

The most important step in a crack repair is knowing which cracks are not yours to fill, so this is the honest stopping point: if the crack shows any sign of being structural, moving, or tied to a foundation, put the filler down and get a professional assessment before you spend another dollar. Filling a structural crack does not fix it, it hides it, and a smooth cosmetic patch over an active problem can delay the exact inspection that would have caught it early, which is the expensive way to learn this lesson.

Call a professional when the crack is wide (commonly cited around a quarter inch or more), when one side of the crack sits higher than the other so the slab has displaced vertically, when the crack runs through a foundation wall or footing, when it keeps growing over weeks, when it leaks water into a basement or crawlspace, or when it arrives with doors that stick, floors that slope, or stair-step cracks in block or brick. Widespread spalling, flaking, or a heaved slab is also past a filler job. A concrete or foundation professional can tell settling from a serious structural problem and prescribe the real fix, whether that is underpinning, slab jacking, or drainage work, none of which comes out of a caulk tube.

Watch out for talking yourself into a fill because it is cheaper and faster today. The cost of a wrong call is not the tube of filler, it is the structural damage that keeps compounding behind a patch that looked fine. When a crack is cosmetic, the earlier steps make it a satisfying weekend repair; when it is structural, the bravest and cheapest move is to stop, and the companion below flags the structural cases so you know which job you actually have before you buy.

A worked example: repairing a cracked garage floor

Pull the eight steps together on one realistic job, a garage floor with a single crack running about 20 feet across the slab, roughly 1/4 inch at its widest and tapering to a hairline at the ends, with both sides flush at the same height and no sign the slab has moved in years. Watch the plan flow from an assessment to a filler count and a sealed, finished floor.

Step 1, identify: the crack is flush, shallow, stable, and nowhere near a foundation wall, so it reads as a cosmetic shrinkage crack, a fillable job rather than a structural one. Step 2, measure: the widest section is about 1/4 inch, putting it in the medium band, and the total running length is about 20 feet. Step 3, clean and rout: the crack gets chased open with a grinder into a clean channel, brushed, vacuumed, and blown out until no dust remains. Step 4, choose: because a garage slab still expands and contracts a little with temperature, the owner picks a flexible self-leveling polyurethane over a rigid epoxy, so the repair can move with the floor.

Step 5, apply: at an illustrative 20 linear feet per 10 ounce cartridge for a medium crack, 20 feet is about one cartridge, so the owner buys two to be safe, runs the nozzle along the base of the crack, fills from the bottom up, and strikes it level. Step 6, cure: the repair is left a full day, kept dry, with no car on it. Step 7, seal: once cured, the owner seals the floor so water and any road salt tracked off tires stay out. Step 8, the pro check never triggers, because nothing about this crack was structural.

So one measured crack produces a clean plan: about one to two cartridges of flexible filler, an hour of prep and fill, a day of cure, and a seal. Change any input and it moves predictably: a wider crack needs backer rod and more product, a moving crack rules out rigid epoxy, and any sign of displacement sends the whole job to Step 8 instead. Run your own crack length and width through the crack repair estimator and the filler pick and cartridge count are done for you.

How much crack filler a job needs

It is easy to buy one tube of filler and discover halfway along a driveway crack that you are short, so sizing the product to the crack before you start saves the second trip and the half-cured mess of stopping midway. The amount comes down to the running length of the crack and its width, because a wider crack swallows far more filler per foot than a hairline, and a deep crack needs backer rod to keep you from pouring product into a void.

Roughly, at an illustrative rate for a medium crack of about 20 linear feet per 10 ounce cartridge, the cartridge count climbs straight with the length of crack you are filling. A short 20 foot crack is about one cartridge, a 40 foot run about two, and a long driveway crack of 100 feet closer to five, before you add a spare. A hairline crack stretches a cartridge much further because the bead is so thin, while a wide crack burns through product fast and is where backer rod earns its place by filling the depth you would otherwise pay for in filler.

The chart is illustrative, and the real coverage swings with the bead size and the crack width, so confirm the figure on your product before you buy. The point is to buy from the measured length rather than a guess, add a spare, and never stop a fill halfway because you ran out mid-crack, which leaves a cold joint the repair never fully closes.

Where the effort goes in a crack repair

It is tempting to picture a crack repair as mostly the filling, the satisfying part with the caulk gun, but the filling is a small slice of the real effort, and seeing where the work actually goes helps you plan the job honestly. A crack repair is far more assessing and cleaning than squeezing, which is exactly why the crack, not the filler, decides the result.

Roughly, the assessment and measuring are a real share of the effort even though they use no product, because reading the crack right is what keeps you from repairing the wrong thing. The cleaning and routing, the grinding, brushing, vacuuming, and blowing out, is the single biggest share of the hands-on work, since a crack the filler cannot bond to is a repair that fails. The filling itself is a modest, steady share, the finishing and tooling a little less, and the sealing at the end another share that protects everything before it. The cure then runs passively for hours to a day after all the active work is done.

The chart makes the lesson plain: if you budget your time around the filling, you will underestimate the cleaning, which is most of the work, and be tempted to cut it short, which is exactly the shortcut that makes a repair peel back out. Plan the job around reading and prepping the crack, and the filling itself becomes the quick, satisfying part it should be.

Common mistakes when repairing concrete cracks

A handful of errors account for nearly every crack repair that peels, sinks, or reopens, and almost all of them are decisions made before or during the job, not accidents later.

• Filling a structural crack. Sealing a moving or foundation crack hides the cause instead of fixing it and delays the assessment that would catch it early. Read the crack first, and hand any structural sign to a professional.
• Skipping the routing and cleaning. Filler squeezed onto a dusty, unrouted hairline has nothing to grip and lifts back out. Chase the crack open where needed, then vacuum and blow out every trace of dust before filling.
• Choosing the wrong filler for the movement. A rigid epoxy in a slab that still shifts cracks again beside the repair, and a thin caulk in a wide deep crack sags into the void. Match the product to the width and the movement.
• Filling only the surface. A bead smeared across the top leaves a hollow crack underneath that later sinks or cracks. Fill from the bottom up, pack out the air, and overfill slightly before striking level.
• Rushing the cure. Driving on, sealing over, or loading a repair before it has set pulls the bond loose before it forms. Follow the label’s separate skin, foot-traffic, and full-load times, and lean long on deep fills.
• Never sealing the surface. An unsealed outdoor repair sits exposed to the water and freeze-thaw that opened the crack in the first place. Seal the cured repair, and ideally the whole slab, to keep water out.

Every one of these is a decision about the crack, the prep, the product, the fill, the cure, or the sealing, which is exactly why they get skipped and exactly why they decide whether the repair lasts.

Troubleshooting: cracks that reopen, sink, or spread

Real cracks rarely match the tidy example, so here is how to handle the conditions that complicate them.

What if the repaired crack reopens? A fill that cracks again along the same line usually means the slab is still moving and a rigid filler could not stretch with it, or the crack is structural and the force behind it never stopped. The first fix is a flexible filler such as a polyurethane that moves with the slab; the second, if a flexible repair also reopens, is to treat the recurrence as the signal it is and get a professional assessment, because a crack that keeps returning after a proper repair is telling you the cause is active.

What if the filler sank or left a low, hollow line? A repair that dips below the surface as it cured was usually underfilled, filled only at the top over a void, or made with a product that shrank more than expected. The fix is to clean the low channel and add another layer, filling from the depth and overfilling slightly this time, then striking level once it firms. Prevention is the real answer: pack the crack full from the bottom, use backer rod in deep cracks, and overfill to allow for shrinkage.

What about a crack that leaks water or keeps spreading? A crack that weeps water into a basement or crawlspace, or one that visibly lengthens and widens over weeks, is past a cosmetic repair. Sealing the surface will not stop water finding a moving crack, and filling a spreading crack only masks it, so these belong with a professional who can address the drainage or structural cause. The companion below flags the structural cases, and our how to seal a concrete driveway manual handles the surface protection once a sound crack is repaired.

Your concrete crack repair checklist

Before the filler is bought and before the first bead goes down, run down this compact checklist. It is the save-this asset of the whole job.

• Assessed the crack honestly and confirmed it is cosmetic, not structural, moving, displaced, or on a foundation.
• Measured the widest width and sorted it into a band, hairline, medium, or wide, and measured the total running length.
• Bought a filler matched to the width and the movement, plus backer rod for any deep crack and a spare cartridge.
• Chased the crack open where needed, then brushed, vacuumed, and blew out every trace of dust and cleaned any oil.
• Confirmed the crack is dry, or the product is rated for a damp crack, before filling.
• Filled from the bottom up, packed out the air, seated backer rod in deep sections, and overfilled slightly before striking level.
• Let the repair cure fully to the label’s times, kept it dry, and put no load on it during the cure.
• Sealed the cured repair, and ideally the whole slab, on a dry mild day to keep water and freeze-thaw out.
• Set a note to watch the repair, and to call a professional if it reopens, grows, or leaks.

Work top to bottom and the crack you repair is one that was correctly judged, cleanly prepped, properly filled, fully cured, and sealed, which is the difference between a repair that protects the slab for years and one that peels back out by the next season. The companion below sizes the filler on your own crack, so you can buy right in one trip.

The bottom line

Repairing a concrete crack is eight steps worn into a sequence: identify the crack type, measure it, clean and rout it, choose a filler matched to its width and movement, apply it from the bottom up, let it cure, seal the surface, and know when to stop and call a professional. The whole job rests on a truth that is easy to rush past, that the assessment matters more than the filler, because a smooth patch over a structural crack is worse than no patch at all. For the common cosmetic crack in a slab, driveway, patio, or garage floor, the repair is a satisfying weekend job when you prep the crack properly, size the filler to the width, pick flexible for a moving slab and rigid for a dormant one, cure it fully, and seal it against the water that would otherwise reopen it. Read the crack first, fill the ones that are yours to fill, and hand the structural ones to someone who can see the cause, which is the cheapest insurance a slab ever gets.

Read this walkthrough as bench notes meant to teach the method, not as a product spec you can follow without checking the label. The filler types, width thresholds, coverage rates, and cure times here are typical illustrative values, and your real numbers shift with the specific product, the crack, the slab, and the climate. This walkthrough covers cosmetic crack repair only: any crack that is structural, moving, displaced, growing, leaking, or connected to a foundation is outside its scope and should be assessed by a qualified concrete or foundation professional before you attempt a repair. Measure your own crack, confirm the filler, coverage, and cure time against the manufacturer instructions, and consult a professional whenever the crack shows any sign of movement or structural cause.