Quick Answer: Epoxy Restores Structure, Polyurethane Stops Water
Concrete crack repair epoxy Toronto homeowners search for is really two different jobs sold under one name. Epoxy injection bonds a dry crack back together and restores strength. Polyurethane injection seals a crack that is leaking and does not restore strength. Choosing the wrong one wastes the money, and the crack comes back. For commercial slabs and active joints, our concrete repair Toronto service uses flexible materials matched to how the crack behaves.
Quick answer: use epoxy injection when the crack is dry or damp and stable and the goal is to re-bond the concrete. Use polyurethane injection when water is actively seeping and the goal is to stop the leak. Use neither when the crack is still moving or the cause is settlement, because injecting a moving crack only moves the crack.
What this guide is, and is not: it explains injection as a standalone repair method for walls and slabs. It does not cover filling and grinding cracks as a prep step before coating, which our guide to concrete floor repair and cracks before epoxy already handles, and it does not repeat the moisture testing in our article on basement epoxy flooring and moisture vapour barriers.
| Crack condition | Epoxy injection | Polyurethane injection | Neither |
|---|---|---|---|
| Dry, stable, structural crack | Best fit. Re-bonds the concrete | Not needed | No |
| Damp crack, no flowing water | Possible with a moisture-tolerant epoxy | Possible if the goal is sealing only | No |
| Actively seeping or leaking | Poor fit. Water blocks the bond | Best fit. Reacts with the water and seals | No |
| Crack still opening and closing | Will re-crack | Flexible foam may hold a seal for a time | Find the cause first |
| Crack from settlement or a bowing wall | Cosmetic only | Cosmetic only | Call a structural engineer |
How Epoxy Injection Works
Epoxy injection uses a very thin, two-part resin that flows into a crack the way water wicks into a paper towel. Once it cures it is as strong as, or stronger than, the surrounding concrete in tension. The goal is to glue the two faces of the crack back into one piece.
The process follows the same sequence on a wall or a slab:
- Clean the crack. Dust, efflorescence and loose material are brushed and vacuumed out so the resin can reach the full depth.
- Seal the surface. A thick epoxy paste is applied over the crack face, which keeps the liquid resin from running back out.
- Set the ports. Small plastic injection ports are bonded along the crack, usually spaced about the same distance apart as the wall is thick.
- Inject from the bottom up. Resin goes into the lowest port until it appears at the next one. That port is capped and injection moves up the crack.
- Cure and remove the surface seal. After the resin cures, the ports are cut off and the paste is ground flush.
This works because epoxy is rigid. A rigid, strong resin is what you want when the crack is a loss of structural continuity, such as a shrinkage or load crack that has stopped moving. It is also the reason epoxy is the wrong choice for a crack that keeps opening and closing: a rigid repair cannot flex with the movement, so the concrete next to it cracks again.
Epoxy also needs the crack to be dry. Standard injection resin does not bond reliably to wet concrete, and flowing water can wash the resin out of the crack before it sets. Moisture-tolerant formulations exist for damp surfaces, but they are not designed for a crack that is visibly leaking.
How Polyurethane Injection Works
Polyurethane injection is a water-control repair. The resin is a liquid that reacts with water. When it meets the water in a leaking crack it foams, expands and fills the void, then cures into a closed-cell seal. The more water present, the faster the reaction, which is why this method is chosen for cracks that are actively seeping.
The injection steps are similar to epoxy: ports are set along the crack, and the resin is pumped in, but there are two differences worth knowing. First, the crack is often injected through a drilled port at an angle so the resin crosses the crack deep inside the wall rather than only at the surface. Second, the crack is often not surface-sealed with paste, because the resin needs the water and the crack face to react.
There are two broad families. Foaming polyurethanes expand strongly and are used where a lot of water is moving. Flexible gel-type polyurethanes expand less and stay elastic, which makes them better where a crack moves slightly with the seasons. Either way the result is a flexible waterproof plug, not a structural repair. If the crack was a structural crack, it is still a structural crack after polyurethane has sealed it.
That is the source of most of the confusion. A homeowner is told the crack was injected, the leak stops, and they assume the wall is repaired. The wall has been made watertight. Whether it has been made stronger is a separate question, and the answer with polyurethane is no.
How to Tell Which One Your Crack Needs
Four observations decide the method, and none of them needs special equipment. A contractor who recommends a product before looking at all four is guessing.
1. Is water actually moving?
Look at the crack after a heavy rain or spring melt, and again in dry weather. Active seepage, dripping or a damp stain that spreads after rain points to polyurethane. A crack that is dry or only stained from old leaks can usually take epoxy.
2. How wide is it?
Hairline cracks under about 0.3 mm (0.012 in) are often cosmetic and may not need injection at all. Epoxy injection typically suits cracks from hairline up to roughly 6 mm (about 1/4 in). Wider cracks usually need a different material or a cause investigation, because a crack that wide has moved.
3. Is it moving?
Crack monitors are inexpensive, and a simple pair of pencil marks across the crack, dated, tells you a lot. A crack that has not changed over a freeze-thaw season is stable. A crack that opens in winter and closes in summer is a movement crack, and a rigid repair will fail.
4. Where is it?
A vertical crack in a poured wall, a crack through a slab and a crack at a cold joint behave differently. The next section covers each. Location also tells you the likely cause, which matters more than the product.
Rule of thumb: water present, seal it with polyurethane. Dry and stable, bond it with epoxy. Moving or settling, fix the cause first. If you cannot tell which one applies, a short site visit and a few weeks of monitoring is cheaper than a repair that fails.
Wall Cracks, Slab Cracks and Cold Joints
The same product behaves differently depending on where the crack is, so the repair plan changes with the location.
| Location | Typical cause | Usual injection choice | Watch for |
|---|---|---|---|
| Vertical crack in a poured basement wall | Shrinkage as the concrete cured | Polyurethane if it leaks, epoxy if dry and stable | Cracks wider at the top or bottom, which suggest movement |
| Crack through a slab on grade | Shrinkage, curling or load | Epoxy or flexible polyurea if dry. Polyurethane for seepage | Vertical offset between the two sides |
| Cold joint in a wall | Pour interruption, a plane of weakness | Polyurethane for leaks | Repeat leaks that return after heavy rain |
| Wall and floor joint | Normal separation between two pours | Often sealed rather than injected | Hydrostatic water pushing up at the joint |
Wall cracks. Poured foundation walls in the Greater Toronto Area commonly show one or two vertical shrinkage cracks, often near the middle of a long wall or at a window corner. These are the classic injection candidates. If the crack leaks, polyurethane is the usual answer. If it is dry and the owner wants the wall re-bonded, epoxy is. Injection works from the inside, so it does not require digging up the exterior.
Slab cracks. Cracks in a basement or garage slab are often curling or shrinkage cracks. Epoxy can bond a stable slab crack, but a crack that is a control joint in disguise will reopen. In commercial floors with traffic, the crack is frequently filled with a flexible polyurea instead, which is the approach described on our concrete repair page. For a slab with seepage coming up from below, injection treats the symptom and the real fix is drainage or vapour control.
Cold joints. A cold joint is a seam where one pour met the next. It is not a crack in the usual sense, and it leaks because there is a path for water, not because the concrete has failed. Polyurethane injection is the usual fix, but the expectation should be a good seal, not a permanent one in every case.
When Injection Fails
Most failed injections trace back to one of three causes, and all three can be spotted before the work is booked.
- The crack was moving. A rigid epoxy bond cannot follow a crack that opens and closes with the seasons. The concrete next to the repair cracks, usually within a year or two. Polyurethane gel can tolerate small movement, but it is a seal, not a fix for the movement itself.
- The water source was never addressed. Sealing a crack on the inside pushes the water to find another path. If the soil outside is saturated, a downspout dumps beside the wall or the grade slopes toward the house, water will find the next weak point. A new damp patch beside a repaired crack is a drainage signal, not a bad repair.
- The crack was a settlement crack. A crack caused by the footing moving is the building telling you something. Injection will fill it, and the footing will keep moving.
Poor workmanship is the fourth cause, and it is less common than people assume. Ports spaced too far apart, a surface seal that is not fully cured before injection, and resin that was not allowed to reach the full depth all leave voids. A good contractor checks that resin appears at the next port before moving on, and records the quantity used against the expected volume.
Cracks in a slab that will later be coated also deserve a note: a crack that is sealed but still moving will telegraph through the coating. If you are planning a coating, the sequence and the crack-to-coating rules are covered in our guide on concrete floor repair before epoxy.
Structural Red Flags: Stop and Call an Engineer
Some cracks are not a repair-contractor question. They need a licensed structural engineer (P.Eng.) to assess the building before anyone injects anything. Injecting a structural crack without a diagnosis can hide the evidence the engineer needs.
- A horizontal crack in a basement wall, especially at mid-height, which can mean soil pressure is pushing the wall inward.
- A wall that bows or leans inward, or a crack that is wider at one end.
- Stair-step cracks in block or brick that follow the mortar joints.
- One side of a crack sitting higher than the other, called vertical displacement.
- Cracks wider than about 6 mm (1/4 in), or cracks that are visibly growing.
- Doors and windows that suddenly stick, or cracks that appear in several places over a short time.
This is not a reason to panic. Many cracks are shrinkage cracks with no structural meaning. But if any item above applies, an engineer's letter should come before a repair quote. Injection can follow the engineer's recommendation. It should not replace it.
What Injection Costs in Toronto and How It Is Priced
Crack injection is priced by the linear foot of crack, not by the square foot of floor or wall. Most contractors also apply a minimum charge, because the crew, equipment and setup time are the same whether the crack is 1.5 m (5 ft) or 6 m (20 ft). That is why a single short crack can feel expensive per foot, and a longer one cheaper.
We do not publish a per-foot injection figure on this page, because the number depends on the method and on access, and a figure quoted without seeing the crack would be a guess. What we can say honestly is what moves the price:
| Factor | Effect on price |
|---|---|
| Method (epoxy or polyurethane) | Material and port cost differ, |
| Crack length and depth | Thicker walls need more ports and more resin |
| Access | Finished basements, shelving and tight spaces add labour |
| Number of separate cracks | Each crack adds setup time, so several cracks cost less per foot than one |
| Active leaking | Water control adds time |
| Warranty terms | Watertight warranties cost more than workmanship-only terms |
Two quotes that differ widely are usually pricing two different things. One may be epoxy for a crack that leaks, which is cheaper now and costs more later, and the other polyurethane. Ask which resin is being used and why. A contractor who cannot explain the choice in two sentences has not looked at the crack.
For surface damage on commercial slabs, such as spalls and chipped joints, the repair work published on our concrete repair page is quoted separately from injection. A short site visit is the fastest way to a real figure, and the quote should name the resin, the crack length and what the warranty covers.
What to Do About Cracks Before Coating a Floor
If a floor crack is part of a basement or garage you plan to coat, decide the order of work before booking anything. A leaking crack needs water control first, because a coating on a wet slab can blister and peel. A dry, stable crack can be injected or filled, ground flush and then coated. A moving crack needs a flexible repair, and the coating system should be chosen to tolerate some movement.
For basements, the crack decision belongs alongside the moisture decision. Our basement epoxy floor Toronto page explains how we assess a slab before coating, and the moisture article linked above explains vapour testing. In short, resolve leaks, then cracks, then coat.
When you call a contractor, describe the crack in five facts: location, length, width, whether water has been present and whether it has changed in the last year. A photo with a coin or ruler beside the crack helps. Those five facts are enough to tell a legitimate quote from a sales pitch.
Questions to Ask Before You Hire
- Which resin are you using, and why is it right for this crack?
- Is the crack stable? How do you know?
- Are you addressing where the water comes from, or only the crack?
- How will you confirm the resin reached the full depth?
- Does the warranty cover leaks, structure or workmanship only?
- Do you recommend an engineer for any of the cracks you saw?
A straightforward contractor will answer all six and may talk you out of the job. If a crack looks structural, the best advice is often to hire an engineer first, and that is the advice we give.