Two homeowners in La Jolla can get the same recommendation, “inject the crack,” and mean completely different things. One proposal calls for epoxy. The other calls for polyurethane. Sometimes a single bid lists both for different cracks on the same wall. The two resins are not interchangeable brands of the same fix. They do different jobs, and most disappointing crack repairs come from using a good material for the wrong job.
This post explains what each resin is for, the two questions that usually decide between them, and what neither one can do. It is an education piece for La Jolla homeowners citywide: hillside, canyon, coastal, and mesa lots. It is not a do-it-yourself injection guide. For the commercial side of crack repair, meaning how we evaluate and scope it, see foundation crack repair. For vocabulary and monitoring before anyone injects anything, read our earlier post on hairline vs structural foundation cracks.
Injection is a repair technique, not a diagnosis. A crack that injects cleanly can still belong to an active settlement or drainage problem. A crack that looks dramatic can be dormant shrinkage that needs no structural work at all.
The short version
The U.S. Bureau of Reclamation’s Guide to Concrete Repair puts it about as plainly as anyone: epoxy resins are used to structurally rebond cracks that are dormant and relatively dry. Polyurethanes (and some related resins) are used to seal cracks and joints that leak water. The same guide adds that it is usually not possible to structurally rebond cracks that are leaking, dirty, or very wet.
So the first fork is simple to state and harder to answer: are you trying to make a cracked piece of concrete act as one piece again, or trying to stop water coming through it? Some cracks need both, sometimes in stages.
Epoxy: strong, stiff, and intolerant of movement
Epoxy cures into a hard, strong, stiff solid that bonds well to clean concrete. Reclamation’s guide notes that, properly applied, it can restore the original structural strength of cracked concrete. That stiffness is the point. It is also the catch.
- It doesn’t tolerate movement. Because the bond can be stronger than the surrounding concrete, a crack that keeps “working” tends to re-crack right next to the epoxy line. Reclamation’s wording is blunt: epoxy should not be used to rebond working cracks.
- It wants dry, clean cracks. Moisture-tolerant epoxies and special wet-crack techniques exist. They are specialized, and silt or water in the crack can reduce how well the epoxy bonds.
- It has a width window. Reclamation gives a typical injectable range of about 0.002 inch to 0.25 inch. Narrower cracks are hard to fill, and wider ones are hard to keep the resin in. Those are application limits for the material. They are not a severity scale for your foundation.
- There is a recognized specification. Rebonding epoxies are expected to meet ASTM C881, the standard for epoxy-resin bonding systems for concrete. ACI 224.1R points to C881 Type IV for structural crack repair.
The most important line in the American Concrete Institute’s crack-repair guide (ACI 224.1R) is not about chemistry. Unless the cause of the cracking has been corrected, new cracks will probably form near the original one. If the cause can’t be removed, ACI describes three options: treat the crack as a joint (rout and seal it), build in a joint that accommodates movement, or add support or reinforcement so the crack stops moving.
Polyurethane: built for water
Polyurethane injection resins are mostly about water. Many react with water to foam or gel, which is why they are a natural fit for damp or actively leaking cracks. Reclamation lists several traits worth knowing:
- Low strength for most types. Except for two-part solid polyurethanes, these systems should not be used to structurally rebond cracked concrete.
- Some movement tolerance. Flexible foams and semi-flexible solids can follow small movements that would break an epoxy bond, within limits.
- Minimum width. Reclamation suggests polyurethane injection for cracks no narrower than about 0.005 inch. Very fine leaking cracks may call for other low-viscosity resins.
- Confinement matters. Expanding resin works best when it can’t freely expand out of the crack.
- No equivalent standard. There is no specification for polyurethane crack-injection resins comparable to ASTM C881 for epoxy, and properties vary widely between products. Reclamation warns that “cookbook” use is likely to fail.
One more distinction trips people up. “Polyurethane foam” also shows up in ads for lifting sunken slabs by injecting foam underneath them. That is a different technique with a different purpose. Polyurethane crack injection seals a crack. It does not lift anything.
The two questions that usually decide
1. Is the crack active or dormant?
The International Concrete Repair Institute defines an active crack as one whose cause is still at work, a crack that is still moving. A dormant crack is unlikely to become active, or its movement is too small to affect a repair.
That single distinction drives most material choices. ACI guidance on leaking cracks groups the typical responses to active cracks as chemical-grout injection, routing and sealing with a flexible sealant, or a flexible barrier. It calls these serviceability repairs, not structural ones. Epoxy rebonding fits dormant cracks.
You usually can’t tell active from dormant in one visit or from a phone photo. Dated photos with a scale, simple crack gauges, and notes about seasons, irrigation changes, and companion symptoms all help. Our hairline vs structural cracks post covers monitoring you can do safely.
2. Is the crack wet or dry, and is water the complaint?
If water is coming through a stem wall or a wall that retains soil, the homeowner’s goal is often leak control. Polyurethane is built for that. If the crack is dry and the concern is that a concrete element has lost continuity, epoxy may be the candidate, but only after someone has looked at why it cracked.
On hillside and canyon lots, walls that hold soil on one side are where leak questions show up most. Those leaks usually trace back to how water reaches the wall. Grade, downspouts, irrigation, and subsurface drainage belong in the conversation; see foundation drainage.
What injection can’t do
ASTM D8109, the standard guide for waterproofing repair by chemical-grout crack injection, says something every homeowner should hear before signing: injection does not affect the source of a leak. It blocks water at that spot. The water is diverted, and it commonly appears somewhere else. The same guide notes that cracks in below-grade walls may be a sign of structural distress and should be assessed before choosing a non-structural repair.
Put plainly:
- Polyurethane won’t make a cracked wall strong again.
- Epoxy won’t stop a foundation that is still settling. It tends to move the crack over a few inches.
- Neither resin fixes the drainage, soil, or support problem that may have caused the crack.
- Reclamation notes that injection is not normally used on shallow cracks, drying-shrinkage cracks, or pattern cracking. Sealing approaches usually work better for those.
When cracks widen over months, show offset or “lipped” edges, or come with sloping floors and racked doors, the conversation often moves from crack treatment to support. That is where foundation settlement repair and foundation underpinning come in. Our companion Day 8 post, underpinning explained for homeowners, walks through what that kind of project means for you and the house.
Where this shows up in La Jolla homes
La Jolla is a City of San Diego community planning area with a mid-century-heavy housing stock. Census ACS 2024 five-year estimates put ZCTA 92037 at roughly 19,688 housing units, with the largest year-built group in the 1970s. Census data doesn’t report foundation type, so nobody can honestly tell you what share of local homes have stem walls versus slabs.
What we can say is where injection questions tend to come up: perimeter stem walls on raised foundations, walls that retain soil on sloped lots, and garage or partial-height below-grade walls on hillside parcels. Slab-on-grade cracks are a different conversation. Many are shrinkage or curling issues where injection is the wrong tool.
Parcel overlays matter too. The City’s Seismic Safety Study hazard categories, Alquist-Priolo strands of the Rose Canyon Fault Zone (the La Jolla quadrangle maps became official on September 23, 2021), Coastal Overlay, and steep-hillside rules can all change whether geotechnical review belongs in the picture before a “simple” injection bid. A crack is not evidence of fault rupture. A mapped hazard just means your parcel may deserve a closer look if the repair turns out to be structural.
Permits and who decides what
The San Diego Municipal Code generally requires a Building Permit to alter or repair regulated structures (SDMC §129.0202), with narrow exemptions in §129.0203. Structural foundation repair generally needs a permit. Whether a particular sealing job falls under an exemption depends on scope, and the Building Official decides that, not a bid and not a blog. Be wary of anyone who promises “no permit needed” before defining what the work actually is.
Roles stay separate in California. Deciding whether a crack is structural, and designing a structural fix, is engineering practice under the Board for Professional Engineers, Land Surveyors, and Geologists. Construction is done under the right Contractors State License Board classification for the scope. Concrete work is commonly in the C-8 classification, and structural foundation work can involve others. La Jolla Structural evaluates cracks on site and repairs them when the diagnosis supports it. We coordinate with licensed engineers as needed when stamped design is required. We don’t stamp plans ourselves.
Questions to ask before you sign an injection proposal
Write the answers down. They make bids comparable.
- What diagnosis came first? Who looked at the crack, and what do they think caused it? Is it judged active or dormant, and how was that decided?
- What is the goal for each crack? Structural rebonding, leak sealing, or both?
- Which material, exactly? For epoxy, does it meet ASTM C881, and which type? For polyurethane, which product, and can you see the data sheet?
- How will it be installed? Surface ports or drilled ports? How do they confirm the resin went through the wall thickness?
- What about water showing up elsewhere? If leak sealing diverts water, what is the plan for the source: grade, gutters, irrigation, drains?
- What happens if the crack moves again? Is monitoring recommended? Is a support evaluation recommended if movement continues?
- Permits and roles. Will this scope need a permit? If structural, who is the engineer, and who pulls the permit?
Proposals that skip the first question are the ones to slow down on.
Cost factors only
This site doesn’t publish injection prices or per-foot rates. Cost depends on factors like the number, length, and width of cracks, wall thickness, access (crawl-space clearance, finished walls, landscaping), whether cracks are wet or actively leaking, the material specified, any drainage work that should come first, engineering and permit needs, and whether the visible crack is part of a larger settlement scope. Anyone quoting a firm number from a photo is guessing. For more on how scope drives cost, see what drives foundation repair cost.
Get a Free Quote
If you are holding an injection proposal and aren’t sure whether it matches the crack in front of you, we evaluate La Jolla homes citywide and can help you sort sealing from structural repair from support work. Call (650) 739-6537, or request online.
Related evaluations & services
- Foundation crack repair: how we evaluate and scope crack repair
- Foundation cracks: symptom guide
- Foundation underpinning: when support, not sealing, is the issue
- Foundation settlement repair: movement mechanisms
- Foundation drainage: the water side of leaking cracks
- Companion post: Underpinning explained for homeowners
Sources (accessed October 2, 2026)
- U.S. Bureau of Reclamation, Guide to Concrete Repair, 2nd ed. (2015), Ch. D §4 Crack and Water Leak Repairs: epoxy resins; polyurethane and methacrylic acrylate resins
- American Concrete Institute, ACI 224.1R-07, Causes, Evaluation, and Repair of Cracks in Concrete Structures, §3.2 Epoxy injection
- ASTM D8109, Standard Guide for Waterproofing Repair of Concrete by Chemical Grout Crack Injection (scope)
- ASTM C881/C881M, Epoxy-Resin-Base Bonding Systems for Concrete (by reference)
- International Concrete Repair Institute, Concrete Repair Terminology (active and dormant crack)
- San Diego Municipal Code §129.0202–§129.0203
- City of San Diego Seismic Safety Study; California Geological Survey Alquist-Priolo program
- U.S. Census ACS 2024 5-year, Table B25034, ZCTA 92037