Short answer
You can fix most small vertical cracks in a poured concrete foundation by cleaning the crack and performing a low-pressure injection with epoxy (for structural bonding) or polyurethane (for stopping leaks). Address exterior drainage to prevent future movement. If the crack is horizontal, wider than 1/8 inch, part of a bowed wall, or linked to settlement, bring in a structural professional—reinforcement or underpinning may be required.
Understand the crack before you repair
Not all cracks are equal. Map and measure the crack, check for moisture, and watch for movement.
Crack width guide
- Hairline: ≤ 1/32 in (0.8 mm)
- Minor: 1/32–1/16 in (0.8–1.6 mm)
- Moderate: 1/16–1/8 in (1.6–3.2 mm)
- Major: > 1/8 in (> 3.2 mm)
What the pattern suggests:
- Vertical crack in poured concrete: common from shrinkage; often non-structural. Usually injectable.
- Diagonal crack from corner of window/door: could indicate settlement or stress concentration; evaluate carefully.
- Stair-step in block: more concerning; may relate to lateral soil pressure or footing movement.
- Horizontal crack: red flag for bowing/lateral pressure; get a pro.
Check for movement by marking each end with a pencil date line or installing a crack gauge; recheck in 4–8 weeks. If widening or leaking under rain, prioritize drainage and consider pro assessment.
Best DIY fix for a typical poured wall crack: low‑pressure injection
For stable vertical cracks up to ~1/8 inch in poured concrete, a DIY injection kit is effective.
Tools and materials
- Dual-cartridge epoxy or polyurethane injection kit (cartridges, static mixers, surface paste, injection ports)
- Dual-cartridge applicator gun (300/300 ml or as required)
- Angle grinder with diamond cup wheel or wire brush
- Shop vac (with HEPA if grinding)
- Painter’s tape, putty knife, utility knife
- Safety: gloves, eye protection, N95/respirator, hearing protection
- Optional: heat gun (cool temps), masonry crack gauge
Cost: $80–$300 per crack for a DIY kit; a pro typically charges $400–$800. Time: 3–5 hours including cure time.
Epoxy vs. polyurethane
- Epoxy: Bonds the concrete and restores strength. Crack should be mostly dry. Use for structural re-bonding.
- Polyurethane (urethane foam): Expands to stop active leaks; tolerates moisture. Use to stop water infiltration. For structural re-bonding, inject epoxy after the leak is controlled (often on a dry day).
Step-by-step
- Prep the surface
- Expose the full crack. Remove paint/efflorescence. Lightly grind or wire-brush to bare concrete; vacuum clean. Dry the surface.
- Install ports
- Space surface ports 8–12 inches apart along the crack, starting at the bottom. Bond with the supplied surface paste. Use painter’s tape to keep paste neat. Bridge the crack with a 1–2 inch wide band of surface paste to seal between ports.
- Allow paste to cure
- Typically 30–60 minutes. Check manufacturer temperature ranges (often 50–90°F). Warm the area if needed with a heat gun on low.
- Inject from the bottom up
- Start at the lowest port. Slowly dispense until material appears at the next port; cap the lower port and move upward. Keep pressure low (hand pressure with a dual-cartridge gun is ideal) to avoid blowouts.
- Finish
- After full cure (usually overnight), remove ports, shave paste flush with a putty knife, and touch up with masonry sealer or waterproofing paint if desired.
Tip: If water is weeping, begin with polyurethane. Once dry, you can re-seal the surface and follow with epoxy on a later day for structural bonding.
Special cases
- Concrete block (CMU) walls: Hollow cores complicate injection. Surface patching doesn’t solve lateral pressure. For minor stair-step cracks, repoint mortar and add interior reinforcement (carbon fiber straps every 4–6 feet) to address bowing. Persistent leaks call for exterior waterproofing and improved drainage.
- Bowing or horizontal cracks: Carbon fiber straps or steel I-beams installed by a pro can stabilize the wall. If bow exceeds ~1/2 inch, expect engineered solutions.
- Settlement cracks: If doors stick, floors slope, or cracks spread diagonally from corners, consult a structural engineer. Underpinning with piers may be needed.
Fix the water and soil pressure
Even a perfect injection won’t hold if water is pushing on the wall.
- Extend downspouts 6–10 feet away; clean gutters.
- Grade soil to slope away at ~1 inch per foot for 6–10 feet.
- Consider a perimeter drain/sump in wet areas.
- Exterior waterproofing (pro-level): Excavate to the footing, patch, apply rubberized membrane, add drain board, and install a perforated drain to sump or daylight. Labor- and equipment-heavy; call 811 before digging and follow trench safety rules.
Safety
- Wear eye protection, gloves, and a respirator when grinding or handling chemicals. Silica dust is hazardous.
- Keep the area ventilated; epoxy and urethane produce fumes.
- Never work in unshored trenches deeper than 4 feet. Call 811 to locate utilities before any excavation.
- Don’t demo or notch load-bearing elements without engineering guidance.
Common mistakes to avoid
- Patching with hydraulic cement alone for structural cracks. It may stop drips but doesn’t bond the wall.
- Injecting epoxy into a wet, actively leaking crack; use polyurethane first or dry the crack.
- Skipping surface prep; dust and paint lead to failed adhesion.
- Over-pressurizing injections, causing blowouts or channeling.
- Ignoring drainage; the crack will return or new ones will form.
When to call a professional
- Horizontal cracks, bulging/bowing walls, or cracks wider than 1/8 inch.
- Recurrent water entry after basic drainage fixes.
- Signs of settlement: sloped floors, sticking doors/windows, multiple diagonal cracks.
- Block walls with displacement or large stair-step cracking.
- If you’re uncomfortable using chemical injection systems.
Quick shopping list (ToolStash-style)
- Dual-cartridge epoxy or polyurethane crack injection kit with ports
- Dual-cartridge applicator gun and static mixers
- Angle grinder + diamond cup/wire wheel, shop vac
- PPE: safety glasses, N95/respirator, gloves, hearing protection
- Painter’s tape, putty/utility knife
Address the cause, choose the right material, take your time on prep, and a DIY injection can last decades. For structural movement or bowing, professional reinforcement pays off in safety and long-term performance.