Short answer
Evaluate foundation wall cracks by their orientation, width, displacement, moisture, and whether they’re changing. Most hairline, vertical cracks in poured concrete that don’t leak can be monitored. Cracks that are horizontal, stair-step (in block), wider than about 1/8 inch, show displacement/bulging, or are growing need repair and often a professional assessment.
What the crack is telling you
Different cracks point to different causes:
- Vertical (poured concrete): Often shrinkage or minor settlement. Usually monitor unless leaking or widening.
- Diagonal from corners: Differential settlement or heave. Monitor closely; repair if movement continues or water is present.
- Horizontal (especially mid-height in block/CMU): Lateral soil pressure. This is a red flag—get a pro to assess.
- Stair-step (in block walls): Settlement or lateral pressure. If you see movement or displacement, call a pro.
- Map/crazing (hairline network on surface): Surface shrinkage; often cosmetic.
- Corner pop (poured concrete): Often related to rebar rust or differential expansion—worth a closer look.
Tools and materials you’ll find helpful
- Bright flashlight/headlamp
- Tape measure and crack width gauge or feeler gauges (ToolStash comparator card)
- Ruler or calipers for repeatable width checks
- 4-ft level or straightedge; laser level for bowing (optional)
- Plumb bob for out-of-plane checks
- Masking/painter’s tape and marker for date-stamping
- Smartphone/camera for photos
- Crack monitors (tell-tales) for tracking movement
- Moisture meter (pinless or pin-type), or at least paper towels for simple checks
- PPE: safety glasses, gloves, N95 if grinding/cleaning
If repair is likely:
- Polyurethane injection kit (for sealing leaking but non-structural cracks)
- Epoxy injection kit (for structural crack welding in poured walls)
- Hydraulic cement for quick weep control (temporary/water only)
Step-by-step assessment
- Clean and expose: Brush loose dust/paint. Don’t grind unless needed; avoid breathing silica dust (wear an N95 if you must).
- Document: Take photos with a ruler in frame. Note date, weather, and recent heavy rain events.
- Measure width: Use a crack gauge/feeler gauges at three points along the crack.
- Check orientation and type: Vertical, diagonal, horizontal, stair-step? Note if it crosses mortar joints (block walls) or runs through concrete.
- Look for displacement: Hold a 4-ft level or straightedge across the crack. Any step or bulge is a concern.
- Check for moisture: Look for dampness, efflorescence (white powder), or active drips. Use a moisture meter if available.
- Mark a baseline: Put small ticks across the crack with painter’s tape and date them, or install a crack monitor.
- Monitor: Recheck monthly for 3–6 months and after heavy rains/freezes. Record changes.
Quick decision thresholds
Crack width:
- ≤ 1/16 in (≈1.5 mm): Typically monitor unless leaking.
- 1/16–1/8 in: Monitor closely; seal if leaking; evaluate drainage.
- > 1/8 in (3 mm) or any step/displacement: Repair recommended; consider pro.
- ≥ 1/4 in (6 mm) or growing: Professional structural evaluation.
Orientation flags:
- Horizontal (any width), or stair-step with displacement: Professional evaluation.
Movement:
- Growth > 1/32 in over 3–6 months: Plan repair; seek advice.
What to do based on findings
- Stable, hairline vertical crack, no moisture: Monitor. Optionally seal from inside for appearance.
- Leaking crack, otherwise stable: Low-pressure polyurethane injection is effective at stopping water. DIY kits run $80–$200; pro service $400–$800 per crack.
- Wider crack in poured concrete, little to no movement: Epoxy injection can restore structural continuity. DIY kits $150–$300; many homeowners choose a pro.
- Block wall with horizontal or stair-step cracking, or any bowing: This can indicate soil pressure. Likely needs engineered solution (e.g., carbon fiber straps, wall anchors, or excavation/relief). Carbon fiber materials run $300–$600 per strap plus labor.
- Recurring moisture regardless of crack size: Improve drainage: extend downspouts 6–10 ft ($10–$30), clean gutters, regrade soil to slope 1 in/ft for 6–10 ft (regrading may be $500–$1,500). Consider interior drain tile/sump if hydrostatic pressure is an issue.
Safety considerations
- Eye protection and gloves when inspecting and handling injections.
- Ventilate during injections; epoxies and foams can off-gas.
- Wet basements may harbor mold—wear a respirator if musty and disturbed.
- Use ladders and work lights safely in tight crawlspaces.
Tips for best results
- Address drainage before or alongside any crack repair to prevent recurrence.
- Track cracks through at least one wet season if not urgent.
- Use a crack monitor or comparator card for consistent measurements.
- Seal from the inside for easy access, but recognize this treats symptoms. Exterior causes still need attention.
- For cosmetic smoothing after injection, sand lightly and paint with masonry paint once cured and dry.
Common mistakes to avoid
- Patching with hydraulic cement alone and expecting structural repair—it’s a water stop, not reinforcement.
- Painting over damp walls without solving moisture.
- Ignoring horizontal or widening stair-step cracks—they rarely improve on their own.
- Measuring once and declaring victory. Movement trends matter.
When to call a professional
Call a licensed structural engineer or reputable foundation contractor if you observe:
- Horizontal cracks of any size
- Stair-step cracks with displacement in block walls
- Crack width > 1/4 inch, or continuous widening
- Noticeable wall bowing/bulging, or a step/offset at the crack
- Multiple doors/windows sticking, new drywall cracks, or sloping floors above
- Persistent water intrusion despite drainage improvements
A structural engineer’s on-site assessment typically costs $300–$700 and gives you a written scope you can hand to contractors, often saving on misdirected repairs.
If you’d like to kit up, look for: ToolStash crack width gauge, 4-ft box level, pinless moisture meter, and a low-pressure polyurethane or epoxy injection kit suited to wall thickness.