Foundation Cracks: Which Ones Are Harmless and Which Need a Pro
why is there a crack in my foundation and is it serious
Most thin, vertical hairline cracks in a concrete foundation are normal shrinkage and only need sealing to keep water out. Horizontal, stair-step, or wide/widening cracks can signal structural movement and warrant a foundation engineer.
ℹ️ Reference only: For general reference only. This guide does not guarantee any result — every home is different. Verify against your local building codes and a licensed professional before acting, especially for electrical, gas, plumbing, structural, or roof work.
Common causes
- Concrete shrinkage as the foundation cured (thin vertical or diagonal hairline cracks, often within the first year). Cosmetic, very common. (most common) Quick check: Crack is vertical or near-vertical, under about 1/8 inch wide, and not getting longer. A nickel's edge (about 1/16 inch) barely fits.
- Soil settlement or heave under the footing (expansive clay drying/swelling, poor compaction, drought then heavy rain). Produces diagonal or stair-step cracks. (common) Quick check: Look for a crack that widens toward one end, plus sticking doors/windows or sloping floors nearby.
- Hydrostatic pressure from poor drainage pushing on the wall (downspouts dumping at the base, ground sloping toward the house, high water table). (common) Quick check: Crack leaks during/after rain; soil grade slopes toward the foundation; gutters overflow or discharge near the wall.
- Frost heave or freeze-thaw cycling (cold climates, footings above frost line, saturated soil). (less common) Quick check: New cracks appear or widen seasonally in winter/spring; common in unheated garages and porches.
- Structural overload or lateral soil/water pressure bowing a basement wall — the serious one. Horizontal crack across a block or poured wall, often mid-height, sometimes with inward bowing. (less common) Quick check: Crack runs horizontally, wall is bowing inward, or a stair-step crack in block is wide (over 1/4 inch). Stop DIY and call an engineer.
How to fix it
- Identify the crack type first. Measure width with a tape or coin. Mark each end with a pencil and write today's date next to it; check again in 2-4 weeks (and after a heavy rain) to see if it is growing. A photo log helps.
- Triage by pattern. Thin vertical/diagonal hairlines = cosmetic, seal them. Horizontal cracks, inward-bowing walls, stair-step cracks wider than ~1/4 inch, or anything actively widening = stop and get a structural/foundation engineer before doing anything.
- Fix the water first (this prevents most repeat cracks and leaks). Extend downspouts 4-6 feet from the house, clean gutters, and regrade soil so the ground slopes away at least 6 inches of drop over the first 10 feet.
- Seal a non-structural vertical poured-concrete crack from the inside with an injection kit (about $20-$80 at a hardware store). Use a flexible polyurethane kit if the crack is damp or leaks; use rigid epoxy only on a fully dry, stable crack. Clean the crack, set the surface ports, smear the paste over the crack face, then inject port to port per the kit instructions.
- For a narrow cosmetic crack you only want to keep watertight, clean it out and fill with a polyurethane or masonry crack sealant/caulk; for wider gaps use a fast-setting, water-resistant hydraulic cement patch (mixes and sets in minutes).
- After repair, keep monitoring. Re-mark the patched crack and watch for new movement. A repair that re-cracks means there is ongoing movement underneath — that needs a pro, not another tube of caulk.
- Do not patch over a structural crack and call it done. Sealing a horizontal or bowing-wall crack hides a load problem and can let it get dangerous. The fix there is engineered: carbon-fiber straps, wall anchors, steel I-beams, or push/helical piers, installed by a foundation contractor.
DIY or call a pro?
DIY is fine for sealing thin (under ~1/8 inch), stable, vertical or diagonal hairline cracks and for fixing the drainage that caused them — polyurethane/epoxy injection and downspout/grading work are well within a careful homeowner's reach. Call a licensed structural or foundation engineer/contractor for any horizontal crack, any inward-bowing wall, stair-step cracks wider than ~1/4 inch, cracks that keep widening, or cracks paired with sticking doors, sloping floors, or chimney separation. When in doubt, a one-time engineer inspection ($400-$800) is cheap insurance against a five-figure mistake.
Tools & parts
- Tape measure or a nickel/quarter for gauging width
- Pencil or paint marker for dating crack ends
- Wire brush and shop vacuum to clean the crack
- Polyurethane (for damp/leaking) or epoxy (for dry/stable) crack-injection kit with surface ports
- Hydraulic cement or masonry crack sealant for wider gaps
- Caulk gun (for tube-style sealant)
- Safety glasses, nitrile gloves, dust mask
- Downspout extensions and a shovel/rake for drainage and regrading
- Phone camera for a dated photo log
Keep a record of every fix you make — what broke, what it cost, how you solved it.
Track your home's fixes in Home Story →Based on: Reputable DIY references (This Old House, Family Handyman, Bob Vila in spirit); Manufacturer guidance for epoxy/polyurethane crack-injection and hydraulic cement products; Building-code grading/drainage norms (IRC R401.3: slope away ~6 inches over 10 feet); Foundation-repair industry guidance on structural vs. non-structural crack assessment
General homeowner guidance, not a substitute for an on-site inspection. Foundation problems vary by soil, climate, and construction; when a crack shows any structural warning sign, have a licensed structural or foundation engineer evaluate it before acting.