Foundation cracks: which ones matter and which don't

When it is serious

• Stair-step cracks following mortar joints in brick or block
• Horizontal cracks in a basement or crawl-space wall (lateral soil pressure)
• Cracks wider at one end than the other, or clearly more than a hairline
• A crack that reopens after being patched, or grows between spring and late summer
• Cracks paired with doors that no longer latch or floors that slope

When it is usually cosmetic

• Vertical hairline cracks in poured concrete, uniform width
• Drywall cracks above door and window corners with no other symptoms
• Slab surface crazing (a fine network) with no displacement across the crack

What the soil under your address does to this answer

On high and very high shrink-swell soil, cracks open in late summer as clay dries and close in a wet spring; a crack that cycles with the season is the soil talking, and drainage and moisture control usually come before piers. On low shrink-swell soil a growing crack more often points to poor fill, a plumbing leak or tree roots.

Horizontal vs. vertical foundation cracks

Direction is the first thing an engineer reads. A vertical or slightly diagonal crack in poured concrete is the concrete shrinking as it cured or the footing settling a little; it runs top to bottom, stays roughly the same width, and is sealed rather than braced. A stair-step crack in block or brick follows the mortar joints and means one part of the wall is moving relative to the rest: settlement or heave under that section.

A horizontal crack in a basement or crawl-space wall is different in kind. Soil is pushing sideways on the wall and the wall is bending; it is structural until an engineer measures the deflection and says otherwise. Treat a horizontal crack with any visible inward bow as the bowing-wall problem, not the crack problem.

A crack in a slab floor is read the same way: a hairline network with no displacement across it is surface crazing; a crack with one side higher than the other, or that lines up with a low corner of the house, is the slab moving with the soil.

A crack flagged during a home inspection

A home inspector reports the crack; they do not diagnose it, and the standard line in their report is a referral to a structural engineer. Before closing, that referral is worth taking: an engineer's letter is published at a few hundred to about a thousand dollars (see the inspection guide) and turns "foundation issues" into either a priced scope you can negotiate, a reason to walk, or a reason to stop worrying.

Sealing a foundation crack: when a sealant is enough

A sealant is the right fix for a crack that is not moving: a vertical or slightly diagonal hairline in poured concrete, uniform in width, dry or only damp after heavy rain. Epoxy injection bonds the two faces and restores strength; polyurethane injection stays flexible and stops water. Either is done from inside the basement or crawl space and does not need the wall excavated.

A sealant is the wrong fix for a crack that is a symptom. Stair-step cracks in block or brick, horizontal cracks in a wall, cracks that are wider at one end, or a crack that reopened after a previous patch are the wall or the slab moving. Sealing those buys a season and hides the evidence an engineer needs. Fix the cause first (drainage, piers, wall reinforcement), then close the crack.

Can I fix a foundation crack myself?

For a hairline shrinkage crack in poured concrete that is not leaking much and has not grown, yes: hardware-store polyurethane or epoxy injection kits are made for exactly that crack. Mark the ends with a pencil and date first, so you know later whether it moved.

Do not DIY a crack you have not diagnosed. If it matches anything on the serious list above, or the wall is block, or the crack is clearly more than a hairline, the injection is not the repair; the movement is. That is the point to get an engineer, not a kit.

Can a foundation crack be repaired from the inside?

Non-structural cracks in poured concrete are almost always repaired from the inside, by injection. Water problems that are bigger than one crack are also usually handled from inside with a perimeter drain and sump. Exterior excavation and membrane is the expensive option, reserved for walls that leak along their length or where the interior side cannot be reached.

Can a cinder block foundation be repaired?

Yes, but block behaves differently from poured concrete. Block walls crack along the mortar joints, so a stair-step crack is the usual sign and it usually means the wall is bowing or settling rather than shrinking. Injection does not bond block the way it bonds poured concrete; the repairs are carbon-fiber straps or steel braces for a bowing wall, piers for settling, and repointing or parging for the joints once the movement is stopped. A block wall with a horizontal crack and visible bow needs an engineer's measurement before any of it.

What the repair costs, if it is one

Full cost guide → · Engineer vs. contractor inspection →

What to do next

Photograph the crack with a ruler, mark its ends with a pencil and date, and re-check in 60 days. If it grows or matches the serious list, an independent engineer's inspection is the next step, not a contractor's free one.

Questions people ask

We are not engineers and this is not an engineering opinion. We report public soil-survey data and published price ranges. Nothing here states that a specific house is or is not structurally sound; a licensed structural engineer who has seen the house can.

Other signs

• House settling: when to be concerned
• Bowing basement walls: what's pushing, and what fixes it
• Sagging and sloping floors: what's underneath
• Sticking doors and windows: humidity or movement?
• Wet crawl space: what to fix first