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Fix a foundation crack that leaks

We'll help you fix drainage first, assess the crack type, then seal hairline cracks or tell you when to call a foundation pro.

Troubleshooting · Home maintenance
1–4 hours
What you'll need
  • Wire brush or vacuum (to clean the crack)
  • Epoxy or polyurethane crack sealant (for hairline cracks)
  • Hydraulic cement (for active leaks)
  • Chisel (optional, to widen crack for sealant)
Step 1 of 6

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Quick triage — pick your path

Choose the option that matches what you see. You can jump straight to that section.

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Steps

Goal: Confirm the leak, fix drainage first, assess the crack type, then seal or call a pro.

  • Check during or after rain. Water seeps through the crack or stains appear on the wall.
  • Good: Water enters through a visible crack—proceed to Fix drainage first.
  • Bad: Water from the floor or unclear source—see fix-basement-floods.

Fix drainage first

Goal: Confirm water drains away from the foundation before sealing.

Assess crack type

Goal: Confirm whether the crack is sealable or needs a foundation professional.

  • Measure the crack width. Hairline cracks are under 1/8 inch and often vertical. Structural cracks are stair-step (follow mortar joints in block), horizontal, wider than 1/4 inch, or growing.
  • Check if the crack has changed over time. Mark it with tape and date it if unsure.
  • Good: Hairline and stable—proceed to Seal the crack.
  • Bad: Stair-step, horizontal, wide, or growing—call a foundation professional.

Seal the crack

Goal: Clean, prepare, and seal hairline cracks from inside.

  • Remove loose material with a wire brush or vacuum. Widen the crack slightly with a chisel if needed to create a V-gap for sealant. The crack must be dry and clean.
  • For cracks under 1/8 inch, use epoxy injection or polyurethane sealant per product instructions. For active leaks while you work, use hydraulic cement first—it sets quickly and can stop water under pressure.
  • Good: Sealant fills the crack. No water after the next rain.
  • Bad: Water still enters—drainage may not be fixed, or the crack may need injection from outside. Call a pro.

When to get help

Call a foundation professional for:

  • Cracks wider than 1/4 inch.
  • Stair-step or horizontal cracks.
  • Cracks that are growing.
  • Bowing walls.
  • Sealing has failed.

Confirm you have fixed drainage first—many leaks stop once water is directed away from the foundation.

Verification

  • No water enters through the crack during or after rain.
  • Drainage is fixed—gutters clear, downspouts extend 4–6 feet, ground slopes away.
  • The crack has not widened or grown.
  • Sealant is in place and dry (if you sealed it).

Escalation ladder

Work from the device outward. Stop when the problem is fixed.

  1. Confirm leak Verify water enters through the crack during or after rain.
  2. Fix drainage Check gutters, downspouts, grading—water must drain away from the foundation.
  3. Assess crack Measure width; check for stair-step, horizontal, or growing cracks.
  4. Seal or call pro Seal hairline cracks; call a pro for structural cracks.
  5. Call a pro Structural cracks, sealing failed, or crack is growing—call a foundation professional.

What to capture if you need help

Before calling support or posting for help, have these ready. It speeds everything up.

  • Crack width and pattern (vertical, stair-step, horizontal)
  • Whether the crack is growing
  • Drainage condition (gutters, downspouts, grading)
  • Steps already tried

Does water enter through a visible crack during or after rain?

Water seeps through the crack or stains appear on the wall. If water comes from the floor, see fix-basement-floods.

Check during or after rain. Water or moisture at a visible crack indicates a foundation crack leak. Good: water enters through the crack—proceed. Bad: water from floor or unclear source—see fix-basement-floods.

Yes No

You can change your answer later.

Is drainage fixed—gutters, downspouts, grading?

Water pooling against the foundation causes leaks. Sealing without fixing drainage often fails.

Check that gutters are clear, downspouts extend 4–6 feet from the foundation, and the ground slopes away. See fix-downspout-overflows and fix-downspout-is-disconnected. Good: drainage is fixed. Bad: fix drainage first—sealing will not hold otherwise.

Yes No

You can change your answer later.

Fix drainage first

Fix gutters, downspouts, and grading so water drains away from the foundation. See fix-downspout-overflows and fix-downspout-is-disconnected. Many leaks stop once drainage is fixed. Recheck the crack after the next rain.

Done

Is the crack hairline (under 1/8 inch) and stable?

Hairline cracks are vertical and under 1/8 inch. Structural cracks are stair-step, horizontal, wider than 1/4 inch, or growing.

Measure the crack width. Check the pattern—vertical (often sealable) vs stair-step or horizontal (structural). Check if it has grown. Hairline and stable: you can seal it. Structural or growing: call a foundation pro.

Yes No (structural)

You can change your answer later.

Seal the crack

Clean the crack with a wire brush or vacuum. For hairline cracks, use epoxy or polyurethane sealant per product instructions. For active leaks, use hydraulic cement first. Check after the next rain. If water still enters, call a pro.

Done

Call a foundation professional

Call a foundation professional for: cracks wider than 1/4 inch, stair-step or horizontal cracks, cracks that are growing, bowing walls, or when sealing has failed. Confirm you have fixed drainage first.

Done

Reviewed by Blackbox Atlas

Frequently asked questions

Why would a foundation crack leak?
Water enters through cracks when hydrostatic pressure builds—usually from poor drainage (gutters, downspouts, grading) or high groundwater. Fix drainage first; sealing alone often fails if water keeps pooling against the foundation.
Can I seal a foundation crack myself?
Yes, for hairline cracks (under 1/8 inch) that are stable. Clean the crack, apply epoxy or polyurethane sealant from inside, or use hydraulic cement for minor leaks. Structural cracks, stair-step patterns, or cracks wider than 1/4 inch need a pro.
When should I call a foundation professional?
Call a pro for: cracks wider than 1/4 inch, stair-step or horizontal cracks, cracks that are growing, bowing walls, or when sealing has failed. Foundation repair often requires excavation and injection from outside.

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