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Reviewed by Marcus Aldridge — Senior Editorial Reviewer — Plumbing, HVAC & Wet Trades

Who to call · Foundation Specialist

Basement Floor Crack Widening Over Time: call a foundation specialist — or fix it yourself

Hairline cracks in a basement concrete slab are common and often harmless — normal curing shrinkage produces them within the first year. But a crack that measurably widens over months, develops vertical displacement on one side (one lip higher than the other), or starts seeping water signals active soil movement beneath the slab. Hydrostatic pressure from a high water table can push groundwater through widening cracks, and expansive clay soils swell and contract seasonally, progressively heaving one side of the slab. If the crack follows the perimeter near the footing, it may indicate differential settlement of the foundation itself. A foundation specialist can measure crack progression with tell-tales or calipers, determine whether the movement is ongoing, and recommend the appropriate repair — from flexible polyurethane injection for stable cracks, to underpinning or pier installation if the slab is responding to footing settlement.

Is this your problem?

Symptoms to look for

  1. 01

    A crack that was barely visible last year is now wide enough to insert a coin edge into

  2. 02

    One side of the crack sits noticeably higher than the other, creating a lip you can feel underfoot

  3. 03

    Water seeps up through the crack during or after heavy rain, even if the basement was previously dry

  4. 04

    Doors on the basement level have started sticking or swinging open on their own as the floor shifts

Who to call

Foundation Specialist

A foundation specialist measures the movement, injects a stable crack with polyurethane, and underpins or piers a footing that is settling; a waterproofer handles a crack that has become the water's route in.

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Fix it yourself

How to fix basement floor crack widening over time

Basement Floor Crack Widening Over Time

easy 30–60 min

Tools: ruler or feeler gauges, pencil, phone camera, straightedge, tape · Materials: crack monitor (tell-tale) or a dab of plaster across the crack, polyurethane crack sealant for a stable, dry crack

Steps

  1. Mark the crack's ends with a pencil and date, and measure the width at a marked spot with feeler gauges; photograph it with a ruler in frame.
  2. Stick a crack monitor across it, or smear a thin line of plaster across the crack — if the plaster cracks, the slab is still moving.
  3. Lay a straightedge across the crack: a lip you can feel means one side has dropped or heaved, not just shrunk.
  4. Check again monthly for three months and after heavy rain; a crack that stays the same width is shrinkage, and you can seal it.
  5. For a stable, dry crack, clean it and fill with a flexible polyurethane sealant so water and radon cannot come through.
  6. Look outside for what could be moving the slab: a downspout at the wall, a new tree, or a dry summer cracking clay soil.

Stop and call a pro if

  • the crack widens between measurements or the plaster tell-tale breaks
  • there is a vertical lip, water seeping up, or the crack runs along the perimeter by the footing
  • doors on the level above have started sticking or new cracks have appeared in the walls

What a pro does that you can't

A foundation specialist measures the movement, injects a stable crack with polyurethane, and underpins or piers a footing that is settling; a waterproofer handles a crack that has become the water's route in.

DIY cost: $10–30 for a tell-tale and sealant

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FAQ

Frequently asked questions

Who should I call for: Basement Floor Crack Widening Over Time?

Call a licensed foundation specialist. Foundation Specialists have the specialized training, tools, and insurance to diagnose basement floor crack widening over time safely and stand behind the fix. Most offer free quotes and can visit the same day or the next.

Can I fix this myself?

Often, yes: the guide above has 6 steps (easy, 30–60 min, $10–30 for a tell-tale and sealant). Stop and call a foundation specialist if the crack widens between measurements or the plaster tell-tale breaks.

How fast can a pro come out?

Most foundation specialists offer same-day or next-day visits for non-emergency issues. True emergencies (burst pipes, gas smell, no heat in winter) are typically handled within 1–4 hours.

How much does it cost to fix?

Costs vary by severity and your location. Submit a free quote request to get real pricing from local pros in your area.

What are the signs of: Basement Floor Crack Widening Over Time?

A crack that was barely visible last year is now wide enough to insert a coin edge into. One side of the crack sits noticeably higher than the other, creating a lip you can feel underfoot. Water seeps up through the crack during or after heavy rain, even if the basement was previously dry. Doors on the basement level have started sticking or swinging open on their own as the floor shifts.

Who to call

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