A heaving crack is a crack where part of the basement floor has been pushed up, so one edge sits higher than the other or the floor humps along the crack. It's caused by something under the slab, usually water pressure from poor drainage or clay soil that swells when wet. It's a job for an engineer, not a crack filler.
What is a heaving crack in basement floor?
It’s a crack where the floor has been pushed up from below. One edge of the crack sits higher than the other, or the slab rises into a hump with the crack along the top. ASHI inspector Tom Feiza puts it simply: floors that are heaved and cracked are a problem (opens in a new tab).
A normal shrinkage crack has flush edges and the floor around it stays flat. A raised crack in basement floor concrete means the ground under the slab has changed shape. The crack is the symptom, so filling it doesn’t fix anything.
What causes basement floor heaving?
Usually water. Either water collects under the slab and pushes up, or clay soil soaks it up and swells. Both lift the floor unevenly, and the slab cracks where it bends.
- Water pressure under the slab. Feiza says heaving indicates hydrostatic pressure from a hampered drain tile system (opens in a new tab) or a sump pump that’s not working or not set right. When the drains can’t carry water away, it builds up under the floor.
- Swelling clay. Some clay soils swell when wet and shrink when dry. The Colorado Geological Survey explains that water molecules work their way between the clay’s flat plates and push them farther apart (opens in a new tab), which creates high swell pressure in the soil.
- Trees close to the house. The same guide warns that a tree near the foundation can draw moisture from beneath the floor slab, and that the alternate wetting and drying can heave and crack it.
- A wet hole. A basement dug into tight clay can collect water like a tub. The survey calls this perched ground water, and notes it can form in a basement excavation where no ground water existed before.
Swelling clay isn’t only a Colorado problem. The survey itself notes that Colorado is only one of many areas in the United States that have swelling soils (opens in a new tab). A soils report from when your house was built, if you can get one, will say whether yours is one of them.
How do you tell heave from settlement?
Look at which way the floor moved. A heaved floor rises, often in the middle of the room, with the crack along the top of a hump. A settled floor drops, often along a wall or footing, and the crack sits at the edge of the low area.
Feiza notes that when a footing drops, there often will be a crack in the floor where the footing has started to drop (opens in a new tab), and the floor may be raised or dropped along it. A straightedge across the crack tells you which side moved.
You don’t have to get it right. Both mean the ground under the slab has changed, and both go to an engineer. The when to worry guide shows how to check the edges and the slope.
Is a raised crack in basement floor serious?
Yes. It means the soil is moving, and moving soil can keep going. The Colorado Geological Survey’s homebuilding guide says that differential heave, rather than total movement (opens in a new tab), is generally responsible for major structural damage.
How serious depends on what’s attached to the slab. A bare floor in an unfinished basement can rise without hurting the house. A floor with walls, posts, stairs or pipes standing on it pushes all of them up too.
The survey lists the range: swelling soil damage runs from cracked driveways, sidewalks, and basement floors (opens in a new tab) to severe structural distress and broken pipes and sewer lines. Have a structural engineer look before anyone sells you a fix.
What should you check before calling an engineer?
Check where the water is coming from. Most of it is in your control, and the engineer will ask anyway. The Colorado Geological Survey says control of surface and subsurface drainage will reduce the risk (opens in a new tab) of future problems with foundations and basement floors.
- Sump pump. Lift the float and make sure the pump runs and the water leaves. If the pit stays full or the pump never runs after heavy rain, tell the engineer.
- Downspouts. The survey’s homebuilding guide says roof runoff should be carried at least 10 feet from the building (opens in a new tab).
- Grading. Its homeowner guide recommends a slope of 12 inches of vertical fall for 10 feet (opens in a new tab) next to the foundation, with 6 inches as the minimum in planted areas.
- Pipes. Look where water and drain lines go through the floor. Feiza notes that crushed lead plumbing pipes can show up where they enter the concrete floor.
- The crack. Photograph it, mark the ends and measure the step with a ruler. The crack monitoring guide shows how to track it.
How is a heaving basement floor fixed?
By stopping the water first, then dealing with the slab. Fixing the floor without fixing the cause means it heaves again. The engineer’s report decides which repairs you need.
Drainage work comes first: repairing or adding drain tile, fixing the sump, and moving roof and surface water away from the house. Once the floor stops moving, a badly lifted section may be cut out and replaced.
On swelling clay, the floor may need to be built to move. The survey’s homebuilding guide describes a floating slab where complete isolation of the floor from bearing walls, columns, nonbearing partitions, stairs, and utilities (opens in a new tab) lets it move with minimum damage to the building. Partition walls are hung from above with a gap at the bottom, so the floor can rise without lifting them.
Can you seal a heaving crack?
Not until the cause is fixed and the floor has stopped moving. Sealing hides the step the engineer needs to see, and a rising slab will tear most fillers apart anyway.
Once the engineer signs off, a flexible urethane filler is the usual choice for floor cracks that may still move a little. The basement floor crack repair guide covers how to fill one, and the basement floor cracks guide explains the other crack types.
Questions
Can a heaved basement floor go back down?
Sometimes, partly. Swelling clay shrinks again when it dries, so a floor on clay can rise in wet seasons and settle in dry ones. Measure the step at the crack each month for a year to see which way it's going.
Can I grind down the raised edge?
Grinding the lip flat removes a trip hazard, but it doesn't stop the movement. Do it only after an engineer has looked at the floor, so the evidence is still there for them to see.
Does homeowners insurance cover a heaving basement floor?
It depends on your policy and on what caused the heave. Many policies treat earth movement and groundwater differently from sudden water damage. Read your policy and ask your agent before you pay for repairs.
I have a finished basement. What should I watch for?
Doors in basement walls that start to stick, drywall cracks at the top corners of door frames, and gaps opening at the top or bottom of partition walls. A wall standing on a rising slab gets pushed up into the floor framing above it.