What Causes Concrete to Heave in a Basement?

Water Pressure

• Basements built in areas that receive heavy rains or flooding regularly are trickier to keep dry. When soil under the basement becomes completely saturated, the hydrostatic pressure pushes upward on the bottom of the slab floor, according to Popular Mechanics, which lifts the concrete, causing it to crack and creating a spot that heaves above the rest of the floor. Adding a sump pump at the bottom of a sand-lined hole in the concrete floor prevents your basement from flooding or heaving.

Lack of Reinforcement

• Concrete is strong but still cracks under pressure. Embedding steel rebar as the concrete of your basement is poured ensures that even if surface cracks appear, the concrete is held together by the reinforcements, says Inspectapedia. Leaving out steel bars isn't enough on its own to cause serious heaving, but it does play a large part in preventing it. If you didn't add reinforcement and your basement isn't built to handle ground frost or soil flooding on its own, the lack of extra support can cause a major problem with heaving.

Frost

• Water gathers under concrete, either directly under the slab of your basement if you didn't lay a drainage layer of gravel below it, or in the soil below the gravel. When this water freezes into ice in the winter, it expands in all directions. Frost expansion in the soil is strong enough to push with thousands of pounds of pressure upward on your basement floor, according to Inspectapedia. Frost heaving is the most common form of heaving in concrete and requires footers poured below the frost line to prevent damage.

Tree Roots

• Although not as common as frost or water heaving, if large tree roots grow extensively below your concrete basement, they may be the cause of your uneven floor, says National Inspection Services. Tree roots constantly seek out and grow towards moisture. Inadequate drainage under the basement lets water gather and draws the large, destructive roots of hardwood trees. If the root system below the basement is extensive enough to crack the foundation, you'll have to cut down the offending trees to stop further damage.