If you have seen cracking floors or walls, sticking doors and windows, and uneven surfaces in your home, you could have a foundation heave problem. Foundation heave occurs when a foundation or concrete slab is forced upward by the expansion of soils underneath.

heaving floor

What causes basement floors and/or interior footings to heave?

Heave is the upward movement of a foundation or slab caused by underlying soils that expand or swell. This occurs due to an increase in moisture or by freezing forces. Heave is more common with slabs than foundations because slabs have less weight to resist heaving forces. Unless there is a long period of drought, heave most commonly occurs within the first few years of the building’s construction.

When a house is built, a hole is dug and the hole remains exposed to sunlight, usually for several weeks.

With so much direct sun exposure, moisture is removed from the soil and the soil shrinks. When the basement floor slab is poured, over time, the moisture that once was in the soil returns, causing it to expand, pushing upwards on the basement floor slab and footings. To avoid heaving, the ideal setting to pour a basement floor slab is on undisturbed soil.

Heaving can cause cracks in the basement floor slab and uneven floors, which can make finishing a basement much more difficult. When the soil heaving really gets invasive is when interior framed walls, load bearing and/or non-load bearing, get pushed upwards affecting the main floor framing system. Drywall cracks begin to appear, floors become out of level, and doors begin not opening and closing properly. Like foundation settlement, heave can be very invasive and can cause major damage to the home.

Slab and foundation heave is most commonly caused by these four forces:

Expansive Soils

A house built on soils with high clay content will be susceptible to heaving forces because clay-rich soil expands significantly when it gets wet.

The excavation for a foundation often gives the soil around and under a house a chance to dry out and shrink.  After the house is built, clay-rich soil that gets soaked during wet spells can cause heaving problems while also damaging foundation walls.

Frost Heave

Water increases about 9% in volume when it freezes, wet soil can expand by at least that amount upon freezing. When this happens, the resulting pressure on a slab or foundation can cause it to shift and crack.

The colder the temperature, the deeper the soil will freeze. In frigid temperatures, foundations that were not built below the “frost” or freezing line may heave.  Frost forces can lift a foundation upwards dramatically, sometimes by several inches or more, leading to serious frost heaving damage to a foundation.

Plumbing Leaks

Leaks or breaks in plumbing lines that run underneath a concrete slab or through a foundation wall can deposit moisture underneath the foundation and slab. This moisture can be from supply lines, waste lines, or even HVAC systems.

Prolonged or Heavy Precipitation

Too much precipitation combined with too little drainage away from the house will lead to increased moisture underneath your foundation. Moisture that comes into contact with expansive soil can result in heaving of your foundation and slabs.

Precipitation can also cause expansion in the soils on the sides of your foundation, leading to bowing, buckling walls.

One word of caution

While heave is the opposite of foundation settlement, the symptoms are often similar. To an untrained eye, it may appear that the exterior walls are going down — when the reality is that the basement slab is being pushed up near the center of the house.

expansive soil
frost heave
leak under slab

How to fix a heaved floor

To apply the right solution to a foundation problem, make sure you’re getting the right diagnosis.

After the cause of heaving has been removed, a good way to repair the floor is to cut it along lines about 12 inches on each side of the cracks and remove the resulting 24-inch-wide strips of concrete. Then fresh concrete can be placed in the same area and sloped from the high side to the low side, thus getting rid of the abrupt change in level. A completely level surface can be obtained by placing a concrete topping, at least 2 inches thick at the highest point, over the whole floor.

Before undertaking any repair of the basement floor it is a good idea to trowel a thin layer of plaster of Paris over a number of short lengths of the crack to see if the floor is moving. If the plaster of Paris finally cracks, add another series of patches. When a set of patches do not crack during a period of 3 or 4 months, or just barely crack, it can be assumed that heaving has been arrested and it is safe to repair the floor.

As with any foundation or floor heave, if you are unsure about how to remediate the situation, we recommend that you seek the services of a professional.

*Schematics courtesy of Carson Dunlop