Renovation & Design

ASK THE INSPECTOR: Soil moisture key to sinking basement floors

QUESTION: I have a 2800-square-foot, split-level home on the Assiniboine river in Westwood. Over the past few years, with dry summers and limited rain, the floor in one section of the lower level has dropped around two inches. There are no cracks in the walls or window frames. We were told when we purchased the home 20 years ago that it was on piles and the floor was not anchored into the foundation.

Is this normal for river property and, with rehydration of the soil, will the floor elevate to pre-existing level?

Thank you, Jim Thliveris

ANSWER: Unfortunately, it's quite normal for there to be significant swelling and shrinkage in the soil under basement floor slabs in our area. Being in close proximity to the river may be a contributing factor, but cracking and shifting of concrete floors can occur almost anywhere in the Red River Valley.

You may see some rebound in the sunken concrete once normal soil moisture levels return, but likely not enough to prevent the need for eventual repairs.

Basement concrete floor slabs primarily serve two functions in most homes. They prevent moisture, insects and dirt from entering our homes by providing a solid surface over the soil, and they provide lateral strength to prevent the bottom of the foundation from bowing inward due to soil pressure.

In most homes, these thick, non-reinforced concrete slabs are poured directly over a thin polyethylene sheet installed on top of a layer of pea gravel or round stone. Pea gravel is used because it lacks sharp edges and has good flexibility, which prevents damage to weeping tiles and plumbing pipes below this area.

The drawback of using this type of granular fill on top of the naturally occurring expansive clay soil underneath, is the lack compressive strength. Changes in soil moisture may occur in varying degrees in different areas of the hollowed-out basement excavation. This can create uneven areas of expansion and contraction, which creates different forces on different areas of the basement slab above. This uneven movement may also be partially due to different soil composition in various layers, especially if there is silt in some locations. That is more likely in areas near rivers, like your home.

The uneven movement of the soil beneath your basement floor can cause some areas of the concrete to heave upwards, while others may stay put or drop. Because this "floating slab" is not reinforced with steel or anchored into the foundation, these uneven forces will cause the cracks you are seeing.

In some cases, soil erosion underneath the slab occurs because of differing soil compositions, damaged weeping tiles or poor backfilling. That's what has likely caused your floor to sink in one area only. As the moisture in the soil outside your foundation drops because of hot, dry summer weather, major shrinkage can occur.

Uneven soil shrinkage can occur because many different variables can affect moisture levels, including direction of foundation walls, vegetation outside the area, grading, downspout termination locations, and proximity to the river.

South- and west-facing foundation walls have a tendency to have drier soil because of the warming effects of the sun. Also, erosion of the soil on the riverbank may not be visible to the naked eye, but may cause settling of your entire yard in that direction.

All of this could be causing a loss of soil underneath your foundation walls, which may be partially replaced by fill sliding in from under your basement floor. In that scenario, a small void could be created under the concrete, causing your slab to sink.

This is similar to the "sinkholes" we sometimes see on the nightly news, which occur under roadways and swallow up vehicles or large sections of pavement when they collapse. While a similar catastrophe is unlikely in your basement, taking some steps to prevent a worsening of the situation may be wise.

The first thing to do is to ensure that soil outside your foundation, particularly on the south side and river side is replaced if it is eroding or shrinking. Simply shovelling soil against the foundation and in low areas, covering it with sod or grass seed, should help prevent further shrinkage.

Regular watering of these areas and any nearby trees is critical to prevent loss of soil in dry weather. While it may not be possible to water enough for a replenishment of the soil underneath your foundation, keeping the upper layers of soil moist may prevent excessive drying and shrinking of the soil deep down. Keeping trees properly hydrated will not only prevent soil erosion, it will help stop deep roots from removing what little moisture is still in the clay near the bottom of your foundation.

Ultimately, the only real solution to your problem is to remove the concrete, replace the missing fill and re-pour new concrete to provide a more even surface. But since there's no guarantee that this type of repair will be permanent, it should only be done if the slab is seriously damaged.

Maintaining better soil moisture outside your home is a better alternative to a messy and costly concrete repair.

Ari Marantz is the owner of Trained Eye Home Inspection Ltd. and the President of the Canadian Association of Home & Property Inspectors - Manitoba ( Questions can be e-mailed to the address below. Ari can be reached at (204) 291-5358 or check out his website at


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