What is the Gap between the Basement Floor and Wall?

I was asked this question while teaching my seminar “Helping Seller with their Cellars” at the Prudential New Jersey Properties in Westfield, NJ last week. Homes with basements often have a gap between the basement walls and floor. This is fairly common. The answer to the question is two-fold: it is either a floating slab or it is a French drain.

What is a Floating Basement Slab?

A floating slab or floating floor is part of a typical stem foundation in areas where there is freezing and expansive soils. If the foundation is monolithic, the floor is not a slab but actually part of the foundation itself. If the home has a stem foundation, where the wall is constructed upon a footing, then it may have a floating slab which also sits upon the footing.

The floating slab is created by placing a rigid object, such as a 1x6” board against the wall before the slab floor is poured. After the concrete floor sets the board is removed a clean even gap is left between the basement wall and floor. This is a floating slab or floating floor.

Some areas have very expansive soil which means the soil expands and contracts. This expansive soil puts pressure on the walls when the soil expands and releases pressure when the soil contracts. These soil pressures cause the foundation to move. The wall actually rocks back and forth in very slow motion.

When the slab, or basement floor, is directly adjoining the stem foundation wall sitting upon the footing it is subject to the same movement that the wall is. If the slab is touching the stem walls on all sides, and the wall moves in a thirty-second of an inch, the slab has nowher to move to and cracks as a result of this pressure. Having the gap of space that the floating slab provides allows movement of the walls without the floor slab cracking.

What is a French Drain?

The French drain basement waterproofing system is designed to prevent water from flooding your basement. It is installed inside the basement, below the floor, around the perimeter. This French Drain controls the water, delivers it to the sump pump(s), and discharges it out of the house.
If your house has hollow block masonry walls, an interior French Drain system can capture the water entering the block system and drain the water into a sump where a sump pump will discharge it from the building. Weep holes are drilled into the lowest course of block, allowing the drainage to occur.

How can I tell the difference between a Floating Slab and a French Drain?

While both may have a gap between the basement floor and walls it is easy to tell the difference between the two. A floating slab is part of the original construction and a French drain is installed post construction.

Because the French drain requires a sub floor drainage channel, the slab must be cut around the perimeter to excavate this channel. When the channel is closed with concrete, it sets and cures with a different consistency than the original floor. The basement with the French drain will have a discernibly different strip of concrete around the perimeter.