Gap between slab and basement wall

[bryvander]

I know this is an old thread but registered in case someone else is looking for information about this type of gap and finds this thread. The space between the wall and the poured concrete is not a French drain nor is it space created by concrete shrinkage as suggested here. And I am certain the space was planned for in the original plans. It is normally called a floating basement slab and is pretty common in the North East or in areas that have deep frost.

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.

[Mack Knife]

What a great contribution. ^^^^^

Just add to that that often is it mistakenly thought that the expansion takes place in the hot weather or summer season when it actually happens in the wet season or mostly cool weather. In some homes you will notice doors sticking in one season but not another and it can change throughout the home.

We'd all like to think our homes sit on non-moveable ground when it isn't anything like that unless you drove piles into bedrock and nothing touches the ground. Even then, things move.

[DanBev]

In Denver my basement slab was that way,floating because of expansive clay I was told.I could not attach walls directly to slab,had to be floating for expansion.

[Fred74]

I have this same situation in my basement near Philadelphia. I'm putting new walls up. Now that we know what it is, would anyone recommend sealing the gap with concrete in case of moisture coming up? Like the very first post, I have never had water come in and am only concerned about having moisture behind my new walls. Thanks

[DanBev]

In Colorado our slab had open expansion gap because of betonite (expansive clay).my walls in basement were floating,not fastened to slab,so slab could move up or down.

[b-nasty]

Originally Posted by Fred74

I have this same situation in my basement near Philadelphia. I'm putting new walls up. Now that we know what it is, would anyone recommend sealing the gap with concrete in case of moisture coming up? Like the very first post, I have never had water come in and am only concerned about having moisture behind my new walls. Thanks

No, don't fill it with concrete. No form of sealing will do much, because any hydrostatic pressure from surrounding soil will bust right on through.



Basically, you either have a water problem in your basement or not. If you do, proper drains/pumps will be needed to remove the water from the soil before it comes through the concrete. About the only legit reason to seal that gap is if you have a radon fan.