Floating Slab Foundation Problems

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The floating slab foundation is the most primitive of all foundation types. In fact, it is not technically a foundation at all, but a simple poured concrete floor that sits right on the grade, or underlying soil. In some cases, a more sophisticated design will pour the slab on either prepared soil or a gravel bed. Since the floating slab is normally a maintenance nightmare, it is usually only used for garages or sheds, not homes. The main feature of the floating slab is that the poured concrete floor is not attached to anything. It stands, or floats, by itself.

  • Analyze and examine the soils of the area the slab rests upon. This is the main issue because there is normally immediate contact between the soil and the slab. If the soil has not been prepared — that is, has not been properly drained — the wet soil, after it freezes, can cause the slab to lift and heave. This causes cracks. If the soil is actually clay, then, since many clays expand, the danger is even greater. Since most clays react to the freeze-thaw cycle, this can play havoc with the floating slab. In such environments, unless extensive preparation of the ground has been made, a floating slab, even for a garage or a shed, is never a good idea.

  • Examine the moisture content of the soil. This is another important variable in dealing with cracking and heaving problems in floating slabs. If the soil has not been drained prior to pouring, this heavier soil is liable to change its density during dry seasons when its moisture evaporates. This can render the soil less stable, less dense and, as a result, unable to withstand the weight of the slab and the furnishing sitting atop it. This, in turn, means that the ground will shift, leading to tilting, sinking and, as a result, extreme cracking.

  • Drain the soil and lay down a bed of gravel before pouring the concrete. This is the only way to ensure a basically trouble-free floating slab foundation/floor. Since the structures are not to be lived in, construction of such free slabs is often done haphazardly. Workspaces and garages still require a certain level of integrity to function at all. If the crack in the floor matches cracks in the free-standing walls, then the entire structure is tilting or shifting. This is the worst sign of all, since it strongly implies that the ground and soil of the structure is unsuited, or was unprepared, for the weight of the floating slab-on-grade. The probable result is, after several winter freezes, that the floor will shift on its own, with the walls likely staying in place. After a time, the floor might even become uninhabitable.

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