Diagnosing Downstairs Floor Slopes

Three types of downstairs floor slopes

Inside corner downstairs floor slopes

If you walk diagonally into an inside corner in a house that is, say, 40-years old, it is common to find that it slopes perceptually downward as you walk into the corner. This sloping is easily explained. First, you have to keep in mind that you are experiencing what is called two-way bending. There is bending from front to rear and from side to side.

Second, the bending in each direction adds together in the diagonal direction.  

Third, in the standard model used in slab foundation design,  the slope due to expansive soil movement grows as you move toward the edge of the foundation. This is mathematically dictated by the model. The model is widely accepted by the engineering community.

Fourth, when there is a drought the supporting soil pulls away from the foundation causing it to bend downward in both directions. When the drought ends, the supporting soil will swell pushing the foundation back up, but it is unlikely to be able to push the slab back to its original position because it must overcome gravity. In other words, gravity pushes the slab corner downward but makes it more difficult for the soil to push the slab corner back up to where it started.      

Wavey downstairs floor slopes

This type of floor slope is not common until the house is at least 40-years old. It is caused by the normal settlement of the pad. As the soil settles it pulls away from the foundation leaving one to two-inch gaps between the supporting soil and the underside of the four-inch-thick slab. The result is that the four-inch thick slab sags between the stiffening beams. Thus, the perception of repeatedly going up and down. 

This sloping is expected and is a consequence of the design method which treats the areas between the stiffening beams as plain concrete.    

Downstairs floor slopes due to tilt

When you take measurements on the surface of a slab-on-ground foundation, it is common that the measurements show a difference in elevation, sometimes a large difference.

Consider a case where the side to side profile is 60-feet long. The elevations are 0, -1.5 and -3-inches. The calculated deflection is 0-inches for a deflection ratio of 0/360. The tilt is 3-inches over 60-feet or .4%. In this example, we see zero deflection and tilt that is less than half of the smallest allowable tilt I can find in the literature. Yet, the slab surface is out of level by 3-inches. 


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