Lowering a Basement Floor

By Nancy Nichols, P.E. | October 12, 2022

Have you ever considered making living space in your unfinished basement?  It’s a possibility if the ceiling height is high enough.  The International Residential Code requires a minimum 7 ft. ceiling clearance except under beams, girders and the like, which can be not less than 6 ft. 4 in.  What if your ceiling is currently too low?  Is lowering the floor an option?

Yes, the floor can be lowered, but only if the inward directed forces on the outside of the basement wall are adequately resisted by opposing forces.  Refer to the schematic below.  For a stone masonry wall, the bottom of the wall is the footing.  The inward directed force is due to active lateral soil pressure.  The soil on the outside of the wall pushes the wall inward.  The resisting forces are provided by the concrete slab, if above the bottom of the wall; friction at the base of the wall; and passive soil pressure below the slab and above the bottom of the wall.

So, the balance of forces depends mostly on the location of the bottom of the stone masonry wall relative to the new floor.  Before deciding to lower your basement floor, there should first be exploration to determine the location of the bottom of the footing, and characterization of soil and water conditions.  If the bottom of the stone wall is not deep enough, then there are engineered methods to resist the inward directed forces given suitable subsurface conditions.  What method to use will depend largely on the configuration of your structure relative to the landscape topography and available space in the basement.

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