Expert: Bruce E. Johnson - 5/16/2009

Question
I'm planning an addition onto my home which has an unusually solid 8" slab foundation.  I would like the addition to include a sunken living room, half of which will be in one portion of the current existing structure.

Is it possible to cut and remove 3" from the top of the current slab foundation to accommodate for this sunken floor?  I would not extend the cuts to any of the walls, but would rather stop 6" from any supporting structure.

If this is not advised, would there be another technique that I'm overlooking that could provide this solution?

Thanks,
Dave

Answer
Hi Dave, Are you sure the entire existing slab is 8" thick?  Sometimes just the edges of a slab foundation are thickened and the slab itself is only 4" thick.  But, whatever style you have I don't see why you can't remove three inches from the thickness.  If your slab has a thickened edge and is only 4" thick in the main body then you would probably want to cut the full 4" thick portion of the slab out, dig the dirt below it down and repour it at the required elevation.  In the other instance where the entire slab is 8" thick you can probably get by with removing 3" of the thickness especially if the work doesn't involve any bearing walls.  The best way to do this is to rent a street saw and cut the slab into thin slices at the required depth, both directions, prior to any jackhammering.  This will put less strain on the existing foundation...I hope this information helps feel free to write again regarding this or other matters, sincerely bruce e johnson..bejohnsonconsulting.com