re: is there a standard depth for house slabs

Another thing to consider is if building in an area with soil subsidence issues like they have in parts of N.O. East and Kenner is to run hanging straps around the plumbing drain lines and allow the ends to be incorporated into the slab.

After pilings are driven, pipes are laid and the slab poured, after a while there could be subsidence that allows unstrapped pipes to sag and break.