What is the typical slab thickness requirement for a two level underfloor duct system where feeder and distribution ducts intersect?

A 4-inch slab is used for single-level duct installations and cannot accommodate the stacked depth of intersecting feeder and distribution ducts.

Five inches provides enough depth for some single-level systems but is insufficient to house both duct levels and required concrete cover at an intersection.

Six inches approaches the required depth but falls 1 inch short of the 7-inch minimum needed where two duct levels cross, leaving inadequate concrete cover.

Where feeder ducts and distribution ducts cross in a two-level underfloor system, both duct levels must be embedded within the concrete slab, requiring a total slab depth of 7 inches. This thickness accounts for the height of the lower feeder duct, the separation between levels, the upper distribution duct, and the minimum concrete cover required above the top duct for structural integrity. Insufficient slab depth at intersections risks cracking and conduit damage under floor loads.