77 Ill. Adm. Code 890.1040
Air Gaps
Section 890
Section 890.1040Â Air Gaps
The air gap between an indirect
waste and the drainage system shall be at least two (2) times the diameter of
the fixture drain or drainage pipe served, but shall never be less than one (1)
inch. For requirements in excess of this minimum, see Appendix A: Table C,
and Appendix H:Â Illustration F.