Units and ductwork in which at any point on one horizontal dimension of the equipment on a roof is less than 4` should have a clearance of at least ____ inches from the bottom of duct or unit from the finished roof.

Units and ductwork in which at any point on one horizontal dimension of the equipment on a roof is less than 4` should have a clearance of at least ____ inches from the bottom of duct or unit from the finished roof.