Abstract: A roof-mounted apparatus is disclosed for discharging exhaust gas at a high velocity. The apparatus includes a stack having a tapered annular cross-section for achieving a relatively high discharge velocity for exhaust gas so that the effective height of the stack is equivalent to that of a considerably taller conventional chimney. The tapered annular stack preferably includes an outer wall and a coaxial inner wall which tapers outwardly toward the outer wall from the bottom to the top of the stack. An in-line fan having an annular outlet which communicates with the bottom of the tapered annular stack is preferably included for impelling exhaust gas admitted to the inlet of the fan into the stack.