Designed and tooled to perfection for the chimney professional, the DuraTech Chimney System sets * This product requires: 2 air clearance to combustibles for all diameters. * Joint/Connection Overlap: 1-1/2. * Joint/Connection Type: Twist-Lock standard on all diameters. * Outer Wall: 430 G-90 Galvanized Steel, 0.016 Thick, or 0.021 Galvalume Steel.