6" DuraVent DuraPlus 11" Square Ceiling Support Box 6DP-CS11 11" DuraPlus Square Ceiling Support Box is used with flat vaulted or cathedral ceilings. Square Ceiling Support Box is painted black. Supports up to 35' of DuraPlus Chimney. Starter Section built-in. Trim included. Diameter: 6" Height: 11" Supports up to 35' of DuraTech Chimney Starter Section built-in Trim included 10â€-24†diameter pipes feature an .027†430 stainless steel inner wall and an outer wall of .029†galvalume or .027†430 stainless steel. A Refractory insulation is encased between walls. Only stainless pipes use the stainless end rings. Regular galvalume pipe uses galvalume end rings.