8" DuraVent DuraPlus Square Ceiling Support Box 8DP-CS11 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: 8" 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.