I will be terminating my pipe through a ceing then through a sloped roof. Since it is against the wall I believe a box that is 24" tall will be enough to go through the ceiling and to the underside of the roof (allowing for 18" of clearance). Due the boxes come with an internal support? If the roof protrusion is at least 2 feet and 2 feet of pipe inside the box, I think some other form of suppot would be needed. Also any sugestions on a brand? Thanks for your help in advance.