Which of the following is not classified as a substantial object?

The classification of substantial objects typically refers to items that significantly affect the structural integrity or layout of a building or scene. Plumbing vent pipes, although they serve an important function in a building's plumbing system, are generally not considered substantial objects because they do not contribute to the overall structural framework of a building.

In contrast, sheet metal housing for roofs, TV masts, and newel posts play more significant roles in the structure. Sheet metal housing is essential for protecting the integrity of the roof and preventing water damage, while TV masts can be integral to a structure's functionality and aesthetic. Newel posts, which are usually located at the ends of staircases, provide essential support to handrails and contribute to the overall safety and design of the staircase. Thus, when categorizing these items based on their structural importance, plumbing vent pipes are less substantial compared to the other listed options.