In which situation is it necessary to have a gully trap?

The necessity for a gully trap primarily stems from its role in preventing the escape of sewer gases. Gully traps are designed to provide a barrier that stops these potentially harmful gases from entering living spaces. They are commonly installed at points of discharge – such as where wastewater enters a drainage system – to trap any odors and protect the health and safety of occupants.

In this context, the gully trap maintains the integrity of the plumbing system by ensuring that sewer gases do not breach the building envelope, thereby securing a healthier indoor environment. Proper installation of gully traps is a crucial component of effective drainage design, particularly where there is risk of backflow or gas escape.

The other options, while relevant to various plumbing practices, do not highlight scenarios specifically necessitating a gully trap for gas prevention. For instance, installing above-ground pipes, vertical stacks, or dealing with steep gradients may involve other considerations that do not directly require a gully trap's functionality. Thus, emphasizing the role of a gully trap in mitigating sewer gas risks provides significant insight into its importance in drainage systems.