A single or double sanitary T fitting is prohibited in what type of pipe?

The prohibition of a single or double sanitary T fitting in nominally horizontal soil-or-waste pipes is based on the concern of effective drainage and maintaining a smooth flow of wastewater. In horizontal runs, the flow needs to be uninterrupted to prevent blockages and facilitate proper drainage. A sanitary T fitting, especially if used incorrectly, could cause turbulence or create areas where solids might settle, leading to clogs.

In contrast, other pipe types such as vertical waste pipes, nominally vertical soil pipes, and main drainage pipes allow for more flexibility in the use of T fittings because the gravity and flow dynamics differ. In vertical pipes, waste can move more freely downward, reducing the likelihood of blockages. Main drainage pipes, designed for larger flows, also have a different structure that can handle various fittings more efficiently.

Therefore, avoiding the use of single or double sanitary T fittings in nominally horizontal soil-or-waste pipes is vital for ensuring a functional plumbing system that minimizes the risk of obstructions and maintains efficient drainage.