What is the minimum trap size required for a bathtub with a shower head?

The minimum trap size required for a bathtub with a shower head is 1 1/2 inches. This size is specified in plumbing codes to ensure proper drainage and to prevent clogs that can occur as a result of the flow rate of water from both the bathtub and the shower.

A 1 1/2-inch trap can accommodate the volume of water that a bathtub typically uses, which is especially important in situations where water may drain from both the bathtub and a shower simultaneously. Using a trap that is too small, such as 1 inch or 1 1/4 inches, may not provide adequate drainage and could lead to slow drainage or backup issues, as these smaller sizes cannot handle the increased flow associated with bathtub and shower use.

The next larger size, 2 inches, while appropriate for larger fixtures or systems with high flow rates, is not necessary for a standard bathtub equipped with a shower head, making it an overly large option for this application. Thus, the selection of a 1 1/2-inch trap is both efficient and compliant with plumbing standards, providing an effective solution for managing waste in a typical residential plumbing system.