How much increase is necessary in the size of a waste pipe when a shower head is installed over a bathtub?

When installing a shower head over a bathtub, there is generally no requirement to increase the size of the waste pipe. The existing waste pipe for the bathtub is typically sufficient to handle the additional drainage that results from a shower. This is due to the fact that both the bathtub and shower share the same drainage capacity, and the fixture units calculated for plumbing systems usually account for simultaneous use of both the bathtub and shower.

Therefore, the existing waste system is designed to handle the flow rates from both fixtures without the need for an increase in pipe size. This is in accordance with plumbing codes which aim to ensure that the waste system can accommodate the expected load without introducing clogs or drainage issues. Hence, the correct answer reflects the standard practice in plumbing design.