What size waste pipe is required for 3 lavatories, 3 bathtubs, and 1 kitchen sink?

The requirement for waste pipe size is determined by the total drainage requirements of the fixtures being served. In this case, you need to consider the number of fixtures and their flows.

For the combination of 3 lavatories, 3 bathtubs, and 1 kitchen sink, the total load can be calculated using fixture unit values commonly used in plumbing codes. Typically, a lavatory has a load of 1 fixture unit, a bathtub has about 2 fixture units, and a kitchen sink can have approximately 2 fixture units as well.

Calculating the fixture units:

• 3 lavatories = 3 x 1 = 3 fixture units

• 3 bathtubs = 3 x 2 = 6 fixture units

• 1 kitchen sink = 2 fixture units

• Total = 3 + 6 + 2 = 11 fixture units

When sizing a drainage pipe, 2-inch waste pipes are generally capable of handling up to 20 fixture units in many residential plumbing systems. Given that the total calculated is 11 fixture units, a 2-inch pipe would suffice to accommodate the drainage load effectively.

Larger pipe sizes, such as 3 inches or 4 inches, are intended for