How to Size Black Water Pipes

Black plastic pipes that come into your home are larger than the water supply pipes. These black pipes are for wastewater drainage inside the home and out to the street. Older homes use cast iron pipes, which tend to rust and deteriorate over time, resulting in expensive repairs. Black plastic pipes are cheaper, and they will last much longer. You can figure out the correct size for your home with a few simple calculations.

• 1

  Calculate your water usage. Use the reference chart on the Plumbingbasics.com web site (http://www.plumbing-basics.com/drainage/sizing.htm), calculate your total water usage in the house by adding up all of the different units that use water. For example, a bathtub is two units, while a toilet is six, and an entire bathroom group (tub/shower, sink and toilet) is eight units.

• 2

  Determine the vertical drain pipe size. If your total number of water units is two, you can use a 1 1/2-inch drain pipe. If your total jumps to nine or less (not including a toilet), then you can use a 2-inch pipe. However, if you have a toilet, or up to 35 water units, then you will have to use a 3- or 4-inch pipe.

• 3

  Determine the size of your drain pipe to the street. Your street drain needs to be either the same size as your vertical drain pipe, or one size larger, to provide for the proper flow. The minimum size for a drain pipe to the street is 3 inches, while homes with multiple toilets will require a 4- or 5-inch drain pipe to accommodate the additional water usage, depending on the total number of water units in the home. A 5-inch pipe can accommodate up to 324 water units, so that is only recommended for very large homes.