Drain and Sewer Pipe Size

Limitations of the Material in this Section
Determining the correct size of drain and sewer pipes requires knowledge that is beyond most readers of this book; therefore, we will not discuss in detail how to determine drainage and sewer pipe sizes. Leave calculation of correct drain and sewer pipe size calculations to qualified contractors. We will present some basic information about how to calculate drain and sewer pipe sizes and present an example of calculating drainage and sewer pipe sizes for an average home. Do not assume that your drain and sewer pipe sizes are wrong if your home has different size pipes from the following example. Your pipes may have been sized using different assumptions or sizing methods. In practice, plumbers often use “rules of thumb” to install drain and sewer pipes using a limited number of pipe sizes that may be larger than the minimum required size. This makes drain and sewer installation easier and relieves the plumber from performing detailed pipe size calculations.

Drainage Fixture Units Description
1. Each plumbing fixture and group of related fixtures has been assigned a drainage fixture unit (dfu) value. This value is based on the typical water flow demand of the fixture or fixture group and on the probability that the fixtures will not all be used simultaneously.
2. Use the highest dfu value for a similar fixture if a fixture is not listed in the following table.
3. Use a dfu value of 1.5 dfu for every 1 gallon per minute fl ow of water into a drain system from a continuous or semi-continuous source such as a sump pump.
4. Note that the actual pipe size required for a shower or a fixture group that includes a shower may be different from the pipe size calculated based on drainage fixture units. This is because the shower trap size may be larger when numerous shower heads and body sprays are installed. Refer to Table P3201 for shower trap size requirements and base the shower drainage pipe size on the required trap size, not on the dfu value.

Drain Pipe Size Table
1. Use the following table to determine how many dfu may flow through horizontal branch drainage pipes or through vertical stack drain pipes. Use at least (≥) a 3 inch diameter pipe for toilets regardless of the values in the table.
2. Base the actual pipe size on the trap size required from Table P3201 when there is a difference between Table P3201 and Table P3005.4-1.

Sewer Pipe Size Table
1. Use the following table to determine how many dfu may flow through building drain or building sewer pipes. Use at least (≥) a 3 inch diameter pipe for building and sewer drain pipes that serve toilets regardless of the values in the table.

Drain and Sewer Pipe Size Calculation Method
1. Use the following steps to determine the size of drain and sewer pipes.
(a) Draw a line diagram of the plumbing system showing the relative location of each fixture.
(b) Assign the dfu value from Table P3004.1 to each fixture and fixture group.
(c) Begin at the top floor or most remote fixture. Work downstream adding the dfu values for each fixture or fixture group. Remember to use the
reduced multiple-bath fixture group dfu values beginning where the pipes from the bathrooms connect. Remember to check Table P3201 for shower trap size.
(d) Assign sizes to the pipes using Tables P3005.4-1 and P3005.4-2. You may use larger pipes than those in the table. You may not use smaller pipes and you may not reduce the pipe size in the direction of fl ow even if the diagram appears to allow it.

Drain and Sewer Pipe Size Calculation Example
1. Assume a new, four bedrooms, 2 ½ bathrooms, home with the plumbing fixtures described in the following table. The full bathrooms are on the second floor. The remaining fixtures are on the first floor.

Example 1:
Question: Assuming that both second floor bathrooms drain to one stack, what is the dfu load on the stack and what size pipe should be installed for that stack?
Answer: The dfu load is 12 and the pipe size is at least (≥) 3 inches. Refer to table P3004.1. Remember to use the bathroom group values and add for any extra fixtures.
The dfu load for a two full bathroom group (containing 2 bathtubs or showers, 2 lavatory sinks, and 2 toilets) from the dfu table is 8. Add 2 dfu for the extra shower and 2 dfu for the two extra lavatory sinks. Refer to Table 3004.5-1. A 2 ½ inches diameter pipe can serve up to 20 dfu as shown in the vertical stack column; however, the minimum pipe size serving a toilet is 3 inches.
Example 2:
Question: What is the total dfu load for this home and what size building and sewer drain pipes should be installed?
Answer: The total dfu load for this home is 17 and both the building drain and building sewer pipe sizes are at least (≥) 3 inches. Refer to table P3004.1. Remember to use the bathroom and kitchen group values and add for any extra fixtures. The dfu load for two and one half bathroom groups (containing 2 bathtubs or showers, 3 lavatory sinks, and 3 toilets) from the dfu table is 9. Add 2 dfu for the extra shower and 2 dfu for the two extra lavatory sinks. Add 2 dfu for the kitchen group and 2 dfu for the laundry.  The hose bibbs add no drainage load. A 3 inch diameter pipe is the minimum size for any building drain or building sewer pipe that connects to a toilet.

Drain and Sewer Pipe Slope
1. Install horizontal drain and sewer pipe with a uniform slope and alignment.
2. Install pipe not more than (≤) 2 ½ inches diameter with at least (≥) a ¼ unit in 12 units (2 percent) slope.
3. Install pipe at least (≥) 3 inches diameter with at least (≥) a ⅛ unit in 12 units (1 percent) slope.

Drain and Sewer Pipe Offset Size
1. Use the vertical stack column in the drain pipe size Table P3005.4-1 to determine the size of any off set pipe in a vertical drain pipe that is not more than (≤) 45 degrees from vertical.
2. Use the sewer pipe size Table P3005.4-2 to determine the size of any off set pipe below the lowest horizontal branch that is more than (>) 45 degrees from vertical.
3. Size an off set pipe above the lowest horizontal branch that is more than (>) 45 degrees from vertical as follows:
(a) use the vertical stack column in the drain pipe size Table P3005.4-1 to determine the size of the stack pipe above the off set, and
(b) use the sewer pipe size Table P3005.4-2 to determine the size of the off set, and (c) use the pipe size from the previous step (b) or the total number of dfu draining into the entire stack, whichever is larger, to determine the size of the stack below the off set.

Horizontal Branch Connection Near Stacks
1. Separate the connection point of a horizontal branch drain that connects near the base of a vertical stack by at least (≥) 10 times the diameter of the vertical stack. Make the connection downstream from the stack. Example: if the vertical stack diameter is 3 inches, then connect any horizontal branch drain at least (≥) 30 inches downstream from the vertical stack connection.
2. Separate the connection point of a horizontal branch drain that connects near a horizontal off – set of a vertical stack by at least (≥) 10 times the diameter of the vertical stack. Make the connection downstream from upper part of the vertical stack. Example: a 3 inch diameter vertical stack makes a horizontal off set to get around a beam. Any connection of a horizontal branch drain to the horizontal off set must occur at least 30 inches from where the upper part of the vertical stack turns to make the horizontal off set.