Four Steps to Sizing Sanitary Sewers & Vents

Sanitary sewers and vents play important roles in maintaining the quality of the indoor environment of a building. Correct sizing is crucial to their proper functioning, so you want to be sure you know how to get it right. Fortunately, understanding how to properly sizing sanitary sewers and vents in a facility is made easier using the set of charts published in the International Plumbing Code (IPC). Using these charts, there are four steps to accurately size uniform, acceptable and easily built drainage and venting systems in buildings:

1. Add up the total number of drainage fixture units (dfu) for each sanitary branch

2. Determine the sanitary branch sizes using the dfu values

3. Calculate the size of the main building drain using the total of all dfu values

4. Size the vents using an IPC table and the dfu values

Let’s assume you are assigned to add a new restroom with 10 water closets and four lavatories to an existing building. To determine the dfu’s for the project, use IPC table 709.1 (Figure 1), which lists the dfu value as a load factor and the minimum size of the trap in common plumbing fixtures. Just locate the type of fixtures to be used in the facility and add up the total dfu values for all plumbing fixtures. In our example, the table indicates that “Water Closets, Public” have a wfu of four while the listed value for lavatories is one. Using this information, here is the formula to determine total dfu:

10 WCs = 10 X 4 dfu = 40 dfu

4 Lavs = 4 X 1 dfu = 4 dfu

Total dfu = 40 + 4 = 44

The next step is to size the branch using IPC Table 710.1(2) (Figure 2). You will need to know the number of branch intervals tying into same the stack and remember to use the pipe size with the allowable dfu value greater than the dfu value calculated in step one. Returning to our example, according to the table, the minimum pipe diameter for a horizontal branch with 44 dfu is 4”. A simple tip to remember is that water closets have 4” waste connections, so any branch with a water closet will be at least 4”. 

In calculating the drain size, let’s assume there is only sanitary waste line in the building and use Table 710.1(1) (see Figure 3) to size the building sanitary main. You can determine the desired slope using the pipe size with allowable dfu value greater than the dfu value calculated in step one, or 44”. The table indicates that the minimum slope for a building drain with 44 dfu is 1/8” per one foot of pipe and the minimum pipe diameter at 1/8” slope is 4”

Finally, you can use IPC Table 916.1 (see Figure 4) to size vent piping using the dfu values from step one and waste stack sizes from steps two and three. Note that you must also know the total vent piping length to select the appropriate vent pipe size. Concluding the example, we will assume the restroom has its own vent to the roof with a total piping length of 25’. Consulting IPC Table 916.1 (see Figure 4), we see that for a 4” waste stack with a dfu up to 140 and with 27’ or less of vent piping length the correct vent pipe size is 2”.

Understanding how to use the tables provided in the IPC enables engineers to quickly determine the appropriate, and typically code approved, sizes for sanitary mains and vents. So understanding how to use these tables is an important skill for MEP engineers.

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