Drain Pipe Slope Calculation

Accurately calculating the slope of your drain pipe is essential to the proper working of your plumbing fixtures. According to plumbing code, your drain pipe has to be sloped a minimum of ¼-inch per foot and a maximum of 3 inches per foot or vertical. A slope of less than ¼-inch per foot will cause constant drain clogs and a slope of more than 3 inches will allow the water to drain without the solids. Calculating the slope of your drain pipe is easy.

Step 1—Take Measurements

The first thing you need to do is take measurements. You need to know the distance the waste line has to travel before you do anything else. This can sometimes be very difficult, depending on the location of the line. However, it is absolutely necessary.

Step 2—Plan Drain Route

Once you know the length of the waste line, you can plan the drain route. Some drains will travel in a straight line. Others will bend and others will make 90 degree turns. Use a pencil and paper to map the drain route. Remember that you want to keep the drain as short as possible. The longer the drain, the greater the chance of clogs.

Step 3—Determine Drain Length

Now that you know the route of your waste line, you can determine the length of the pipe. You need to purchase plumbing pipe in the length you calculated from taking measurements and routing the waste line. When you go to your local home improvement store, don’t forget to get the necessary components for bends and turns. They will have a wide variety of pipe connectors.

The final drain length is absolutely necessary to calculate the proper slope of the waste line.

Step 4—Calculate the Slope

With the final drain length available, you can calculate the slope of the waste line. Remember, the waste line has to slope a minimum of ¼-inch per foot and a maximum of 3 inches per foot or vertical.

Here is an example, if your final drain length is 15 feet and you are going to slope the line at the minimum ¼-inch per foot, the drain has to slope a total of 3 ¾ inches from the start of the pipe to the end. To determine the slope, multiply the slope by the length of the line, in this case, .25 by 15.

Calculating the slope of your drain pipe may be that simple, or it might be a bit more involved. If your drain route requires a bend or turn, you have to calculate the slope of each piece of pipe separately.

If you have to route a longer drain pipe without a lot of space, you may opt for a deeper slope up to the maximum of 3 inches per foot. For example, if you want to route that same 15 foot drain pipe at a slope of 3 inches per foot, the drain has to slope a total of 45 inches from start to finish. To determine the total slope, you will multiply 3 by 15.

TIP:Doityourself’s plumbing consultant Mark Vander Sande suggests, “When you purchase your pipe and fittings, get some pipe hangars, there are J hooks the size of the piping, these are the simplest type of hangar, start at the beginning of your run with the first hangar and use the torpedo level on your next hangar location, usually 4 feet from the first one and continue this as you get to the end of your run, the bubble on the level should be breaking the lines on the level opposite the direction your pipe is running…..the bubble should be about an 1/8 of an inch past the line on the high side of your drain or where you started the run.”

Mark Vander Sande, professional plumber, contributed to this article.