Accurately calculating the slope of your drain pipe is essential to the proper function of your plumbing fixtures. According to plumbing code, drain pipe has to be sloped a minimum of 1/4-inch per foot and a maximum of three inches per foot or vertical. A slope of less than 1/4-inch per foot will cause constant drain clogs and a slope of more than three 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 can 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 your drain route, remembering that you want to keep the drain as short as possible. The longer the drain, the greater the chance of clogs in the future.
Step 3 - Determine Drain Length
With the length and route of your waste line, you can determine the length of the piping. 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 any 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 finally calculate the slope. Remember, the waste line has to slope a minimum of 1/4-inch per foot and a maximum of three 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 1/4-inch per foot, the drain has to slope a total of 3 3/4 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, 1/4 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 three inches per foot. For example, if you want to route that same 15-foot drain pipe at a slope of three 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 three by 15.
Tip: When you purchase your pipe and fittings, get some pipe hangers. There are J hooks sized for the piping and these are the simplest type of hanger. Start at the beginning of your run with the first hanger and use the torpedo level on your next hanger location, usually four feet from the first. Continue this and as you get to the end of your run, the bubble on the level should be breaking the level lines opposite the direction your pipe is running. The bubble should be about 1/8-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.