Water flows downhill and in order for a drain or waste pipe to work properly, it must slope downhill. Building codes for plumbing specify the amount of slope a pipe should have in order for the drain or waste pipe to move water and solid wastes to the main sewer or septic pipe. The slope on a drain or waste pipe is called the "fall" in the plumbing trades. Measuring the fall requires a level. A 4-foot level is suitable for long runs of pipe, but a shorter level works better on shorter lengths of pipe.
Select a level that will rest against the drain or waste pipe you are measuring without contacting fittings or other obstacles. The level must rest only against the pipe.
Set the level on the top of the pipe or hold it against the bottom. Note the position of the level's bubble -- it should not be centred. Lift the end of the level until the bubble is centred.
Measure the distance between the end of the level and the pipe. The distance is the "fall" of the pipe along the length of the level. The standard fall for most drain and waste pipes is 1/4 inch per foot of pipe. On a 4-foot level, the fall should be 1 inch and on a 2-foot level, the gap between the pipe and the end of the level is 1/2 inch.
Verify the fall is in the correct direction. The pipe must slope downward from the source of the water or waste toward the main sewer or septic pipe.
Some levels have bubbles that indicate slope and make it easier to measure the fall of a pipe. Tape a spacer with a thickness equal to the minimum fall to the end of a level. Then rest the level and spacer on the pipe. If the bubble is level, the fall is correct. The spacer for a 2-foot level should be 1/2-inch thick.
A fall greater than 1/4 inch per foot might seem better, but this is not always so for pipes that move solid waste. Use 1/4 inch per foot as a general rule and your pipe will drain correctly.