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The following diagram illustrates problems had with improper drain systems. Here, we see a kitchen sink drain line having excessive bends and length of pipe in the composition of the drain line.

This kitchen sink drain diagram illustrates inefficient piping.

  1. Too many elbows (This diagram illustrates 7 bends, not including the trap. Sometimes, there is no other choice but to use the number of bends you end up using when we consider trying to get the drain from the sink to the stack.)
  2. Length of the drain line (Again, this problem may not be avoidable)
  3. Inside diameter of the pipe for the application
  4. Poor venting


From the kitchen sink, the piping generally disappears into the wall behind the cabinet. The piping will run horizontally to a tee. From the tee, the pipe will become vertical. Gravity will lead water down from the branch of the tee. And, air will flow in from above the tee, through the sewer or plumbing vent. Proper installation of the plumbing system will determine problems or efficiency in the use of the drain. In the diagram above, the vertical pipe from the tee leads down to a 90 degree bend. The pipe travels horizontally for a short distance and then drops from a second 90 degree bend. Following the drop, there is a third 90 degree bend then the pipe is horizontal until it runs over to the stack and meets a 45 degree bend and then drops into the sewer.

The use of 90 degree bends should be avoided where possible. What occurs is, the grey water from the kitchen sink becomes vented when it drops through the branch of the tee. However, the ventilation is lost as soon as the waste water piles in the 90 degree bend after dropping from the tee. From there, the grey water, levels out and is now vented from the air in the sewer. But, then there is a second horizontal drop where the water meets another 90 bend. The water will level off and then travel to the sewer.

You may be wondering why the 90 degree bends are a problem. The answer is, the vent is choked off, water stops flowing until the air inside the pipe can vent it. The waste water may carry debris that will lodge itself in the bends of the pipe or in imperfect pitched pipe. Once this occurs, the build up will become a restriction and then, eventually a blockage. If the use of 90 degree bends is necessary, consider using long sweep 90 degree bends. Or, use 2 – 45 degree bends to make a long sweep 90. Using 2 – 45 degree bends will allow a longer turn radius. The affect a long turn radius has, is to allow water and air to have a more laminar flow through the bend then would occur if the bend was a tight 90 degree turn radius. Where possible, use 45 degree bends to move the sewage through vertical to horizontal transitions. This allows the vent line to be maintained. Also, determine the amount of volume that will pass from the sink. The standard for drains found in a home is based on a 1- 1/4″ to 1-1/2″ drain from the fixture. Your drainage line should be 11/2″ to 2″ diameter, depending on the use. If you are piping a bathroom sink drain, you can reliably use 11/2″ drain pipe to discharge water from the sink to the sewer. If you are piping a kitchen sink with more than one compartment, be sure to drain each compartment to an 1-1/2″ pipe then to a larger pipe when branching to the other compartments. This will allow air flow to continue when two compartments drain simultaneously.

The following video is taken from a project we were involved in, to re-pipe a drain line. The total linear footage for the kitchen drain line was approximately 75 feet. Piping a drain line for this distance is impossible to keep proper pitch (1/4″ fall for every 12″ of horizontal travel) in residential joist. In order to give the pipe proper pitch, the pipe needs to be routed to an outside wall, while remaining in the joist, at 1/4″ per foot pitch. As soon as a wall is reached, drop into the wall ( or, in our case…onto the basement wall where studs will be placed later) and use the vertical space inside the wall to run the distance.

In running excessively long lines for drainage, one of the most common problems is blockages occur as the drain line remains the same diameter throughout the distance of the drain line. What eventually occurs is occlusion. The pipe will eventually lose its vent line as the pipe fills with debris. If we maintain our pitch and increase the pipe size, adding clean outs (keeping them accessible) we can maintain these long lines. Also, we have the ability to monitor the flow in the line with clean outs.

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