From our sister site, the Journal of Light Construction, a guide to plumbing a drain cleanout.

A cleanout must have at least 18 in. of clearance for drains 6 in. and smaller.

  • Cleanouts are generally required every 40 ft. wherever drain lines change direction more than 45 degrees.
  • Cleanouts are typically the same size as the drain lines they serve.
  • Each cleanout must be installed with the flow of the drain so that a snake inserted into it will also go with the flow.
  • Horizontal drains must be equipped with cleanouts at least every 100 ft.
  • All cleanouts must be accessible. They can’t be buried in the wall and must have clear space in front of them. For any drain 6 in. and smaller this clear space must be at least 18 inches

Visit the JLC Field Guide for a complete professional reference guide on Plumbing Drains.

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