What is the 135 rule in plumbing, and why does it matter?

The 135 rule in plumbing is a common shorthand that means keep total changes in direction gentle (often using 45-degree fittings) and add a cleanout once a run has turned significantly—around 135 degrees in many jurisdictions—to keep flow smooth and maintenance practical. The main reason is function: sharp or repeated 90s slow wastewater, trap debris, and make snaking harder, while a nearby cleanout gives direct access if a clog forms. This idea shows up in many codes, but details vary by city and inspector. We routinely coordinate with area inspectors in central Illinois so layouts pass review the first time.

Consider these practical cues during layout:

• Use two 45s rather than a hard 90 where space allows.
• Place a cleanout before or after long offsets so a cable can reach the problem area.
• Verify your local code’s exact cleanout and vent rules before rough-in.

When in doubt, have a licensed plumber confirm layout and permitting before you close walls.

Updated: 2026-09-14

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