The 135 rule in residential plumbing and vent design refers to the maximum cumulative degrees of horizontal directional bends allowed in a continuous drainage or vent pipe run between cleanouts or terminal connections without requiring an additional cleanout fitting. Specifically, building codes limit total bends between accessible cleanouts to no more than 135 degrees to ensure that plumbing snakes and hydro-jetting equipment can successfully navigate the piping bends during maintenance and clearing operations without getting permanently stuck.
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