Why does the main drain line have a s-trap? A toilet already has a built-in trap.

Terry Smith • September 9, 2026

Why does the main drain line have a s-trap? A toilet already has a built-in trap.

A toilet already has a built-in trap. That curvy passage you see inside the toilet isn’t there because the manufacturer had extra porcelain and got creative—it’s the trap! πŸ˜‚πŸš½


So why would we install another trap in the main drain line underneath it? Apparently because one trap wasn’t enough excitement.


Now you’ve basically created a plumbing obstacle course. Waste leaves the toilet, makes it through Trap #1, then gets greeted by Trap #2 saying, “Not so fast, buddy!”


And wait until the sewer line stops up. We send a sewer cable down the line, and suddenly it encounters that unnecessary underground trap. Now the cable has to negotiate turns it was never invited to make. Best case, it’s aggravating. Worst case, the cable gets hung up, kinked, or damaged—and now a simple stoppage has turned into an archaeological excavation of your plumbing system. πŸ’°πŸ˜‚


The toilet is already trapped! You don’t need to trap the trap.


Sometimes good plumbing is knowing what NOT to install.


At American Plumbing, we prefer keeping the sewer line accessible instead of building an underground amusement park for the sewer machine.


One toilet. One built-in trap. Keep the drain line simple. πŸš½πŸ”§πŸ‡ΊπŸ‡Έ

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