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Frequently Asked Questions
What does the letter actually describe?
The shape of the pipe after the seal.
A P-trap turns the flow horizontal.
An S-trap turns it back downwards.
The seal itself is the same in both.
Only the exit direction differs.
Which one do I need?
Whichever matches the waste outlet.
A wall outlet takes a P-trap.
A floor outlet takes an S-trap.
The choice is made by the building.
Establish the outlet before ordering.
What is self-siphonage?
The trap pulling its own seal out.
A full bore of water acts as a piston.
Dragging the seal down behind it.
Leaving the trap empty.
And the room open to the drain.
Why is the S-trap more prone to it?
Because of the vertical drop below it.
Falling water gets a straight pull on the seal.
A P-trap turns the flow sideways first.
Which breaks that pull.
Geometry, not quality.
How is an S-trap made to work?
By venting near the trap.
A vent pipe breaks the vacuum.
Or an air admittance valve does it locally.
Where a vent pipe is impractical.
Fitting one with neither is the mistake.