Do I have an S-Trap Problem in this Shower Drain?

[CanadaGuy]

Hello all,

I am looking for advice on the image below that is a cross-sectional view of a drain I installed for our shower remodel. I wanted the initial drop to the trap to be as short as possible to make cleaning it from above easier, but this meant that the P-Trap would not be below a needed piece of blocking under the subfloor.

So after the P-Trap I had to drop under the blocking, and then drop again to clear under the I-Joists before connecting into the existing 4" drain stack.

My concern is after looking at it, I wonder if I have accidentally created an S-Trap???

Thanks in advance for any advice.

(Drawing is obviously not to scale, but I hope it conveys the design. I have room to do it over again in case I need to insert a length of horizontal pipe before the Street 90. ... Drain is 2" PVC connecting into 4" PVC stack)

 

[wwhitney]

Short answer: This not an acceptable routing for a trap arm, which is the fixture drain between the trap outlet (just before your Street 90) to the vent connection for the trap (which appears to be the 4" stack). You have violated the requirement that the trap arm may only fall one trap diameter, no more. If it falls more, and it fills with water, it will very likely siphon the trap.

Longer answer: What is draining from above into the 4" drain stack? If you have any fixtures from a story above the the story with the shower, or any fixtures other than bathroom fixtures on this story (lav or tub would fine; I'd have to check if a WC is allowed above the shower or if the WC would have to connect below the shower), then the usual wet venting rules would not allow you to use the stack for the vent.

As MI uses the IPC, if the wet venting rules don't work for the using the stack as a vent, we could check the IPC stack venting rules to see if it might qualify. But you'd need to diagram the entire stack from the shower connection up to the roof. If the stack doesn't work as a vent, you'd need to connect a wet or dry vent to the shower trap arm before it hits the stack.

If your stack is allowed to vent the shower trap arm, both of your vertical downward offsets are a problem, and you need to use a san-tee at the stack, not a wye. The wye has extra drop that eats into your total allowed drop between trap outlet and vent connection. So you could fix both those problems by dropping the trap (the 5" length you have labeled is allowed to be up to 24") and changing the wye at the stack to a san-tee.

Cheers, Wayne

 

[Sponsor]

 

[CanadaGuy]

Short answer: This not an acceptable routing for a trap arm, which is the fixture drain between the trap outlet (just before your Street 90) to the vent connection for the trap (which appears to be the 4" stack). You have violated the requirement that the trap arm may only fall one trap diameter, no more. If it falls more, and it fills with water, it will very likely siphon the trap.

Longer answer: What is draining from above into the 4" drain stack? If you have any fixtures from a story above the the story with the shower, or any fixtures other than bathroom fixtures on this story (lav or tub would fine; I'd have to check if a WC is allowed above the shower or if the WC would have to connect below the shower), then the usual wet venting rules would not allow you to use the stack for the vent.

As MI uses the IPC, if the wet venting rules don't work for the using the stack as a vent, we could check the IPC stack venting rules to see if it might qualify. But you'd need to diagram the entire stack from the shower connection up to the roof. If the stack doesn't work as a vent, you'd need to connect a wet or dry vent to the shower trap arm before it hits the stack.

If your stack is allowed to vent the shower trap arm, both of your vertical downward offsets are a problem, and you need to use a san-tee at the stack, not a wye. The wye has extra drop that eats into your total allowed drop between trap outlet and vent connection. So you could fix both those problems by dropping the trap (the 5" length you have labeled is allowed to be up to 24") and changing the wye at the stack to a san-tee.

Cheers, Wayne

Thanks for the analysis and suggested changes.

To answer your questions.... The other fixtures on this 4" line are the 2 sinks and a toilet in the bathroom, and the drain from the washer, and it does lead to a roof vent.

As for the choice of Wye vs. San-tee... I went with the Wye as I modeled the connection to the stack in a manner similar to how the builder (or their contracted plumber) connected a different bathroom drain to the other 4" drain pipe on the opposite side of the house.... Maybe they did their plumbing wrong, which lead me to think it was Ok?

I TRIED to follow the code, rather than create the "handyman's special" but looks like I'll just invest another $20 in new fittings, and do it right, with your advice.