Need to raise shower p-trap

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kevreh

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I'm finishing my laundry room and would like to maximize the ceiling height. Can I raise the P trap as shown in my rough drawing, or does this become an S trap? Those two grey boxes are joists.

Thanks-
Kevin
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Reach4

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I'm finishing my laundry room and would like to maximize the ceiling height. Can I raise the P trap as shown in my rough drawing, or does this become an S trap? Those two grey boxes are joists.
No. It it would become an S-trap. So no go as drawn.

You could maybe add a horizontal vent pipe after the P-trap. That could prevent the siphoning, and give you that headroom. Maybe you could even have an accessible AAV under there. I am not at all sure. I am not a plumber.
 
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Jadnashua

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The trap arm after the p-trap MUST be vented before it turns down. Running the line through the joists might be able to be done, but it's really tough to get it where it's structurally sound and able to be installed. The drain must run through the middle of the joist, and can't be any holes within 2' or so (not positive on the exact number) of the ends. Unless those are engineered I-joists...probably won't be able to go through them other than maybe one joist bay.
 

kevreh

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I wanted to follow up with my post as I'll be tackeling this project soon.
  • Jim, are you saying that IF I want an s-trap like this, then the only way to do it is venting between the p-trap part and the downturn part?
  • Is the notion of s-traps being bad have more relevence with a toilet where there's a lot of water being flushed quickly? In this case, is it critical since its for a shower and see a consistant rate of water?
 

Terry

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A toilet is designed to siphon it's trap and then it gets refilled from the tank.
A shower, tub, lav, sink and washer have traps with venting to prevent the trap from siphoning. A dry trap will stink up the home. The vent comes after the trap, not somewhere down below where it has already siphoned itself dry.

If the trap is raised, then the connection at the vent connection gets raised with it. Either that, or leave the trap where it is, and fir down the ceiling to hide it below.
 

kevreh

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Thanks for that explanation Terry.

God I feel dumb, check out this photo, with the first image being how it is now and the second what I plan to do. Basically, if I cut the 4" iron waste pipe I can tap into it running the new pipe (blue line) BETWEEN the joists and thereby raising it. I will remove the current stretch of pipe (red line) and cap off where it meets the toilet junction. As a bonus the new shower pipe will be closer to the main vent stack.

The pipe leading out of what will be the new p-trap is about 7" above the 4" cast iron pipe. Only remaining question is do I bring the new pipe above the 4" cast iron and drop the 2" shower pipe straight down with a sweep 90, or step it down with a couple 45's.

Reason why I'm doing this is the current p trap was in unfinished space. There will now be a shower under it. Raising it gets me about 9" of ceiling height.
 

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