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 1-1/4" vs. 1-1/2" p-trap
Author: nicholas123 (FL)

Hello everyone,


1. I'm relocating my bathroom's old pedestal sink about 42" from the vent stack. I'm in the process of installing the trap arm behind the wall with 1-1/2" pvc. It will have the proper slope and is not too long.

2. The new 1-1/2" p-trap I assembled is about 6-5/8" long, which is about the same clearance I have under the sink (between the back of the pedestal sink to its drain). I don't have room to add the 1/4" thick escutcheon.

So, I have a couple of options...

1. Use a smaller, 1-1/4" trap, which should be shorter and fit in the space available. It may clog easier, but then again the tailpiece is also 1-1/4". However, a smaller trap won't vent as well as a larger 1-1/2" trap.

2. Offset the drain (trap adapter) a few inches, so I can swing the p-trap out and give me more room. I'll have to cut and glue the trap arm a little shorter.

3. Recess the trap adapter into the wall and use caulk to seal the penetration. Note: drywall and tile not installed yet.

4. Buy a vanity sink, which I think has more room. (my other bathroom has a cheap vanity sink with 7" clearance).

Any advice? I'd prefer the larger trap because it should clog less and seems to be more standard. However, the smaller trap would be easier to install and I could probably use my shiny escutcheon and keep the trap pointing straight back to the wall so it's out of the way under the sink.

Edited 1 times.

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 Re: 1-1/4" vs. 1-1/2" p-trap
Author: Backyard plumber (OK)

The arm can be as long as you want it to be. For every 1/4 inch size of yr P-trap counts as a foot from the vent. So a 1 inch 1/2 trap can only be 6ft from the vent. Cause there's on 6 1/4 inches in 1 inch 1/2 trap. :) that tips free bud. On slope, remember @#$%& rolls downhill and you'll be fine. Good luck on the remodel

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 Re: 1-1/4" vs. 1-1/2" p-trap
Author: hj (AZ)

1. 1 1/4" Trap IS standard for a lavatory. Traps do NOT "vent" so that has nothing to do with the trap size.
2. If you offset the trap it may not fit inside the pedestal.
3. How will you tighten the nut on the trap adapter if it is inside the wall and the sink is in front of it?
4. Traps are designed so they do NOT "clog".

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 Re: 1-1/4" vs. 1-1/2" p-trap
Author: nicholas123 (FL)


1. You're right about the trap not venting. I was really thinking about the smaller diameter trap adapter for a 1-1/4" trap, but after looking at a sketch of the trap arm and how the water could back up in this arm and cut off the venting air, I can see how the size of the trap adapter shouldn't affect the venting ability. The trap adapter is on the high side of this 'air venting path', so it doesn't really affect the venting.

2. And you're right about the damn trap not fitting within the pedestal if it's offset and spun a little. My pedestal sink only allows the trap to be turned maybe 30 degrees.

3. I could only partially recess the trap adapter into the wall, so I could still tighten the nut. I'd rather not, but I think the old installation was that way. I could also move the pedestal sink out away from the wall and lay a thick layer of caulk between sink and wall, like the old installation, but that looks like crap.

I'm having second thoughts about the pedestal sink, because it doesn't have much counter top space and no storage underneath the sink. I guess the problem is I'm trying to position the drain stub-out where it will accommodate either type of sink...pedestal or vanity. Space is very limited and that's why the last guy choose a pedestal sink.

If I try to install a standard cheapo 24" wide sink, which seems to be actually 24-1/2" wide, and add for the tile on the side wall, I actually have less than 15" for the toilet clearance, maybe closer to 14". So, I may have to stay with the pedestal sink.

I guess I should reconsider a 1-1/4" trap.

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 Re: 1-1/4" vs. 1-1/2" p-trap
Author: North Carolina Plumber (NC)

I never use 1 1/4" traps. Sometimes I'll use a street trap adapter in the 90 where it turns out of the wall. That gives room for the trap and the nut is exposed for tightening purposes.

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 Re: 1-1/4" vs. 1-1/2" p-trap
Author: hj (AZ)

Back in the day, before plumbers got lazy, and cheap, we always used 1 1/4" chrome "P" traps. Now, we usually use 1 1/2" plastic traps, we they are inside cabinets.

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 Re: 1-1/4" vs. 1-1/2" p-trap :-)
Author: nicholas123 (FL)

NC Plumber, Thank you for the tip. I just checked and since my wet wall is wide, I have room for a normal elbow.

I just came back from Lowe's. Apparently, most high-end vanities have at least 8 inches of clearance so normally this isn't an issue. I guess cheap vanities and especially pedestal sinks are tight like that.

I checked out their p-traps too and unfortunately, both size traps have the same takeout, so a smaller trap will not really help me. So, I'm going to keep the 1-1/2" trap I already bought and just recess the trap adapter about 1/2" into the wall. The threads will still stick out for the nut.

Take care.

Edited 1 times.

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