Moving shower drain P-trap question

[ianrwesley]

Hi,

We currently have a center located point drain in our bathroom shower stall. We are in the process of installing a linear drain and I have a question on best practice of what to do with the P-trap.

The current P trap is located in the center of the shower directly under the old drain. The new drain location will be 3 ft away. Looking for some advice on the best way to handle this. There are a couple options I have from contractors. I have some crude drawings below.

Option 1: Put a 90 deg long sweep on current vertical pipe coming off existing P trap and run under slab to new location. Install another 90 to come up and connect to new drain. Ensure 2% slope from old drain to new.

Option 2: Relocate P trap so that it is directly under the new drain. Can the P trap height be raised at all or does it need to remain the same depth below the slab? If the P trap moves what is the max slope for the line that goes from P trap to existing drain tie in?

I also am not sure how far away the vent is from the existing trap. Our lavatory sinks have under cabinet air vents.

Thanks.

 

[wwhitney]

Option 2 is almost always better.

You do need to figure out how the shower trap is vented. The total fall from the p-trap outlet to the vent connection is limited to one trap diameter to avoid siphoning the trap. Assuming a 2" trap, if you have to extend the shower trap arm 3', then you would need to confirm that the existing trap arm is using at most 1-1/4" of the allowable 2" of fall. If you can confirm that, then if you run your extra 3' at the minimum 1/4" per foot slope, it will add only 3/4" of fall, so the total fall will be below the 2" maximum allowed.

If you find that the existing fall exceeds 1-1/4", then you'd need to reroute the shower trap arm and/or relocate the vent connection so that you can comply with the maximum 2" of fall while still maintaining a minimum 1/4" per foot of slope.

Cheers, Wayne

 

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[ianrwesley]

Option 2 is almost always better.

You do need to figure out how the shower trap is vented. The total fall from the p-trap outlet to the vent connection is limited to one trap diameter to avoid siphoning the trap. Assuming a 2" trap, if you have to extend the shower trap arm 3', then you would need to confirm that the existing trap arm is using at most 1-1/4" of the allowable 2" of fall. If you can confirm that, then if you run your extra 3' at the minimum 1/4" per foot slope, it will add only 3/4" of fall, so the total fall will be below the 2" maximum allowed.

If you find that the existing fall exceeds 1-1/4", then you'd need to reroute the shower trap arm and/or relocate the vent connection so that you can comply with the maximum 2" of fall while still maintaining a minimum 1/4" per foot of slope.

Cheers, Wayne

How would I confirm how much fall the existing trap is using? If I can't confirm or I would exceed 2" of fall to relocate the P trap should I go with option #1?

Thanks.

 

[wwhitney]

How would I confirm how much fall the existing trap is using?

Plans of the existing construction, stick a camera down the existing trap arm, or break up the slab to investigate. Or maybe something else I've not thought of.

If I can't confirm or I would exceed 2" of fall to relocate the P trap should I go with option #1?

So, the 3' distance makes option #1 a code violation (I had to look that up). Florida uses the IRC/IPC, and IRC P3201.6 says in part "The vertical distance from the fixture outlet to the trap weir shall not exceed 24 inches (610 mm) and the horizontal distance shall not exceed 30 inches (762 mm) measured from the center line of the fixture outlet to the centerline of the inlet of the trap."

Chapter 32 Traps: Traps, 2023 FBC - Residential, 8th edition | UpCodes

UpCodes offers a consolidated resource of construction and building code grouped by jurisdiction

up.codes

Now, you might be tempted to do a modified version of option #1 where you move the trap just one foot, say, and have a 24" horizontal run between the shower pan outlet and the trap inlet, and that would comply with the above code section. But that should really be a last resort. That horizontal section can retain material and develop odors just like any other part of your drain system, but it is on the fixture side of the trap, so any odors would be come into the bathroom. And being under a slab, it would be harder to service, as opposed to say the horizontal plumbing under a double kitchen sink.

I assume you've selected a linear drain with the outlet located at a point along its length closest to the location of the existing trap?

Cheers, Wayne

 

[ianrwesley]

Plans of the existing construction, stick a camera down the existing trap arm, or break up the slab to investigate. Or maybe something else I've not thought of.


So, the 3' distance makes option #1 a code violation (I had to look that up). Florida uses the IRC/IPC, and IRC P3201.6 says in part "The vertical distance from the fixture outlet to the trap weir shall not exceed 24 inches (610 mm) and the horizontal distance shall not exceed 30 inches (762 mm) measured from the center line of the fixture outlet to the centerline of the inlet of the trap."

Chapter 32 Traps: Traps, 2023 FBC - Residential, 8th edition | UpCodes

UpCodes offers a consolidated resource of construction and building code grouped by jurisdiction

up.codes

Now, you might be tempted to do a modified version of option #1 where you move the trap just one foot, say, and have a 24" horizontal run between the shower pan outlet and the trap inlet, and that would comply with the above code section. But that should really be a last resort. That horizontal section can retain material and develop odors just like any other part of your drain system, but it is on the fixture side of the trap, so any odors would be come into the bathroom. And being under a slab, it would be harder to service, as opposed to say the horizontal plumbing under a double kitchen sink.

I assume you've selected a linear drain with the outlet located at a point along its length closest to the location of the existing trap?

Cheers, Wayne

Thanks. I have one more follow-up question... I was looking at the florida building code table (included below) and it seems that code allows further distance from vent for larger traps. The question I have is exactly what is it referring to with trap size?

For instance, could I use a 3" trap but then reduce to 2" on the pipe coming up and on the tie in to the existing 2" line? Would this qualify or does it need to be a full 3" all the way from trap to vent?

TABLE 909.1

MAXIMUM DISTANCE OF FIXTURE TRAP FROM VENT

SIZE OF TRAP(inches)​	SLOPE(inch per foot)​	DISTANCE FROM TRAP(feet)​
11/4​	1/4​	5​
11/2​	1/4​	6​
2​	1/4​	8​
3​	1/8​	12​
4​	1/8​	16​

For SI: 1 inch = 25.4 mm, 1 foot = 304.8 mm, 1 inch per foot = 83.3 mm/m.

 

[wwhitney]

Can not reduce drain size when going downstream, so a 3" trap would require a 3" trap arm (= drain between trap and vent). Would also be very unusual on a shower.

Cheers, Wayne