Shower drain size

[jay156]

Trying to replace an old tub with a new shower. It had a 1-1/2" drain, and there's still a portion of that left that can't be easily replaced with 2".

In the attached picture, the shower is going to be on the left, where the small osb board is. The 1-1/2" drain is on the right in front of the toilet drain. It goes under the floor another 2'-6" before it turns down to the basement.

My question is, if I put in as much 2" as possible and then reduce right by the existing, do you think I'll have problems with water backing up in the shower?

 

[Twowaxhack]

I’d use all 1.5” if you can’t change it all to 2”. Don’t go from larger pipe to smaller pipe.

If this is being inspected or needs to be code “ accepted “ get prior permission from inspection office.

Showers will work fine with a 1.5” drain, especially so with a standard 2.5 gallon per minute shower head.

I’ve done it many times and it works despite what the internet says.

 

[MicEd69]

You never want to reduce any drain size going with the flow.

And 1 1/2" will work as a drain as Twowaxhack said. However, you may have a problem with venting the P-trap. IPC allows a vent to be 6 feet from a 1 1/2" P-trap, but the UPC requires the vent to be within 3 1/2 feet of a 1 1/2" P-trap.

So, assuming the vent is where the 1 1/2" drain turns down into the basement, you will need to have the shower drain either 1 foot or 3 1/2 feet of the existing stub, depending on which code you want or need to follow.

 

[jay156]

Thanks for the responses, guys. Okay, so I won't install any 2-inch runs of pipe. One question though, the shower pan that we bought has a 2-in drain that came with it. Would it be okay to install a 2-inch P-trap and then immediately reduce it to 1 inch? Or should I just reduce it right at the shower pan?

The vent is actually in the wall towards the left in the photo. you can see it there where we cut it off when we removed the tub drain. Once the shower is in it will be about 3:00 or 4 ft from the shower drain, then the drain will go off to the right and into the existing.

 

[Twowaxhack]

Thanks for the responses, guys. Okay, so I won't install any 2-inch runs of pipe. One question though, the shower pan that we bought has a 2-in drain that came with it. Would it be okay to install a 2-inch P-trap and then immediately reduce it to 1 inch? Or should I just reduce it right at the shower pan?

The vent is actually in the wall towards the left in the photo. you can see it there where we cut it off when we removed the tub drain. Once the shower is in it will be about 3:00 or 4 ft from the shower drain, then the drain will go off to the right and into the existing.

https://www.homedepot.com/p/JONES-S...VgSfUAR2EiSvYEAQYFCABEgKrVvD_BwE&gclsrc=aw.ds

 

[MicEd69]

Thanks for the responses, guys. Okay, so I won't install any 2-inch runs of pipe. One question though, the shower pan that we bought has a 2-in drain that came with it. Would it be okay to install a 2-inch P-trap and then immediately reduce it to 1 inch? Or should I just reduce it right at the shower pan?

The vent is actually in the wall towards the left in the photo. you can see it there where we cut it off when we removed the tub drain. Once the shower is in it will be about 3:00 or 4 ft from the shower drain, then the drain will go off to the right and into the existing.

I saw that but didn't know what it was for.

As a dry vent cannot run horizontal until it reaches 6" above the flood plain of the fixture, I'm assuming the shower drain will run towards that vent line and the shower vent will then angle up at 45 degrees (which the codes consider vertical) to meet with that vent line. The shower drain line will then turn to the right in the picture to meet up with the existing 1 1/2" drain line, correct?

You also need to consider what fitting to use for the vent connection. Some interpret the codes that a Sanitary tee on its back can be used for a dry vent connection to a horizontal drain line. Others think a Wye and 1/8 bend or a Combo fitting is the only connection that is acceptable for a dry vent to a horizontal drain line. The latter is easier to snake should you need to do that. It's also advisable to add a clean-out in the vent line.

 

[Twowaxhack]

The vent is actually in the wall towards the left in the photo. you can see it there where we cut it off when we removed the tub drain. Once the shower is in it will be about 3:00 or 4 ft from the shower drain, then the drain will go off to the right and into the existing.

Fur the wall out if needed.