Correct pipe fall, toilet flange height and washing machine questions

Users who are viewing this thread


Reaction score
Raleigh, NC
So After much effort designing the dwv system for my total remodel we got the parts and got started (thanks to a few of you for your excellent advise). I did a lot of studying and reading both the code and guidebooks from IPC and am feeling more comfortable in understanding what is code and why.

first question is about pipe fall. I understand what fall is required, and set up a level with a block so im not guessing. Question is this : for example on the main 3” drain, if a 3” combo is installed plumb and square to the drain line, will it have correct fall for the branch or does it need a little turn and test it with a 2’ length of pipe ? I read that the dwv fittings have the correct “Fall” built in to the fitting, but havent been able to tru verify this. We are measuring and double checking before gluing but it would be good to know.

one other fall question. On a run of 2” connecting a lavatory and bidet to main the drain, A section of 2” pipe connecting the wye from bidet to 233 combo at the main drain (about 18” long piece of 2”) the fall is slightly less than the 4’ section on the other side of the wye back to the lavatory. With hangers we got the fall looking good, without requirements. How exact does the fall need to be to work properly? are there acceptable tolerances or does it need to be dead on?

2nd. When installing toilet flanges, finished floor for both will be approximately 11/2” above subfloor (tile, durock, membrane thinset layers). So when placing the flanges, do you cut that pipe to length and install flange for inspection and use a plug for rough in, or do you set it to 11/2” with built in test knock out and hope the layers fit well? Last flange - do you sit it on finished floor? That is what seems to make sense based on toilet and wax rings thickness.

last question. If the washing machine trap is above 6” and under 18” by code, the standpipe (IPC) should be 30” above that, can it be taller? I couldn’t find an answer for this

thanks all. Happy plumbing .


In the Trades
Reaction score
Berkeley, CA
1) Regardless, you always check it with a dry fit piece of pipe and a level.

2) Fall is a minimum, 1/4" per foot. The two drains joining the wye don't need identical fall, as long as they are at least 1/4" per foot. If that answer is insufficient, I'm not sure I follow the question, how about an annotated picture?

3) 1-1/2" seems a bit thick for just tile, durock and thinset. What is the membrane? What would work best would be to leave the riser long, have the final flange you will install on hand so that you leave any required gaps around the pipe during the buildup, and then after the finish floor is set, cut the riser to height and place the flange. Or possibly cut it just before tile is set, if your flange requires you to cut it below the finish floor elevation.

A stainless flange (which is recommended) should sit on the finish floor. A thicker flange (plastic) might be better off partially above and partially below the finish floor.

4) The trap being between 6" and 18" above the floor isn't an IPC requirement, that's from the UPC. The IPC doesn't care. The standpipe needs to be 18" to 42" in length. And your washing machine manual will specify the allowed elevation of the top of the standpipe, relative to the surface the washing machine sits on.

[Looks like NC amended the IPC to allow 48" long standpipes.]

Cheers, Wayne
Hey, wait a minute.

This is awkward, but...

It looks like you're using an ad blocker. We get it, but (1) can't live without ads, and (2) ad blockers can cause issues with videos and comments. If you'd like to support the site, please allow ads.

If any particular ad is your REASON for blocking ads, please let us know. We might be able to do something about it. Thanks.
I've Disabled AdBlock    No Thanks