My old cast iron/steel 2" drain which my kitchen sink, and basement laundry wash tub and washing machine ran through kept plugging and backing up so I ran a new 2" PVC drain overland along the wall to the 4" main sewer drain. I didn't know how much slope I would need so I abandoned the wash tub in order to gets lots of drainage.
I want to include the wash tub now and redo the drain. I have the washing machine drain hose hard plumbed into the drain with a barbed PVC fitting so it can't back up and leak. I also have the drain line connected to the existing steel vent line. There is also no dish washer or garbage disposal.
Will 5/16" / 12" slope be enough? Should I convert to 3" line? This is about a 28' total run.
3" would be better and help with eventual blockages. 5/16" per foot is more than enough of slope for proper drainage.
1/4 inch of foot is the minimum, and you can get away with less if you upsize the pipe. PVC does not plug as quick as cast iron because it's smoother. And if you have a choice then run the washing machine drain in at the end of the other drains because that one is pumped and will help to flush anything else that's moving more slowly or sticking.
This is a link to the section of the international plumbing code that covers slope. It says for a 2" pipe, 1/4" per foot is the minimum, but doesn't say anything about a maximum. I didn't read through the whole section but I assume it has pipe size recommendations as well.
https://codes.iccsafe.org/content/IPC2018P5/chapter-7-sanitary-drainage
This is a drain for a sink so there should be a minimal amount of solids. I believe slope gets trickier for a toilet drain or the main drain to the city sanitary system; you don't want it to slope not enough, or too much, to make sure that the water carries solids all the way out and doesn't let them fall out of suspension and collect in the pipe.
In reply to stuart in mn :
Thanks.
2.5" and less: 1/4" per foot
3" to 6": 1/8" per foot
8" and more: 1/16" per foot
Edit: I couldn't find anything about a maximum slope but I could have missed it easily enough.
