Joining parallel horizontal drains -- some questions!

[twofish]

Hi! This site has been a great resource, thank you for making it available. I have a few questions if anybody is willing to help. Nearly all the drain plumbing for my new residence will be along one wall. Single story, slab on grade. One bathroom, one half bath (shared wall) with laundry/laundry sink, and kitchen (sink and dishwasher). The concrete is not poured yet, I am busy getting the plumbing sorted. In Idaho, which is a UPC variant.

I lived for years in a fairly unconventional (and un-permited) structure where we used composting toilets and everything else was plumbed out subsurface into mulch pits for irrigation. It worked very well, and I eventually want to utilize greywater again in my more conventional and more permitted structure. To that end, I am future-proofing by running parallel 3" main drains under the wall so I don't have to dig up any concrete to route off the greywater later. Both W.C. are on one drain (directly under the wall) and everything else on the other drain (slightly offset) that joins the first. The drains are parallel with flow in the same direction, at the same elevation, and can be some variable distance apart up to about 8" apart (limited by trench width). The two drains will join and continue in the same direction as 4" to the septic tank. Venting is shared and will be joined somewhere in the wall and/or attic.

So, some questions!
1) Though the above plumbing is awkward and uses more material it should be OK under the UPC, at least in principle. Is this right?
2) What is the most appropriate way to join these two parallel 3" drains? I have several ideas in mind but my fluency with these fittings is not as good as a professional's. Can I just roll a wye flat and use a 45?
3) Similarly what is the most appropriate way to offset the vertical drains after they come through the bottom of the wall to get to the other 3" line. It looks like rolling a wye 45 off the main drain and then a 60 to come back vertical might do it, if that is code compliant...
4) I see a lot of whinging about the 1.6g flush toilets not having enough flow to adequately move waste. In my case at least one section (and perhaps at a later point the whole main drain line) will only have the two W.C. draining into it. Is this a concern?

Thank you!

 

[wwhitney]

Both W.C. are on one drain (directly under the wall) and everything else on the other drain

FWIW, I had heard it is best to put the kitchen sink on the black water drain rather than the grey water drain, as the food water/grease tends to be problematic for grey water systems.

1) To my knowledge, yes, as long as everything is properly vented.
2) Yes
3) I don't follow the question, question two implies you are joining the two drains while they are parallel, so what are you trying to do here?
4) Don't know.

Cheers, Wayne

 

[Sponsor]

 

[twofish]

Thank you Wayne.

Regarding kitchen sinks, I was debating that as well and may make that change. We did not have any issues with this previously but it probably depends on the nature of the greywater routing and infiltration method. As far as I can tell Idaho code at the moment considers kitchen sinks, laundry sinks, dishwashers, and laundry machines (everything in my house except for the tub pretty much) to be blackwater anyway, though that wasn't the case previously and if I were to wager probably won't always be the case given the trend of other jurisdictions. To be clear I am not installing any greywater yet, just trying to make my life easier later if I do.

Regarding (3) I probably phrased it poorly, sorry. The two 3" lines do join while parallel; I was referring to the 2" vertical stack drains from fixtures that need to join the 3" "greywater" main drain before that union point. Those vertical drains come through the wall bottom plate but the "greywater" 3" drain is offset parallel to the side underneath the wall because the the "toilet" main drain occupies that location right under the wall. The lines are deep (25-30") so I have some room to play with. I could do (starting at the main 3" drain): 3x3x2 wye with a 2" street 45 to get pointed vertical, then a street 45 turned to face the wall, a short length of 2" pipe to get underneath the wall (and also climb a bit), and a final 45 to connect to the vertical drain. Or perhaps those last two fittings should be sweep 90s instead of 45s. My gut tells me this can be done with fewer fittings by rolling the wye but perhaps I am wrong about that.

 

[wwhitney]

Yes, you can eliminate a fitting by rolling your combo (the wye plus street 45, which can be had as a single fitting). You roll it 45 degrees to eliminate the first upstream 45 from the combo. Then you just need one 45 to go vertical.

Cheers, Wayne