Moving a 3 piece bathroom - drain and vent plan

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TorontoTim

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I'm about to do the rough-in drains for a 3 piece bath I'm relocating at a cabin. It drains into a septic system. The 'old' bathroom is the one on the long back wall of the cabin. The 'new' bathroom is on the short side wall. Once the new one is up and running I will disconnect the drains from the old bathroom and cap the pipes under the cottage or cut the fittings out altogether.

There is a 3" stack beside the current bathroom that I will leave in place, at least for now. The kitchen sink is clearly not vented at all currently. It drains OK, and I'll deal with the venting when I re-do the kitchen at some point in the near future and know where the sink will end up.

The distance from the w/c flange to the point where the 2" lavatory/shower drain/wet vent connect will be 4 feet or so. I believe I'm good with that distance.

The 2 inch vent will come up from the lavatory, take a couple turns and then continue up through the roof.

I would plan on long sweep 90's for the horizontal to vertical changes as the drain comes close to the septic connection below grade. I could do 45's instead if that is preferable to a fully vertical drop, and would be easier to snake if ever needed.

For the 90 degree connections between the 2" drain and existing 3" drains, am I correct that I use a Sanitary Wye and a 1/8 bend combo? Does it have to be angled up 45 degrees from the drain or can it be on the same plane?

Last question - in the long haul, given I have a 3" main drain leaving the cottage into the septic system, I assume I need to maintain a 3" stack going through the roof. Or, can it be a combo of the new 2" vent through the roof and perhaps an additional 2" vent for the kitchen sink going through the roof as well?

The septic tank 'breathes' through this stack. As air is displaced, it exits through the stack. The tank is entirely underground with no other venting. So I want to ensure it has plenty of opportunity to exhaust the air being displaced by incoming water.
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Hey, wait a minute.

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