Joining Drain Vents

[ShadowAviator]

I have a 2 in vent coming from the bathroom group to the south and a 2 in vent from the kitchen/laundry group to the north. I want them to meet and go to a 3 in pipe before exiting the roof. I only want one roof penetration. I suppose I could just keep it 2in pipe all the way. There are not very many fixtures.

Shower, toilet, lavatory.
Utility sink, floor drain.
Kitchen sink, washing machine (maybe in the future).

But the main drain out is 3in, so my understanding is that the final vent should 3in as well. Is my thinking correct?

This is on a 27' x 30' shed-home with 16' tall walls. Its not very big. The roof is just 2x6 purlins with metal roofing and insulation on one side and interior siding on the other. So I have a 5 1/2 in space to work with. The distance between purlins is roughly 3 ft. The slope of the roof is a rise of 1 ft for every 4ft of run. So the roof is more horizontal than vertical. The peak of the roof runs north and south, so the 2in pipes have to join below the 3in exit in order to maintain a slope back to the drains.

This is in a area where there is no code (on a farm in the country), but I want to make sure the system works well. Do I need to maintain slope back to both drains or just one drain? I guess I am just trying to figure out which combination of fittings to use and how to orient them.

I don't mind not following code, as long as, whatever I do is still safe and works well. (Typically thats what code is for, but sometimes it makes us scratch our heads and wonder who thought it up.)

I hope I explained that well enough, I might try to get a picture if I can.

 

[Terry]

(Typically thats what code is for, but sometimes it makes us scratch our heads and wonder who thought it up.)

For a 3" line going out, code wants also a 3" through the roof. In my area, that can be two 2" vents and a 1-1/2" vent, which if you add up the area of the pipes, it's still 3".

When learning about plumbing, they don't let the helpers or apprentices even touch waste and vents for a long time. It looks simple, but everything about it has decades of reasoning behind it. It's not obvious when you're sitting eating your Wheaties for breakfast, so we have plumbing inspectors come and look at our jobs and they slap us in the face if they see something out of place. Now there were a few times I wanted to wrestle them and throw mud on them, but in the end I decided that they knew best, and so I would ask them for a solution instead. At that point they would smile and tell me I was an idiot and then explain what they wanted. It was all good.

 

[Sponsor]

 

[ShadowAviator]

I figure going with the 3" vent is also good future proofing. At least it won't ever be the bottleneck.

I may have thought of something. Heres what I drew up. Thoughts?

 

[Terry]

As long as the vent pipes have grade downward. Any water in the lines needs to find it's way all the way down.

Also, medium bends are cheaper than "vent" 90's. I figured that out in the 70's and quit stocking the vent fittings on the truck.

 

[Jadnashua]

A larger vent also helps in cold country to minimize the possibility of the line becoming closed off from hoar frost from below and through the roof. There's a constant high humidity level, and under the right circumstances, it can cause frost to accumulate when it's cold enough.

 

[ShadowAviator]

I think I should be able to maintain slope in both directions. Most rain water will probably go through the curved part of the sanitary tee. Anything that makes it past will slope to the other direction.

Regarding the 2" to 3" reducers, they need to be eccentric reducers to maintain flow, right?