Venting and drainage in an attic bathroom

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Giantsfan301

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I am finishing my attic and I need some help making sure I install the drainage and venting for the bathroom correctly. In the picture I plan on putting the bathroom as close as I can to the water heater and the ducts you see. Picture a rectangle pushed up against the water heater and the air duct. It will contain a sink a shower and a toilet. I have a 3 inch waste line that is off the left(you can barely see it. It has some pink insulation near it in one picture)20190303_202002.jpg 20190303_201958.jpg 20190303_201949.jpg 20190303_201939.jpg 20190302_143852.jpg which runs through an exterior wall and comes out just above my master bathroom makes two 45 degree turns and then immediately goes out my roof. I also have the 2 inch vent for the sink in that bathroom coming up in that area but it is just on the other side of that duct. My first question is how can I tie into that 3 inch drain pipe. I need the point where I tie into with my drain to be as low as possible to get my slope. Now, as far as venting should I just put a dry vent from the new plumbing straight out my roof as soon as I get past the drywall from the new bathroom. I am getting a new roof so this would be very simple. I hope the pictures help. Thanks20190303_201949.jpg 20190303_201939.jpg20190303_202002.jpg 20190303_201958.jpg
 
They are 2 by 8s. The whole house is 2 by 8s. Is it different for a third floor or updated code since the 80s.
 
https://www.mycarpentry.com/joist-span-table.html

ok....I can help you out but I have questions that need answers
that 3'' pipe, that is a vent, you need to tie onto it. below
what ever it is tied into.
so i need to know. where it comes from.


what i am talking about is this.

2.jpg
YOU
WHY?????
Can't I tie into that 3'' pipe in the attic...it is right there and it would save me lots of money and time
ME
it is a vent, You can not tie a drain into a vent.
you have to tie in below the fixture that it is a vent for
it sucks, but that is the code
 
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Ok that's what I was afraid of. Haha. That pipe goes down and connects to my plumbing in the 2nd story bathroom just below it, just like you have in the diagram and then goes to the crawlspace.
So would the plan then be to take my 3 inch drain from the attic bathroom run it along the existing pipe through the wall and the just tie into it with a Y fitting? Just curious but what are the reasons that code wont allow this. Is it because the toilet sink and everything will be to high?
I really appreciate your help!
 
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Took this photo awhile ago of my plumbing in the bathroom below the proposed attic bathroom. Everything drains into that pipe that comes out the attic. There is also another bathroom on the second floor. From what I can tell both of these full baths go to this pipe. They are also vented through that 2 inch pipe in the original pictures I posted.

Im still kind of confused as to why I cant tie into that 3 inch pipe and then create some dry vents that go out the roof right near the new bathroom. If there was a bathroom just below the second story bathroom that would be fine right? So what's the difference? Thanks again!
 

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This bathroom has been finished for a couple years. I didnt plan on finishing the attic then but yes the 3 inch vent pipe is right behind that corner where the pipes go. It comes straight up from crawlspace to the attic.
 
I am going to attempt to explain why not
the vent systems main job in life is to protect the p traps water seal.
if you did not have a vent, when you washed your face, the water would drain out of the sink
and the water in the p trap would be siphoned / pulled out of the trap.
by having a vent. it equalizes the air pressure on both sides of the trap.
therefore allowing the water to stay in the trap,

with me so far?
I am a plumber not a freaking physics teacher, so hang with me, we will get there

ok..
now, if you drain a bathroom into the vent of the bathroom below it
you are cutting off the vent of the down stairs bathroom every time you use the upstairs bathroom
so,,when JR flushes the toilet down stairs and Daisy May flushes the toilet upstairs
the water in the sink and shower p trap is going to be sucked/siphoned out.
and then,methane gas will start till fill the bathroom.
the bathroom will smell like someone farted, your Lady will be looking at you funny and JR will get in trouble for teasing Daisy May calling her fart fart not smart
You will claim innocence and blame Fido,
 
Ok. That makes sense. I guess I got confused when I saw stack venting diagrams through my research and they had bathrooms on top on bathrooms but they all had dry vents which prevents what you just described. Is that correct?
What should my plan of action be? Do you need more information?
 
Graphic example of how it works...
 

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I am not going to argue
If you want my help, It will be done correctly and up to code
If you want a Jack Legged job then go to home depot at 7 am and you will find lots of plumbers
willing to do it anyway you want.

the sheet rock down stairs has to be removed, in order to install the piping
 
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Im sorry if you interpreted my response as me trying to argue. Im just trying to understand. You've been very helpful and I was just asking if you had a plan in mind. Should I just take a 3 inch pipe and run in down along the existing and y it in below the second story? Will I need to run dry vents up through the attic roof?
 
Not a problem...
What you are doing is not something where I can say, do this do that
with out seeing where the toilet on the 2nd floor ties into the waste line
You need to open the wall on the 2nd floor.
so, go ahead and open the wall, and lets see if this is going to be simple or are you going to have to drop down
and open the ceiling on the first floor. it all depends on how the house was plumbed
 
It ties in just near that corner where you pointed to in that picture I posted. It then goes to crawlspace.
 

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