question regarding plumbing vents

[river place]

Prior to building the main house e're putting a bathroom and den in our 30x40ft barn.  The barn  has a concrete floor poured except for the 10x10 left open in the corner so I can run plumbing.  The area is about 1ft deep and will be poured with concrete once plumbing is done.

here's the layout

4" septic line comes into the barn on the 40' wall 2ft' from the end of the wall.  I then have two junctions for 2 inch pipe to be used for two sinks (one in shower area and on for toilet area).  Each of these lines will vent up the wall to an extenal pipe. 

The 4' septic line then turns 90  about 2ft short of the concrete.  The run now goes to the toilet flange.  The run has a tee prior to the next 90 bend.  the toilet flange attaches to a 90 and the 90 goes into the tee.  after the toilet t connection the septic line goes into another 90 which converts it to 3" pipe.  this line then goes to the shower drain connection then out to the 40ft wall again.  at the 40ft wall there will be a 3" vent line to the roof and a capped end that will go through the barn wall.  This will allow me to run a clean out if required.

My question is should I put another tee in place before the toilet connect to add another vent 1ft down stream from the toilet?  This can be a 3" pipe and go up through the toilet wall.  The toilet can also use the 3" pipe as a vent but I beleive this would be a called a wet vent as the shower uses it as a drain line.

The shape of the drain line is basically a squared off U shape so that I can have a clean out at the wall.

Just not sure if the toilet vent will be OK

[glenn kangiser]

You can get away with a 2" vent taken off the top of that point at a 45 toward the top so it will not bother the flow and return it to the 3" vent at a higher point to prevent the wet vent condition.  Wet venting is allowed in some areas but interpretation by inspectors has always been such a problem for me I avoid it.  Hope I pictured your setup right in my head.

[ScottA]

By code you'd need a vent for the toilet. It can be tied back into one of the other vents 42" above the floor. Typicaly wet vents are only allowed on vertical drain pipes and the wet vented section must be the same size as the drain. If the drain line is hoizontal then no wet venting allowed. Glen is correct that the vent you add needs to come off the top half of the pipe and it must use drainage fitting until it gets 42" above the floor.

[MountainDon]

Without a correctly placed vent won't the toilet gurgle when flushed?

[glenn kangiser]

It'll suck. 

Creations my not flow well in the pipe from lack of easy air movement and the toilet trap may get sucked pretty dry.

Don't ask me how I know.

[John_M]

Did one of your toilets poop out Glenn or was it just a crappy plumbing job?   

[glenn kangiser]

It was a matter of waiting to long before finishing.  You know how bad that can be. Sometimes when you wait to long before finishing you go ahead with something else ... you know ...it slips your mind then things deteriorate from there . 

It may come back to haunt you in the end. 

[river place]

I appreciate the help I think  .  I will try to move the toilet vent connection to the back of the flange/90.  There isn't much room but I'll try.  One thing I'm trying to do is make sure all the bends from the access and turned junctions (not straight tee's) so if I need to run a drain cleaner down the pipes it'll go all the way through.

I'll be back up there this weekend to complete the job.  trying to get the concrete poured by mid April.

[JRR]

Riverplace, would a wall-mounted toilet work better for you?

[river place]

The wall behind the toilet will be the concrete that's already poured so it needs to be a floor mount.