Plumbing questions

[rocking23nf]

Hey all, im getting to the plumbing part of my reno and have a few questions. All pipe is ABS.

My sink/bathroom sink/shower, all have existing 1.5" drain pipes which connect to septic tank and has no air vent.

Now code in my area calls for 2" pipe and a drain vent, however this would be a massive job and im not pulling a permit, this 1.5 inch pipe has been in place for probably 30 years and its in great condition.

my shower has a 2 inch drain so im thinking of adding a reducer to shrink it, am i playing with fire by doing all of this with 1.5 inch drains? keep in mind this is very light use.  I might add cheater valves if you think its neccessary. My toilet is a straight shute to a composter with 3 inch pipe so thats not a concern.

[flyingvan]

  Boy---there are so many reasons to vent it.  Is it really that hard to run a pipe up through the ceiling?  You want air to follow the water down for good drainage.  Also if you're gone for awhile and your p trap dries out, your septic gasses have nowhere to go but in the house.  With a vent pipe, they'd rather go up and out....Another option, if there are vents elsewhere in your system, is an Air Admittance Valve (AAV)  all you have to do is T your pipe right above where it's draining away and glue it in place.  It's a one way valve that will allow air to suck in when water's flowing down.....
  Your instincts are correct on reducing drains.  Never a good idea.  Only go small to big.  Also---the cumulative cross section area of your theough-roof venting needs to be at least equal to the cross section area of the pipe feeding your septic. 

[rocking23nf]

I can probably hook up some sort of vent, but since my plumbing is all under my cabin, (its raised on blocks) I was hoping i could just install cheater valves above each trap.

Regarding the reducer, i dont have much of a choice, its eiter escavate and dig up the septic tank and maybe even replace it (since its 1.5 inch going into the ground), or reduce it from 2 inch on the shower to 1.5 inch.  The old tub was 1.5 inch so i figured a new shower would use less water then a tub.

[flyingvan]

Well, can you at least cut in a sweep right where your drain passes the outside wall on its way to the tank, and run a vent up the exterior wall through the roof?  Your 'cheater valves' will help with the air introduction but do nothing to keep sewer gasses out of the structure.  I'd have really good grates over every drain, too---1 1/2" ABS is just begging to be plugged up with hair

[rocking23nf]

yeah that seems reasonable, do they make 1.5 inch roof stacks (not sure if im using the right term)

[BADB0Y]

You can make it from 1.5", but really it need to be sized off the system.

[flyingvan]

The roof stack is just plain old ABS pipe.  The make the flashing for 1 1/2", if that's what you're asking.  Just glue ABS into the sweep and have it run up and through your eave.  It should be the same or greater diameter than the pipe feeding the septic.  This is very cheap insurance.
Back to the 1 1/2" drain from the shower---you are asking for more hair plugs than Joe Biden