Working on the ABS drain today, which is neither fun nor glamorous, but necessary.
We chose to bring the toilet, sink, and shower drains straight down through the floor and connect them underneath the trailer with flexible couplings. Doing it this way gives us the maximum possible flexibility to reconfigure for gray water separation, composting toilets, or whatever is required in future locations. We can also just loosen all the couplings and remove the entire drain tree and then reattach it in a new location. The only real downsides are losing a bit of precious drop due to the length of the elbow fittings, and the aggravation of putting it all together in such a tight space.
For now we will be connected to city sewer by means of a temporary RV drain, but all the same rules for normal ABS drain plumbing still apply.
Pipes should be sloped at a quarter inch per foot, however, that’s assuming your tiny house is perfectly level so I like to go half inch per foot if I can. The pipe needs to be supported every 4 feet, and if you are using metal strapping you need to wrap the pipe with pipe tape.
I’m venting the whole system through the 1 1/2 autovent attached to the sink stack. Code for this would be a 2” vent, so I added a 2nd 1 1/2” capped off vent pipe underneath the trailer. It’s overwhelmingly unlikely that would be necessary but it never hurts to leave yourself options. I also tend to install a few more cleanouts than code requires because not only does it make it easier to deal with possible blockages, but it also makes it easier to find where the problem actually is.
I like to dry fit as much of the pipe as possible and then go back and glue. A couple pieces of blue tape with an alignment mark helps to make sure you’re gluing things in the orientation you planned. The big can of ABS cement has a bigger brush which lets you goop both sides of the pipe in a single dip which translates into about 10 more seconds of working time when you stick the pipes together which can be hugely helpful. A sawzall is crap for making straight ABS cuts (at least how I do it) but a japanese saw works great.
– Brian
