Tiny House Plumbing: Gray Water Disposal

An important question all “tiny housers” must consider: Where will our waste water go? 

Truthfully, I don’t like to even call it waste water. Waste typically refers to something that can’t be recycled, restored, or reused. This doesn’t have to be the case with gray water. Depending on the system set up and the substances that go into the gray water, it can be used for other purposes, such as watering a garden or lawn. But that’s a discussion for another time. For now let’s focus on how to get gray water out of the house safely.

We’re only talking about gray water here, meaning water from our kitchen sink and shower. We use a composting toilet, so we actually do not have black water to contend with, and I will not be discussing the disposal of black water in this post. As for our gray water, Mr. Epic and I have decided to try to move away from chemicals as much as possible so that none of it enters our gray water. This means we use biodegradable soap for laundry, dishes, and as a body wash, such as Dr. Bronner’s Castile Liquid Soap. Neither do we use shampoo and conditioner anymore (water, baking soda, and apple cider vinegar is all you need).

All that being said, during the few months that we have been living in our Epic Tiny House, we have already had two different gray water disposal systems. The first portion of our gray water system (what is connected to our shower and kitchen sink) have remained the same. The connections made on the outside of the house have changed somewhat, and I will go over each set up and the reasons for each.

First, let’s start with the gray water from the kitchen sink.  

Most sinks use 1 1/2″ size pipes, and you can choose from a variety of materials to use, but most either use ABS or PVC pipes. Both have their pros and cons. As I had never installed plumbing before, I opted for the ABS as it’s a bit easier to install and seems to be a little cheaper in price. I used 1 1/2″ ABS to connect my kitchen sink drain down to a p-trap, then another piece of 1 1/2″ ABS pipe to connect to a 1 1/2″ ABS Sanitary Tee.

The tee serves three purposes; first, as to be a completion of the arm coming off of the p-trap, which is necessary for the function of a p-trap; second, to allow the attachment of an air admittance valve (or AAV); and third, to allow the gray water to continue it’s path out of the house.

Picture showing our p-trap under the kitchen sink

Before I continue on, let’s discuss the purpose of a p-trap and air admittance valve. At first glance, a p-trap appears to be an unnecessary excess of pipe that just takes up valuable space inside a tiny house, and in some limited cases, that might be true. But overwhelming so, these are just as necessary in a tiny house as they are in a traditional home. The reason being is sewer gases.

In a traditional home and sewer set up, all of the gray water (sinks, shower, laundry) and black water (toilets) connect through a series of pipes and enter through one main line into a city sewer system. This sewer creates gas that is smelly and hazardous to your health if allowed to enter the main home. A p-trap is used to trap these sewer gases so that they can’t enter your home. The name comes from the shape the pipes make, which looks like a letter “P”. The part of the pipes that create the rounded part of the “P” is shaped thusly so as to trap water in that section of the pipe. The water creates a seal that does not allow sewer gases to come back up through the drain and into the home. Extremely simple and extremely effective!

As for the air admittance valve, this is an additional measure used to ensure sewer gases do not enter the home. On traditional homes, you’ll notice on the roof there are usually several vents, some small and some larger. At least one of these (and sometimes several of them) are actually vents to allow air to enter the gray and black water pipes inside a house.

This is done to break the siphoning action caused by draining water through the pipes by regulating the air pressure in the pipes. Without this, water could not remain in the p-traps and sewer gases would enter the home. An air admittance valve is used in a traditional home with a vent system can’t be used through the roof (such as a kitchen sink in an island).

Air admittance valve under kitchen sink

For a tiny house, I suspect that most don’t use air admittance valves. The argument is that if you are not connected to sewer lines, or only have gray water, it isn’t necessary. I’m not a plumber, but here’s what I do know: Being in a tiny house on wheels, you never know what your next connection point might be (which could be a city sewer system), and for a few dollars, it’s worth it to go ahead and install one under the kitchen sink.

Moving on then from the 1 1/2″ Sanitary Tee, I attached another piece of 1 1/2″ ABS Pipe that extends through the subfloor and out beyond the bottom of the trailer about 4″. That is where the permanent portion of the kitchen sink gray water plumbing ends, and then begins the transition into the removable and changeable portion of the gray water disposal system. I’ll come back to the removable section in a bit.

Now, let’s discuss the shower drain.

Most shower drains are designed with a 2″ drain, so we chose a short piece of 2″ ABS pipe to attach to our shower drain, and just like the kitchen sink drain, it extends through the subfloor and out beyond the bottom of the trailer about 4″. And just like the kitchen sink, that is the end of the permanent portion of the tiny house plumbing for the shower.

Here is where our two different disposal systems came into play. In our first location, we were parking on someone’s property and were not connected to a city sewer line, thus we did not have to worry about sewer gases. Ideally we would have been able to dig a trench and allow our water to disburse underground to water a garden, but that was not an option at that location. Our only option was to disperse the water as far away from the house as possible onto the ground. Please note that before you do this, you should check your local laws because it is not legal to disperse gray water on the ground in many places. That being said, here was our set up.

For ease of disposal, we decided it would be best to connect the gray water coming from the kitchen sink and the shower into one drain pipe. From the kitchen sink 1 1/2″ ABS pipe protruding from the bottom of the trailer, we attached a 1 1/2″ to 2″ flex coupling and from the 2″ ABS pipe protruding from the bottom of the trailer from the shower, we attached a 2″ flex coupling. These couplings use metal bands that can be tightened and loosed with a screw driver, which allows us to remove the remainder of the plumbing when we’re on the move and adapt it to our new locations.

On the other end of the flex couplings, we attached a series of 2″ ABS pipes and fixtures that are glued together so as to make them leak proof. The pipes are as follows: 2″ ABS pipe sections coming down from both sets of flex couplings, and both attached to 90 degree ABS elbows, which then both connect to 2″ ABS pipe that are connected by a 2″ ABS vent tee, and then another short section of 2″ ABS pipe is attached to the bottom of the vent tee.

At this point we transition from those glued pieces to another temporary connection.  I used a 2″ to 4″ flex coupling to attach the 2″ ABS pipe to a 4″ Flexible/Expandable Landscaping Drain Pipe. This drain pipe was placed on top of a 20′ Sidewinder Plastic Sewer Hose Support to allow for a specific angle for drainage purposes. At the end of the drain pipe, we cut a small piece of insect screen and placed it over the mouth of the pipe via a rubber band to keep insects and rodents out of the pipe. Once a week we removed the screen to clean it, but aside from that, it worked like a charm!

We used the setup described above for several weeks and we didn’t have any problems with it, but when we moved, we knew we were going to be connected to a city sewer system, which changed our design just a bit.

To start, we had to change out the 2″ to 4″ flex coupling and 4″ Flexible/Expandable Landscaping Drain Pipe for something that would work at and RV connection site. For this I switched from a 2″ to 4″ flex coupling to a 2″ flex coupling, which I then attached to a short piece of 2″ ABS pipe followed by another 2″ 90 degree ABS elbow, followed by another short piece of 2″ ABS pipe and lastly followed by a 2″ to 3″ coupling.  

Then, as the distance from my pipes to my sewer drain was longer than I anticipated, I had to run to the local hardware store and picked up a 2′ section of 3″ PVC pipe. I connected the 3″ PVC pipe to the coupling, then attached a 3″ flex coupling to a 2′ section of 3″ ABS pipe that I already had, and then this was connected to a Termination Adapter with 3″ Bayonet and 3″ Slip Hub.  From the bayonet end of the fitting I connected a Camco 39741 RhinoFLEX 20′ Sewer Hose Kit with Swivel Fitting, which extended all the way to my sewer connection.

And that was the end of our changes….initially. Remember what I said about sewer gases entering the home and the importance of p-traps?  As you recall, we had on installed under the kitchen sink but not under the shower. In our first location, this was fine as we did not have to contend with sewer gases but in our second location….whew!!  

The first two days here we could not figure out why it smelled so badly in our house. It wasn’t until the smell woke me up on the second night that it dawned on me that there was no p-trap under the shower drain! The next morning I ran out to the hardware store and picked up the necessary items. Five dollars and 10 minutes later, and the smell was gone!  

So here’s the rest of the changes we made to our set up:

Under the shower drain, I removed the 2″ flex coupling and instead attached a 2″ PVC p-trap. I connected this to a short section of 2″ ABS pipe that I had, followed by a 2″ 90 degree PVC elbow. The elbow then attached to my pre-made ABS pipes that connect the shower and kitchen sink.

I had limited room under the trailer at this point, so I didn’t have room for the couplings and I didn’t want to make it a permanent fixture either, so I just wedged some extra wood under the pipes so that they would not come apart. Also, the only reason I used PVC instead of ABS was because the hardware store didn’t carry ABS for whatever reason. I needed a solution ASAP, so I opted for the PVC.

The PVC p-trap under our shower drain helps keep out the sewer smells.

We’ve been using this new set up for about 2 weeks now and it’s great! There are no sewer smells inside the house or at the sewer connection site, and there are no leaks! In the future I hope that we can dig a trench and dispose our gray water safely into the ground, but for now, this tiny house plumbing setup is working perfectly.

I hope this detailed walkthrough of our system can help you in designing your own.  If you have questions, feel free to shoot us an email or leave a comment. If you’re interested in reading about plumbing as it pertains to water coming into a tiny house, read this post.

 Thanks and happy tiny house plumbing!

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