Monthly Mechanics: Introduction to Septic Design

So you want to design septic systems for a living? Your answer is probably “omg no.” But consider: almost every household in rural America lacks a sewer connection and needs to provide its own wastewater treatment. In Vermont, that applies to over 50% of homes – maybe 150,000 units in all. And the number of people licensed to design septic systems in the state is relatively tiny – about one hundred professional engineers, and a few hundred other individuals who have passed a qualifying exam.

Moreover, wastewater systems – unlike the houses themselves – are strictly enforced in all communities. It’s an issue of public safety. If your house collapses, you’ll only hurt yourself. But if you dispose of your waste improperly, you’ll poison the water supply, potentially sickening lots of people and wildlife for years to come. So those several hundred designers have a captive market.

Usually the system takes the form of a septic tank, where solids (aka POOP) are allowed to settle out, followed by a field where effluent (the remaining wastewater without solids) safely drips into the soil. The first step in designing a wastewater system is to run a percolation test, or perc test, to find out if the existing soil on site is sufficient.

Here’s how a perc test works. First, dig a hole 10 inches deep and 6-8 inches wide. Place 1 inch of crushed stone on the bottom. Then start your timer and pour 6 inches of water into the hole as steadily as possible. (Experts recommend using a siphon.) You need to record how long it takes for the water level to drop a certain amount, with the distance depending on the type of soil you have. As soon as the water level drops the required amount, refill to 6 inches. Repeat a total of 7 times. The percolation rate is the average speed at which the water level drops.

If the percolation rate is faster than a prescribed rate (often 120 minutes per inch), then you can say that the soil percs, and it may be possible to construct a leach field on the existing ground. There are other factors to consider, including ground slope and isolation distance, to determine if a site is appropriate as is. If the soil doesn’t perc, then a mound system is required, which means trucking in appropriate soil to a depth determined by the needs of the system.


Every person living in a house is assumed to generate wastewater in the amount of 70 gallons per day. Since the number of occupants can change over time, the design flow for a house depends on how many bedrooms it has. In Vermont, the first three bedrooms count for two persons each, and additional bedrooms count for one person each. Thus, a three-bedroom house has a design flow of 6*70=420 gallons per day, and a four-bedroom house has a design flow of 7*70=490 gallons per day. This is the volume a septic designer must prove the system can handle.

Interested yet? Read about septic designer licensing at the Vermont ANR website.

Tiny Tuesday: The Nomad

My dear friend MK lives in a house she built on a 24-foot-by-8-foot trailer. The Nomad (as she named it) is parked in her hosts’ sloping backyard in the Finger Lakes region of New York; legally, it’s an accessory dwelling. It’s not a house on wheels at the moment, because she jacked and leveled the house and removed the wheels.

MK previously served as an intern at Yestermorrow Design-Build School, and the courses she took there gave her the expertise to design the house herself. She drew plans with Sketchup. She cut material costs and overall weight using advance framing: rafters align with studs and floor joists to eliminate plates, and strategic placement of windows and doors allows for light header beams. She hired local talent to build a custom trailer (with floor joists in the right places), and she leveraged a building class to contribute labor.


MK’s Sketchup plans for the Nomad.

I love the house’s wedge shape, using a shed roof with a shallow 1.5-on-12 pitch to create headroom without complicated roof lines. As you enter through double doors near the low end, an eat-in kitchen stretches out to your left, with open cabinets, a 3-burner stove, and a huge chest freezer (which MK intends to convert to a fridge). To your right is the bathroom and the mechanical area, including an array of batteries for a future PV installation. The far end contains a steep stairway to a landing that serves as a dressing room, then turns to a loft, which is entirely filled by a queen-size mattress. The stairs and the loft floor frame a living room with built-in shelving and a loveseat tucked into the far wall.


Eat-in kitchen, stairs/dressing area, and living room with loft above.


Handsome finishes on the living room’s built-in shelving.

Right now the house is pretty much off the grid. There is no running water or electricity, although the hookups are all there. (MK uses a composting toilet and showers at the gym.) Insulation is tight with no thermal bridges, and MK expects her marine wood-burning stove and her heat recovery ventilator will be more than sufficient to keep the place comfortable as she prepares to spend her first winter there. In the meantime, the Nomad helps her save money for her dream of one day running a farm and rustic B&B.


MK says it was a challenge to level the house on a wooded, sloping site.

Tiny Tuesday: Go Prefab and Go Home

One of the most appealing things about a tiny house on wheels is that you don’t have to build it where you intend to put it. Construction can happen in your driveway, or in a friend’s yard, or even inside a garage, protected from the elements. When work is complete, you can tow your house to the site. The result is faster construction, lower cost, and better workmanship.

Recently, hotel developers have taken this concept to a new order of magnitude. An article on Hotel News Now describes an emerging market for hotels made of modular rooms, which can be pre-fabricated in a factory and lifted into place onsite. It’s a brilliant concept: a mid-range hotel room is about the size and shape of a shipping container (or tiny house), and factory construction lends itself to producing lots of identical rooms. The article explains how prefab can standardize all the tricky trades – electrical, plumbing, and HVAC – allowing each room to install “like a dishwasher.”

Indeed, connections are the toughest design issues when you’re stacking together lots of prefab pieces or connecting them to a core (lobby, hallways, elevators) built in place. A tiny house is in essence just one prefab piece, so connection issues don’t come up. For a hotel, you’ll need excellent quality control to ensure the stubs of cables, pipes, and ducts land in the right places for easy hookup. Structural issues may crop up as well: a shipping container may be designed to stack ten high, but the consequences are pretty minor if that stack tips over. Not so in habitable spaces, where the building code may require strong shear connections between units for the occupants’ comfort and safety. If the designers in the article have solved these issues, and still produced an attractive-looking hotel, then their work has applications across the building industry.