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A Guide to Running Plumbing and Septic Lines in a Barndominium Kit
You have the perfect floor plan in your head. You know where the kitchen sink goes and where you want the shower. But in a barndominium kit, turning those ideas into reality requires careful planning before the first board is nailed down.
Barndominium plumbing is different from standard home plumbing mainly because of the foundation choices. Whether you are building on a slab or a wood floor affects when and how you install your pipes. This guide explains the process of getting water in and waste out of your Best Barns kit efficiently and safely.
Key Takeaways
- Foundation First: You must install the main drain lines before pouring a concrete slab, as fixing mistakes later requires jackhammering.
- Raised Floor Advantage: Wood floor systems (pier and beam) allow you to run plumbing easily in the crawlspace after the shell is built.
- Interior Wall Routing: Keep water supply lines inside interior partition walls to prevent freezing, especially in colder climates.
- Septic Gravity: Ensure your building site has enough elevation drop for a gravity-fed septic system, or budget for a lift station pump.
- Venting Matters: Every sink, toilet, and shower needs a vent pipe going through the roof to prevent sewer gases from entering your home.
Phase 1: The Rough-In (Under the Slab)
If you are building on a concrete slab, your plumbing starts before the concrete truck arrives. This is the "rough-in" phase.
Map Your Drains: You must know exactly where your toilet flange and shower drain will be.
- Precision is Key: If you miss your mark by six inches, your toilet won't fit against the wall. You need to stake out your bathroom walls on the dirt pad first.
- The "Stub-Up": Your plumber will install the PVC drain pipes in trenches dug into the gravel base. The pipes will "stub up" (stick up) vertically through where the concrete will be poured.
Protect the Pipes: During the concrete pour, workers will be walking everywhere. You should wrap the exposed pipe ends with foam or cardboard to protect them from concrete splatter and accidental kicks.
Phase 2: Raised Floor Plumbing (The Flexible Option)
f you choose a wood floor system (like the FortressFloor™ often used with Best Barns kits), plumbing is much more forgiving.
Crawlspace Access: You build the floor deck first. Then, you can crawl underneath to run your pipes.
- Easy Changes: If you decide to move the kitchen sink a foot to the left, you just drill a new hole in the subfloor and reroute the pipe.
- Insulation: You must insulate pipes in the crawlspace. Use heat tape and foam pipe insulation to prevent freezing, as the crawlspace gets cold.
Phase 3: Running Supply Lines in Walls
Avoid Exterior Walls
In a traditional house, this is a golden rule. In a barndominium, it is even more critical.
- The Risk: The exterior walls of these kits are 2x4 framing. If you put a water pipe inside an exterior wall in a freezing climate, it is very close to the cold outside air.
- The Solution: Run main supply lines through the floor or ceiling and drop them down through interior walls to reach your fixtures. If you must use an exterior wall, keep the pipe on the "warm side" of the insulation.
Second Story Plumbing (Colossal Models)
If you are adding a bathroom to the loft of a Colossal model, you need to hide the pipes.
- The "Wet Wall": Stack your upstairs bathroom directly above the downstairs bathroom or kitchen. This creates a shared "wet wall" where all the drain and vent pipes can run straight down to the foundation.
Once the kit shell is erected and you have framed your interior partition walls, you run the PEX or copper supply lines.
Phase 4: Venting Through the Roof
Drains don't work without air. Every time you flush, air must enter the system to prevent a vacuum lock.
The Stack Vent: You will need a main vent stack (a 3-inch or 4-inch pipe) that goes up through the roof.
- Flashing: When you cut a hole in your new roof for the pipe, you must use a rubber "pipe boot" flashing to seal it against rain.
- Alternatives: In some areas, you can use Air Admittance Valves (AAVs) under sinks to reduce the number of holes you need to cut in the roof. Check your local code.
Phase 5: Connecting to the Septic Tank
The final step is getting the waste away from the house.
Slope is Everything: Sewer pipes must slope downhill at a specific angle (usually 1/4 inch per foot).
- Too Flat: The waste won't move.
- Too Steep: The water moves faster than the solid waste, leaving clogs behind.
The Cleanout: You must install a "cleanout" access point just outside the building foundation. This allows a plumber to snake the line if a blockage occurs between the house and the septic tank.
Have any questions or would like to place an order? We'd love to help! Chat with our friendly customer service team by calling 1-844-945-3625, chatting in on our website or email us at customersupport@wildoaktrail.com. We look forward to hearing from you!
Frequently Asked Questions
Can I do my own plumbing in a barndominium?
In many rural areas, you can do your own plumbing if you are the homeowner, but you still must pass inspections. Check with your local building department first.
What is the best pipe for water supply?
PEX (cross-linked polyethylene) is the standard for DIYers. It is flexible, freeze-resistant, and easier to install than copper because it uses fewer fittings.
Do I need a floor drain in the main area?
If you plan to use part of your barndominium as a garage or workshop, installing a floor drain is smart for washing floors or handling snowmelt from vehicles.
How deep should the septic line be?
The depth depends on your local frost line. The pipe exiting the house must be deep enough so it does not freeze but high enough to flow downhill to the tank.
Can I run plumbing in the Colossal loft?
Yes. The floor joists (2x10s) provide plenty of space to run drain pipes horizontally to a wall where they can drop down to the main sewer line.
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