Plumbing Part I: DWV

The DWV (Drain Waste and Vent) system was the most daunting project of the entire build, in my mind. Most LHBA folks (in fact all of them that I know of) hire this part out to professionals. Even though I knew nothing about plumbing a house from scratch, I decided to tackle it myself. I gained a lot of respect for plumbers – there are a lot of things you have to know to connect everything together. It’s one thing to go in and repair something someone else already built; it’s another to come in and build it from scratch. I knew nothing about plumbing when I started this project; now I know next to nothing. 🙂

Plumbing a log cabin is NOT exciting. There are no “neat looking pictures” to share. Sorry. 🙂 But it needs to be done so we can move in. I researched this topic heavily, bought a few books on the subject, and asked around to cross reference the info I found.

Below, I’ll discuss the basics and the materials needed, and then I’ll do “floor by floor” tour of the DWV system.

Basics of plumbing a drain system

Some things I learned/did:

  • 1/4″ per foot slope is the standard. This ended up being easy to figure out under the house for the main drain – They sell these “J – Hooks” (see below) for hanging PVC pipe that are meant to be spaced every 4 feet under the joists. I started with the main drain touching the bottom of the joist, then put a block 4 feet down the line. Since 1/4″ x 4′ = 1″, I dropped the J-Hook an inch every 4 feet. The J-Hooks have little “inch” markers on the side, so it’s easy to get the slope correct.
  • A pipe set at 45 degrees is still considered vertical. This comes in handy when you have to avoid a joist or some other obstacle.
  • As you connect each fixture drain into the rest of the system, the pipes you use can always get larger (1.5″ -> 2″), but you’re not allowed to go smaller. So you can’t take a 2″ shower drain and run it into a 1.5″ sink drain.
  • You can connect more than one “fixture” to a drain branch. Each fixture gets a number, based on how much water it can discharge at a time (called a “drain fixture unit” or “dfu”). While planning, you decide what fixtures you have, look up the dfu for it, add up the numbers, and that tells you the smallest size drain you can use. A 2″ pipe, for example, can handle drainage for up to 16 dfu’s.
  • In general: minimum 3″ drain for toilets, 2″ for laundry and showers (you can get away with 1.5″ for tubs, but it’ll drain faster with 2″), and 1.5″ for sinks.
  • Sanitary Tees are normally for vertical to horizontal transitions, while Wye’s are for horizontal drainages.
  • Every fixture needs a p-trap, and every p-trap needs a vent.
  • The vents mostly keep the gasses from entering your home, working alongside the p-traps. The vent system handles the gasses and eventually exits the home through the roof, while the drain system gets the solids and liquids out of your home. The whole system is tied together into one main drain and works just on gravity and water pressure alone.


  • You have a few choices for what you can use for a DWV system – cast iron and PVC are the most popular. Cast iron tends to drain more silently than PVC, while PVC is easier to work with. I opted for PVC – it’s also a lot more forgiving if you don’t get the angles of the connectors exactly right. You must use pipes labeled “Shchedule 40” and “DWV”.
  • Fittings: There are elbows, wye’s, Tee’s, reducers, couplers, and a variety of connectors in each style. They come in sizes of 1.5″, 2″, 3″, and 4″ usually.
  • Support: PVC requires plastic hangers. There are a lot of ways to do this. I used a lot of plastic plumber tape and j-hooks.
  • Cement: I used “purple primer” and “clear cement”. You are actually welding the pipes together. If you screw up, you have to cut the entire connection out and start over. You can’t “unweld” pvc pipe. For pipes other than PVC (like black ABS), you have to use the cement indicated for each type of plastic.I decided to go with a 4″ main drain. I have one branch for the main and second floor bathrooms that is 3″, but that’s it.
  • 2″ PVC pipe was my most used pipe (for showers / toilet and other vents), then 1.5″ for the sinks (just enough 1.5″ pipe to get it to either a 2″ vent or drain)
  • I used a lot of “4” x 2″ double wye’s”: for the master bath and main baths. The toilet can drain into the 4″ line, and the sink and the shower or tub can connect to each 2″ side port.
  • There is one AAV (an Air Admittance Valve) in the kitchen sink that takes the place of a normal vent there. I could not see a way to elegantly get the vent up through the ceiling – it would have to be exposed to view, and then go a long way horizontally to connect to all the other vents. An AAV is “to code” if done correctly – it must be at least 6″ above the trap arm.

Under the house

I asked my inspector how he wanted the system to be run under the house, and he said I could just hang it from the joists up until right before it exits the crawlspace. At that point, he said I could just run some 45 degree connections and put it down in the ground to run the rest of the way to the street, which is around 220′ feet away.

The whole idea of the system is to get the waste to the road, and to keep the gasses from entering the home. I found that you mostly want to keep things either vertical or horizontal – you don’t want to do much in between. That said, there are a few exceptions: when the space is extremely tight, you can do a 45 degree drop to bypass an obstacle. With the horizontal runs, you don’t want too much space between the p-trap and the vent. One of the longest exceptions is for a bathtub, where the p-trap is allowed to be up to 5′ from the vent so you can drain the tub on one end, and vent it on the other if you need to.

I started the drain system at the back of the house with the toilet furthest from the road. I tried to make all the runs short and with minimal bends. I also followed code to keep the vents as close to the drains as possible. I pushed just enough pipe up through the floor to allow a connection to the rest of the vents in the room. I did a lot of test fitting before committing to welding it all together with the cement.

First floor system

Once I had the “under the house” system in place for the first floor, I started installing vents for each fixture to go up through the roof – connecting to the stubs I pushed up through the floor while under the house. I used sanitary tee’s for the sinks and ran the vents vertically until they were 6″ above flood level (top of the sink) before running them horizontally in the walls. They then follow the gaps in the log walls around until they meet with a vertical “soil stack” / vent, which takes them to the roof. My area follows UPC (Universal Plumbing Code), which is nearly the same as IPC (International Plumbing Code), but UPC allows horizontal vent pipes to remain level, where IPC requires a 1/4″ / foot slope for them. I decided to go ahead an put a slope in the horizontal vent pipes anyway, since that makes sense to keep the lines drained. Another difference is UPC is less restrictive on AAV’s (air admittance valves) than IPC. That will be discussed later.

For the main bathroom, the venting was easy enough – all the fixtures were on the same wall. Once connected, I pushed a 3″ stack pipe up through the ceiling to the second floor. The lower end of this 3″ pipe functions as the drain for the main bath fixtures. The middle of this pipe is also a drain for the 2nd floor fixtures. The upper end functions as a vent. This is allowed, as long as the vents are always higher than the drain. And you can’t drain the upper fixtures into it, and then use it as a vent for the lower fixtures. So the lower fixtures have their own vent system that goes vertical through the second floor and then connect into this stack up there. Finally, this pipe vents all the fixtures in the house when it exits through the roof.

For the master bathroom, it was a bit more difficult with the logs on two corners and nowhere to hide the pipes – except that we decided earlier – for purposes of keeping the moisture levels low – to just frame in the master bathroom – put 2×4 walls in front of the logs. I didn’t want to cover up my pretty logs, which would look pretty cool in a bathroom – but pragmatism won out. The moisture in a bathroom is just too much of a risk to expose the logs to on a daily basis. But the walls were a huge help with plumbing – they created just enough space for pipes to run behind them.

I was going to run the kitchen vent up between the window and the logs, but an AAV saved the day. Still need to channel that log a bit more for the upper cabinets.

For the kitchen, I was going to run a vent up through the counter, and then next to the window, punch through the second floor then run it over to the main stack. But decided to just do an AAV (Air Admittance Valve) – eliminates the need for a vent in certain situations.

Second floor plumbing

In most homes, there’s a covered space between the ceiling of the first floor and the floor of the second floor. Our home doesn’t have that luxury, so one of the main questions I had to figure out was how to elegantly drain a 2nd floor bathroom when you have exposed beams holding it up. Short answer: you can’t – the drain lines will be exposed. But there’s nothing in the code that says you can’t have exposed plumbing; I just didn’t want to showcase it. A saving grace of our bathroom was that we decided to do a drop ceiling (nail 2×4’s or 2×6’s to the bottom of the joists, and run ventilation and plumbing in the space created) – and then just drywall it. This will help hide the part of the drain lines that we can’t just hide in the water wall (a 2×6 wall used for accommodating plumbing).

The 2nd floor bathroom will just be “stubbed in” for now – going to have the vent and drains roughed in, but not going to attach anything to them yet.

I finished the DWV system by pushing a 3″ vent up through the roof and installed flashing around it.

Next steps

L to R: Dave & Misty, Juanita, Ivy, Julie, Me, Marcia

In the middle of the plumbing, my mom and two sisters and one of their husbands came out to check out our work. First time my family has seen it. We had a weekend of picnicking, touring the cabin, pictures, a hike, and a great southern dinner made by my wife. My BIL owns an AC business and wanted to help me design the air conditioning system. To my great relief, it appears I can do most of this work myself, after he does some calculations for us.

There will be a Part II and a Part III on plumbing. I still need to run the Pex lines (the water lines that will feed the fixtures), and dig a 5′ deep x 200’+ trench to connect to the street. Haven’t decided whether I’ll rent or hire equipment for that (probably hire) or dig it by hand (I’m sure I’ll hate myself if I do this).

Once complete, I can schedule a “rough in” inspection for all of it – electrical, plumbing, and hopefully mechanical. If I pass, then it’s the home stretch for us! Floors, drywall, floor insulation, interior chinking; then a final inspection and we move in.

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