What's the deal with plumbing?

[MushCreek]

I've been trying to design my DWV system, and am running into all sorts of conflicting information. I have 3 different books, and they all seem to have different rules. I designed my system and posted it on a DIY forum, and was met with a chorus of, 'That will never work!'. I re-designed it totally, and got- nothing. I've had plenty of good advice on building, electrical, and HVAC, but the plumbers don't want to talk. You'd think they were part of a mysterious group from a Dan Brown novel. I've talked to a plumber about designing my system- for money- and he seems reluctant.

On the surface, plumbing doesn't seem all the complicated- 'You-know-what' flows down hill, and all that. The venting is where it gets tricky. Interestingly, all 3 of my books use the exact same drawings, despite having different specs. of course, none of the drawings look like what I need to do. I'm a really hard-core DIY'er, and like to learn about the various aspects of house building in detail. It's not just the money, but I want to do as much as I can myself.

Any thoughts (that you dare share on a public forum)? Do I bite the bullet, and just pay a plumber, or can I figure this out? As an aside- my house in FL was built by a builder, and plumbed by a 'professional' has all sorts of plumbing issues and errors.

[mgramann]

I went through many of the same things.  I planned my system off of the IPC 2012, since that is what Michigan uses.  When it came to venting, I generally overbuilt.  If it had a 1.5 inch drain, it got a 1.5 inch vent.  If it was a 2" stack, it got a 2" vent.  So far, all of my pitches have exceeded the minimum, and are well within the required trap to stack maximum distances.

Keep in mind that designing a system and installing it are two different things.  I know some of my plans went out the window due to truss location and other items that can only be seen while onsite, and at that point it's easier to just have a general sketch and build as needed.  That may be why a plumber was hesitant to draw a specific plan.

I'm sorry if I'm stating the obvious here, but one thing that is commonly overlooked, is when to use a Wye, and when to use a Sanitary Tee.  Sanitary Tees should only be used when a horizontal pipe is draining into a vertical pipe.  Otherwise use a Wye and a 45.  I say this only because it was an "a-ha" moment for me.

[Alan Gage]

I had a hard time wrapping my brain around venting as well when doing my house a couple years ago. Like you said, the examples in what I read didn't necessarily fit what I needed to do. It was hard to find much of anything online either. I remember after lots of reading and thinking one day it sort of clicked and I kind of understood how it worked (or needed to work) and then it somewhat made sense. I haven't needed the knowledge since then and I've now forgotten it so I'm not going to be of much help. Just commiserating I guess.

My kitchen sink is my farthest drain from the septic and about 12 ft. from my large vent stack in the bathroom. I didn't want to run a separate vent for it up through the roof and planned on just putting in an AAV under the sink. I forgot to pick one up and was anxious to have a sink so I went ahead without it, figuring it would be easy to add later. Been 2 years now and no problems. No gurgling and water drains like a champ. Go figure.

On the other hand we have a urinal at the shop that I presume isn't vented correctly. It drains incredibly slow.

Alan

[MushCreek]

I'm on site, so I can see what needs to go where. I know about the wye versus sanitary tee, also. There's also a long sweep tee which I think can be used like a wye, although I've designed in wyes everywhere. What I'm worried about is the location of the vent in relation to the fixture. I cooked up a plan where pretty much every fixture is on its own branch, and has its own vent. I combined the lavatory and the washing machine standpipe on one branch, and the two kitchen sinks are on one branch. I have two vents through the roof- one for the kitchen sinks, and the main, which handles everything else. Most of my vents are only 2' or so from the fixture, with the kitchen sink being about 4'.

The books are pretty good about things like being sure that a horizontal vent run is above the flood level of the fixture, and things like that. They disagree on the distance the vent can be from the fixture- probably different codes. None of the books specify which code they are under! We are under IRC 2006, but my inspector hasn't given any of my work more than a quick glance so far. I imagine he'll pass it as long as he sees some kind of vent near each fixture, but I want to be sure it works right.

[flyingvan]

My lesson learned was the total cross section area of your through-roof venting needs to be at least equal to, or greater than, the cross section area of the main drain pipe.  If your main drain is 4" (like mine, wish I used 3") that area is about 12" square.  Each 2" pipe has a cross area of about 3" so you'd need 4 of them. 
Alan---if your workplace urinal has been there awhile, dump some muriatic acid in there and let it sit overnight, then just flush it in the morning. 

[davidj]

My lesson learned was the total cross section area of your through-roof venting needs to be at least equal to, or greater than, the cross section area of the main drain pipe.  If your main drain is 4" (like mine, wish I used 3") that area is about 12" square.  Each 2" pipe has a cross area of about 3" so you'd need 4 of them. 
Alan---if your workplace urinal has been there awhile, dump some muriatic acid in there and let it sit overnight, then just flush it in the morning.

If you haven't got a whole bunch of experience them designing and building DWV systems is as hard as it gets when building a cabin (at least from my experience).   In theory there are a huge number of ways of doing things but, in practice, many just don't work, but you don't know which until you've planned out the whole thing.  It's also not at all intuitive - the rules are pretty weird - and not something you see at all in everyday life as it's all behind walls.  Finally it really is more three dimensional than anything except roof framing (assuming a simple house design where wall and floor framing is mostly a two-dimensional problem). This is in contrast to electrical, where there was much less thinking and much more actual hard work than I expected.

[MushCreek]

The 3D aspect isn't that big a deal for me. It's just trying to interpret the rules in a way that will both please the inspector and work properly. Here's a 2D sketch of the main DWV system, without the kitchen, which is nearly a completely separate system. The letter V designates a vertical vent. All of the vents return to the main stack up in the attic. The one un-labeled fixture is the washing machine standpipe. One book says it doesn't need a vent (?) but i could easily tie it to the lavatory vent. The little arrows are clean-outs- there could be more.