[UpNorth]

Working through the piping scheme so as to make sure the framing will all be OK with the plumber. In doing so I got to wondering about drain and vent size.

The pic shows my hallucination for the drain scheme in the crawlspace of this small 3-bed 2-bath house with washing machine, 16-fixture-units total.

Mainfloor bath toilet drain in my pic is 4" and the main body of the spider drain arrangement is 4", shown in blue. The upstairs bathroom comes down in a 3", turns and flows (red in picture) to the 4". Other stuff in mainfloor bath (shower and lav) goes directly to the 4" drain. Venting for shower and toilet shown dotted. Laundry comes into the 3".

Here are the questions. I know the plumber we'll hire will have an answer, but I'm just curious. Is the 4" overkill and should we use 3" instead?

And when we back all the venting up in the attic and tie it into the one stack that will go up through the roof, does that vent stack size need to match the size of the main drain line in the crawlspace?

 

[KillerToiletSpider]

What does your code book say?

 

[skitian]

Ultimately, code is god. But with advent of low flow toilets and only two bathrooms, I would stay 3" for main until it exits house, then go to 4". And 3" would be fine for main vent as well. But always, code first.

 

[pipelayer83]

Sounds over sized.

Depending on the authority having jurisdiction. You should always consult with the local authority and make sure you know what codes to follow. I live in Washington state. As far as we are concerned up here, your main trunk line is over sized as well as the 3" stack for the laundry. I would suggest hiring a certified real plumber to engineer the system correctly. It will not hurt to have larger drains than needed. However, you will need to adjust the venting size to meet the cross section requirements. This will result in possible trap siphonage and could leed to methane gas leeking into the room. Either way hire a licensed plumber to do the job.

 

[UpNorth]

Many of the jurisdictions in this locale have one person, often part-time, who does permits and inspections. Qualifications? Nothing like what is needed in a large or even medium-sized metro area. Plumbing often is not inspected at all.

The state residential code is what is observed in all the towns, and it says that for my 18 dfu count (I had not counted the dishwasher before), a 3" building drain will suffice. Is sizing up to 4" a good idea?

The code also says, re venting, that "the minimum required diameter of individual vents, branch vents, circuit vents, vent stacks, and stack vents shall be at least one-half the required diameter of the drain served."

I have heard of some jurisdictions (not this one) that cover deep snow country, that the minimum size of the vent at the roof is 3".

The design of the house is such that all three fixtures in the upstairs bath drain down through a mainfloor wall that is not stacked atop with a second floor wall, so there is no 3" vent stack in any of the second floor bath walls. All venting snakes around through floor framing and up through walls, to get to the attic, where all will tie together into a stack that will penetrate up through the roof.

If that single vent is considered the vent for all below, and all below go into a 4" main building drain in the crawlspace, and the largest vent of any of the fixtures is a 2" (that would be both toilets), then one might think (if he or she was unschooled in the plumbing code like me) that a 2" top vent would do. I was thinking of making it a 3" a foot or two under the roof deck using an increaser fitting.

I don't care if you don't want to answer and instead tell me to hire a plumber. That's the plan, but there's no telling how good he will be. I've been involved in a few kitchen and bath remodels here, and have seen plenty of unvented or badly vented or plumbed work.

I'm just curious, is all. Hey, it's the weekend, there is no pro to call on the phone that can answer, and it is too slushy to ski today.

Edit: BTW, there is no certification for plumbers here and no licensing, either at the state or local level. A lot of plumbing gets done by a jack-of-all-trades guy on the contractor's payroll that learned to pipe from someone else, and that guy often does the wiring, too.

Edit2: In that first pic, the three drains at top L are for the main bath shower (2"), toilet (4") and lav (1-1/2"). The red below it is the 3" drain into which has gone everything from the upstairs bath. Adjacent it and wye-ing into the red 3" drain is a 2" from the laundry box. At the R is the 2" from the kitchen sink. The DW drains into the kitchen sink drain as is common.

 

[griz]

3" main & vent should be fine.
If fall gets to be an issue you may need to bump up to 4" main.
Just don't skimp on the clean outs.:thumbsup:

 

[Mike's Plumbing]

A number of years ago Wisconsin went through a big code update. While not getting into detail I'll just say that we used 4" for all under slab plumbing except for a few things of course. When the code changed it virtually eliminated 4" inside the house unless the house has a lot of DFU's (drainage fixture units).

I'm old fashioned and I still use 4" where I can use 3" under the slab, I'm not sure why and I'm not sure it really matters I just do it. I don't charge anymore labor wise and material cost is hardly nothing.

I guess maybe it's because I've run a lot of sewer machines and it's nice to have the room.

On second thought...maybe I'm just stubborn and getting old and refuse to change.:laughing: That's possible.

Mike

 

[Tom M]

Mike is the intent of the code ( using 4" under a slab)be wise in case of additional loads down the road? Just asking if that's the purpose.

 

[Mike's Plumbing]

Mike is the intent of the code ( using 4" under a slab)be wise in case of additional loads down the road? Just asking if that's the purpose.

No, it's based on a formula for probability of usage with the designed fixtures on the system. The old code has a special clause that said "regardless of DFU's no more than 2 toilets on 3"

I always ran 4" just because I felt it was wise, but it probably doesn't matter. I also pitch more than just the minimum 1/8" per foot. I can tell you from experience that 1/8" is BS on underfloor, it should be more. I pitch the hell out of my sewer pipe. When the sh## hits the city sewer main it's screaming fast baby.

Mike

 

[CookeCarpentry]

No, it's based on a formula for probability of usage with the designed fixtures on the system. The old code has a special clause that said "regardless of DFU's no more than 2 toilets on 3"

I always ran 4" just because I felt it was wise, but it probably doesn't matter. I also pitch more than just the minimum 1/8" per foot. I can tell you from experience that 1/8" is BS on underfloor, it should be more. I pitch the hell out of my sewer pipe. When the sh## hits the city sewer main it's screaming fast baby.

Mike

It's funny you mention this, Mike. Last year I did a two bath remodel in a house. When we started the job, homeowners claimed the toilets clogged easily. We put a camera down the underslab and noticed it had a ton of tree roots, as well as a collapse.

We ended up replacing almost all of the underslab, all the way out to the street with 4". My plumber said the same thing you did about extra room for a sewer snake, as well as greater pitch. Good to see great minds think alike! :thumbup:

 

[UALocal1Plumber]

NYC Code requires at least one 4" building house drain and one 4" vent stack per dwelling.

Keith

 

[William James]

Usually for what you have, a 3" vent with everything tied in the attic except a revent (usually for the washing machine) of 2". That goes through the roof also. To further prevent freezing, switching this 2" to a 3" about a foot below the exit point should suffice.

I wouldn't see any need for a 4" line unless you have a long horizontal run. Even then, probably not necessary.

 

[Mike's Plumbing]

Usually for what you have, a 3" vent with everything tied in the attic except a revent (usually for the washing machine) of 2". That goes through the roof also. To further prevent freezing, switching this 2" to a 3" about a foot below the exit point should suffice.

I wouldn't see any need for a 4" line unless you have a long horizontal run. Even then, probably not necessary.

For the vent you would never need 4" in a house

 

[Tinstaafl]

I pitch the hell out of my sewer pipe. When the sh## hits the city sewer main it's screaming fast baby.

I thought the conventional wisdom was that with too much pitch, the liquids will outrun the solids, leaving them marooned in transit. Not so?

 

[griz]

I thought the conventional wisdom was that with too much pitch, the liquids will outrun the solids, leaving them marooned in transit. Not so?

What happens on a second floor bathroom when the main drops straight down to the 1st floor.:whistling

 

[PapaLima]

You are correct. Too much pitch is not good. 1/8 to ¾" is okay but you don't want more than that for the reason you mentioned - liquid will outflow a solid.
and don't give me no crap about a vertical drop, griz. no pun intended.

 

[Mike's Plumbing]

I've never heard of too much pitch being a problem. Keep in mind that on a house I'm responsible for the drain up to the point of connection, usually dead center under the foundation wall. These days I don't do any outside work anymore.

With that said, most plumbers who do exterior laterals give it extra pitch.

One of the problems with 1/8" per foot of pitch often times happens years later after the ground has settled. 1/8" is not much pitch so it's easy to develop flat spots in the lateral. Some of this is natural but most of it is caused from the plumber doing a poor job of bedding the pipe in the correct material and packing it to make sure it's solid. This is also one of the benefits of cast iron.

Over the years I have seen a lot of sewer laterals with a camera and the flat spots (no pitch) show up like a sore thumb. You can't hide sins from the camera.

So I have learned that it's a good idea to give it extra pitch, bed the pipe well by spending extra time packing the trench solid, and keep any soil debris out of the way.

I pitch my sewer between a 1/8 and a 1/4. That may net seem like much but it's a lot for a 4" pipe. Remember, it's a liquid....the solids get busted up pretty well by the time it hits the sewer.

Anyway, I highly doubt too much pitch is a problem. I mean, man, in all these years I have yet to clean a sewer because it had too much pitch. I say the more the better, but keep in mind that after 1/4" on 3 or 4" pipe it becomes unrealistic because the ditch gets pretty deep.:laughing:

You figure that most ranch homes are in the neighborhood of 50 feet so if a stack is at one end and the lateral exits about half way that's 25 feet so the drop is 6 1/4".

Now figure you should be at least 4" under the bottom of the concrete so you don't have cracking problems. Then figure you should have a minimum of a 2" bed under the pipe so you don't get the flats spots I talked about and you end up with a trench exiting that's pretty dam deep.

You can see how unrealistic going past a 1/4" is but like I said, if you hit 1/4" on 4" pipe that motha is screaming.

Mike

 

[Tom M]

I thought the conventional wisdom was that with too much pitch, the liquids will outrun the solids, leaving them marooned in transit. Not so?

I always thought the same logic

What happens on a second floor bathroom when the main drops straight down to the 1st floor.:whistling

Our second floor drains must drop seperately then converge at the basement.

 

[Tinstaafl]

Okay; the way you said it I thought you were talking about something ridiculous. I usually try for about 1/4" myself. :thumbsup:

 

[William James]

For the vent you would never need 4" in a house

Maybe I wasn't specific. I was referring to the drain for 4".