DWV questions

dug · May 18, 2011, 05:16:56 PM

I've been trying to wrap my head around the DWV system and have been doing some reading including a recent thread here. I'm still a bit confused on many issues and could use a little help.

Originally I was not going to install a DWV system because we were not planning to put in an indoor toilet. A friend told me I needed it anyway for proper drain flow, and we may change our minds about the toilet in the future so I thought it best to have everything in place just in case.

I sketched a drawing of how I think it may go but more than likely it's not even close.

I was planning on a 2 inch main vent? Or do I need three?
Also planning to branch the kitchen sink, bathroom sink, and shower into the same main vent pipe using 1 & 1/2 inch pipe. Is this doable- venting all the fixtures into one pipe?? Can the sink and shower share the same vent as I have it pictured?
The bathroom sink is about 7 ft. from the vent stack, the rest of the fixtures are within 5 ft.

I just drew in the one cleanout where the main drain starts, do I need more?

I jogged the main vent a bit at the top over the window so I could exit near the peak. I like the way the flashing goes on there and it seems to be a more foolproof way of doing it as far as avoiding leaks goes. Any problem with doing it that way?

Any and all help or critiques much appreciated! I am even less of a plumber than I am a carpenter.

ScottA · May 18, 2011, 06:08:10 PM

Looks fine except the bath sink drain needs to connect down stream of the shower vent and the bath sink needs it's own vent. You could use a studor vent if it's allowed in your area but I'd advise against it. As it is now you have an S trap on the bath sink which is illegal. The main purpose of the vents it prevent traps from being sucked dry by water flowing through the pipes. Basicly each trap needs a clear path to the outside air.

May 18, 2011, 07:14:23 PM

So like this then?

Do you mean the trap was illegal as was but not when vented like I have it now?

How far does the vent on the bath sink need to penetrate the roof, which would be on the low side of the gable? Would it be possible, or advisable to run that vent through the wall and outside the house? It would be easier to do it that way in the situation I am in now and would be on the back side of the house where no one can see it..

Thanks a lot, its a big help.

ScottA · May 18, 2011, 07:54:49 PM

The trap was illegal because it was an S. It is a P now. Thus the name, P trap. You can vent it where ever you want but most codes require it to be above the roof, 10' or more from an openable window or door etc. I don't know why people make such a fuss about going through the roof. I do it with all vents. I have run them up the outside of the wall and offset them around the overhang before on a remodle where the owner didn't want to cut the ceiling open but not on a new build.You might also see if you can find a way to get it back to the main vent stack.

If your issue is a metal roof it's pretty easy to vent through. Cut a T shaped hole with a sawzall. Slip the top of the flashing into the top of the T so it's under the metal. Screw the bottom of the flashing to the metal and silicone around the edges. So long as the sides of the flashing extend a couple of inches out from the sides of the T it won't leak. Never had a problem.

muldoon · May 18, 2011, 09:09:25 PM

QuoteI was planning on a 2 inch main vent? Or do I need three?

My understanding is the vent needs to be the same diameter as the drain that exits. So if your line to septic is 4inch, your vent should be 4inch. (or two 2" vents). I would also like to get a clearer answer on this if someone can speak to it as I intend to do a single vent, and the boots for 4" vents are pretty slim. Is my understanding incorrect, or is it something that can be tweaked a bit? Any problem with a smaller vent than drain?

May 18, 2011, 09:44:56 PM

One 4" pipe/vent/circle is not equal to two 2"

Re the roof penetration. I'm with ScottA on this. I also do not understand the reluctance to cut a hole in the roof. Properly flashed it is no issue. Either like ScottA suggested or one on the more expensive silicone boots that screw down with a dead soft metal ring molded into the silicone flange. Our home has a total of 7 penetrations and none have leaked since the day the first shingles were applied in Dec 1984.

ScottA · May 18, 2011, 10:09:07 PM

The full size vent is the old code. In most areas it's no longer required. A 2" will work fine. If you want proof look at a gas can. It has a large hole where the gas pours out but a little tiny hold to let air in. Plumbing vents work the same way.

dug · May 18, 2011, 11:06:49 PM

I'm really not so squeamish about the roof penetrations, but I am a knucklehead and got ahead of myself by insulating and closing in part of the area where the second vent pipe I drew would go up.

I can undo it without a whole lot of trouble but I thought that if it were all the same, aesthetics aside, I could more easily go out the wall and up. I would still have to go through the roof, but on the overhanging eave.

Would that work O.K.?

Much obliged for the help!

ScottA · May 19, 2011, 07:10:25 AM

It will be OK so long as your inspector is OK with it. If no inspector then do as you please.

May 19, 2011, 05:13:28 PM

If you are in a area with large snow falls as I am, then placing roof penetrations close to the peak is advisable---2 or 3 feet of snow tends to take vent pipes and chimneys along with it when it cuts loose. I agree that proper flashing avoids leaks effectively.

ScottA · May 19, 2011, 05:59:34 PM

Quote from: rwanders on May 19, 2011, 05:13:28 PM
If you are in a area with large snow falls as I am, then placing roof penetrations close to the peak is advisable---2 or 3 feet of snow tends to take vent pipes and chimneys along with it when it cuts loose. I agree that proper flashing avoids leaks effectively.

I agree with this. There are ways to vent anywhere even in snow country. I can tell you how if you have this problem.