Plumbing (Vent) Penetrations
Wed, Feb 13, 2019If you’re looking to build a house, you’ve probably spent a lot of time thinking about where your drain-waste-vent (DWV) will exit your house. Maybe not, but it’s something I’ve been thinking a lot about, and it may sound silly, but it’s a very important part of your plumbing system.
The DWV allows your waste pipes to maintain a neutral air pressure as waste moves through the pipes. Without proper venting, gases cannot be expelled, and you may get unwanted suction in your pipes (leading to p-traps not able to do their job, and gases backflowing into your house).
The DWV will normally run from your primary waste pipe exiting the house (somewhere below the house) all the way through your house and exit through the roof. Each of your fixtures (sink, toilet, bathtub, etc) will connect their vent pipes to this main stack. Since the main stack can have so much connected to it, it is usually a larger diameter pipe (3”) than the individual fixture pipes (1.5” or 2”). A 3” pipe does not move through walls and floors easily, so you have to think about how this pipe is going to run through your home. That’s in addition to a 3” or 4” waste pipe too. You don’t want to start cutting holes through floor joists.
Also consider the roof penetration. You do not want this pipe to ever become blocked or for air movement to become impeded. Our average snowfall in our area is about 10 feet and given the low pitch of roof, we can expect to have a few feet of snow on the roof throughout the winter. Not to mention with that much moisture on the roof for such a long period, I do not want a big hole punched through it.
Putting the penetration through a wall seems like an obvious work around, but you have to consider what this pipe is exhausting: warm and moist gases. Those gases will rise, and moisture will collect. If you place your penetration on a wall just under an eave or soffit, those gasses and moisture may cause mold growth on the underside of your roof. Even worse, if your attic vents are nearby, that gas and moisture could enter those vents and cause even more damage from inside your attic! Or what if there is a nearby window?
You will have to check your county’s building codes. Kittitas county allows us to place a vent penetration on a wall provided the following:
It is a gable wall The vent stack shall terminate a minimum of 12 inches horizontal from the exterior of the structure 12 inches below any horizontal portion of the structure (soffit, etc) It is at least 10 feet from or at least 3 feet above any operable window, door, air intake, etc.
By requiring this penetration to be on a gable wall, there is unlikely to be attic vents nearby.
Lastly, some jurisdictions will allow the use of Air Admittance Valve, or AAV. These use a mechanical valve to automatically open the valve and bring in air when the pipes are under negative pressure. These can be used in places where you cannot easily or practically run a vent, such as under a detached kitchen island.
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