While recently doing a job, I had to open up the ridge space above the collar beams in a freestanding building. This space had large screened vents at both ends and the collar beams were all insulated with a good vapor barrier on the warm side, so that it was effectively outside. In fact on this winter day it was very cold and drafty up there.
What was odd was that the four-inch plumbing stack, the pipe that normally pokes out through the roof, ended in this vented ridge space, that is opened into it and did not go through the roof. Even so, there was no bad smell up there at all.
Although this was undoubtedly a code violation, I could not think of one good reason why it was a practical problem. Such a vent normally sucks air in as toilets are flushed and is separated from any source of sewer gas by the traps of the system. Even if some sewer gas is expelled, that is a problem for the bees and bats, if any, in the ridge space. Any stink will not be trapped and will pass outside in short order. On the other hand there is one less roof penetration and no unsightly pipe sticking up through the nice standing-seam roof.
Replies
#1) If you spend any time up there you will smell how often that vent doesn't suck
#2) moth flies will start to appear everywhere, they reproduce in the sewers and follow the air currents to the opening
#3) if your in cool climate the moisture from the vent will lead to condensation, ice, and mold ..
need any more??
I kind of thought the same thing as you, as I got behind last fall and moved into a new house before I cut the vents through the roof deck (DIY plumbing job done during the winter before with a nearly constant snow covered roof) after move-in same deal..snow and iced up roof, so I thought what the heck the attic itself is both soffit and ridge vented and besides, there's another vent in the guest suite attached to the garage. And I still had trim to finish. Out of sight out of mind.
Septic system. So when I opened the hatch late last winter I saw lots of ice, moisture, and some black mold on the surface of the OSB near the vents...I got those vents through the roof as soon as there were enough bare patches on the roof to get up there.
Last weekend, back up on the roof to put in a wood stove chimney, boy was it ripe up next to that 3" main vent, yesterday was up there again no smell at all...so atmospheric conditions have alot to do whether that vent just sucks or not.
They do have really good, special roof jacks for bringing vents up thourgh metal roofs.
I may be out of my league here, but the traps prevent the sewer gasses from getting from your stack into the house. In so doing, the only path for the gasses is up through the stack itself, and wherever it ends (your attic). Granted, this may not be a bad place for them, but at least understand that unless you have a whole house trap, sewer gasses will be coming out of that stack sometime.
several years ago I did a project for a customer who had A LOT of rotting wood on an addition.
when I removed the sheating I found that whoever built the addition had simply framed the tie-in over the old roof, covering a waste stack pipe.
they installed a ridge vent---with no soffit vents( cathedral ceiling)
End result----about 12 sheets of plywood just fell apart. you don't even want to know what it looked like around that pipe.
stephen