What are the requirements for venting waste lines from toilets and drains?

Decided I should give you more info...
My septic system seems to me to have a lot of issues with the way it's installed and I am reasonably handy and have fixed many issues. However, I'm not that familiar with the requirements for venting of septic lines.
My system connects to a septic tank if that matters to your answers.
First of all, is it appropriate for the 1.5" vent line to come out of the side of the 3" waste line, and then go down, over, and up? Seems to me that it's likely to fill with waste and be ineffective as a vent. It seems superior to me to have the 1.5" vent come out the top of the 3" waste line, then go over, down, over, and up. This down, over, up is necessary since the waste line is between floor joists, and the path to the roof is through a different joist cavity.
Also, is it acceptable to connect vents together? Does each toilet waste line need its own vent all the way to the roof?
How close to the toilet is the vent supposed to be?
Do sink drains need vents? Can those be connected together at will?
I live in Iowa where pipes in walls are subject to freezing... is it acceptable for a vent tube to go up an outside wall? I wouldn't think so. I have a 1.5" pipe that disappears up into an outside wall from the basement and I can't figure out what it's for... I don't think it shows up in the attic as a vent and I don't think it connects to any drain.

Hi sgseebs and welcome, the additional info will help with a better answer, many users here know the codes but you should post a general location as the codes vary some from place to place. Good luck

What are the requirements for venting waste lines from toilets and drains? The general requirement for all codes is that every fixture that has a trap MUST be vented.

is it appropriate for the 1.5" vent line to come out of the side of the 3" waste line, and then go down, over, and up?] , No! This is a trap and will block the vent, It's also a code violation.

Seems to me that it's likely to fill with waste and be ineffective as a vent. It seems superior to me to have the 1.5" vent come out the top of the 3" waste line, then go over, down, over, and up. This down, over, up is necessary since the waste line is between floor joists, and the path to the roof is through a different joist cavity I don't care how "necessary you think is is you can't take a vent line off the horizontal and "go down and then back up" that's building a trapped vent.

is it acceptable to connect vents together? Yes! If you observe local codes.

Does each toilet waste line need its own vent all the way to the roof? No! Not if you wet vent the bathroom group.(see image)

How close to the toilet is the vent supposed to be? If you're running a single vent In most codes if the drain line's 3" the distance is 10 feet and if the line's 4" it's 12 feet.

Do sink drains need vents? If it has a trap it needs a vent.

Can those be connected together at will? The drains or the vents?

.is it acceptable for a vent tube to go up an outside wall? Do you mean "up the wall" or "in the wall?

where pipes in walls are subject to freezing... is it acceptable for a vent tube to go up an outside wall? I have a 1.5" pipe that disappears up into an outside wall from the basement and I can't figure out what it's for... I don't think it shows up in the attic as a vent and I don't think it connects to any drain. You better hope it doesn't connect to your drainage. Because if it does and it's stubbed out inside a wall you have a direct open line to your septic tank that's allowing sewer gas to enter your home. Sewer gas is a health hazard to your family and the methane content makes it expolosive.
Hope this helps. More questions? I'm as close as a click. Tom

My major take away is that I need to cut through some floor joists to run the septic vent horizontal and up, never down, from the waste line. I gather from your diagram that it is OK for the vent line to enter the side of the waste line... a little waste water may flow up the vent tube, but it'll drain back out.
By the way, I did find that the mysterious run in the outside wall is a vent tube that shows up in the attic. I think they ran more vents than they needed to in this system, but that's not something I need to fix.
Thanks for all the info.

You need to DRILL through the joists, but cannot just drill anywhere... must follow local codes. In most cases, you cannot drill out more than 1/3 of a joist. This means, for example, that if you have a 9" joist you cannot drill a hole larger than 3"... must be relatively centered, too.

Vents must also pitch from the high point (roof) back toward the drains so any rain water that gets in the pipes can flow back to the drain via gravity.

Finally, a vent CANNOT be taken off the horizontal as it must ROLL above the centerline of the drain to be a vent (hard to appreciate this in the above diagram).

More questions just let us know, OK?

Mark

I gather from your diagram that it is OK for the vent line to enter the side of the waste line... a little waste water may flow up the vent tube, but it'll drain back out.
Look again. That's the lavatory drain that you see connected to the 3" toilet drain. It is not acceptable for any of the waste to enter a vent. The vent in the drawing is coming off the tee that picks up the lavatory.

I did find that the mysterious run in the outside wall is a vent tube that shows up in the attic. I sure hope it continues on through the roof because if it was left open in your attic you have a direct open connection between your house and the City Sewer that's allowing sewer gas to enter your home. Sewer gas is a health hazard to your family and the methane content makes it explosive. Check it out ASAP. Good luck, Tom