Principles of Venting

A true vent is a vertical pipe attached to a drain line that travels through the roof with no water running through it. If a fixture is close to the stack and on the top floor, the upper part of the stack serves nicely as a vent. Many fixtures are not so conveniently located, however, and other solutions must be found.
A revent pipe, also called an auxiliary vent, attaches to the drain line near the fixture and runs up and over to the main vent. It may attach directly behind the fixture or to the horizontal drain line.
If two fixtures are on opposite sides of a wall, they may tie into the stack with a sanitary cross; this is called a common vent and can be found on back-to-back sinks.
When a fixture is close enough to a stack, a wet vent may be allowed by code. In the case of a tub that is close to a stack, its drain may empty into a pipe that also serves as a vent.
For a freestanding sink code may allow a loop vent. If reventing is difficult and wet venting isn't allowed, you may have to install a separate vent pipe through the roof.
An air admittance valve (AAV) opens to let air in when waste drains, then gravity closes it to keep sewer gases from escaping back into the room. Codes in many localities allow these relatively new devices to take the place of vent lines. Depending on the size of the unit and any code restrictions, AAVs can be used to vent multiple fixtures. Check codes to make sure they permit AAVs.
The critical distance
When you're remodeling your plumbing system, can you install a wet vent, or do you have to install a revent or a separate vent? Finding the answer can involve complicated calculations, based on formulas that can vary from one locale to another. The size of the pipe that codes require, the type of fixture you want to install, and the number of fixtures that are already wet vented on the same line are three factors that determine the critical distance -- how far the fixture can be from the vent pipe. Measure the length of the pipes carefully and consult a plumbing inspector to determine whether wet venting is possible.
Installing vent pipes
Vent pipes, often narrower than drainpipes, need not slope like drainpipes. Normally they run level or plumb, unless there is an obstacle to work around.
Vent pipes must be installed so they stay dry. This means that they should emerge from the top of the drainpipe, either straight vertically or at no less than a 45-degree angle from horizontal, so that water cannot back up into them.
The horizontal portion of a revent pipe must be at least 6 inches above the fixture's flood level -- the highest point to which water can rise. (On a sink the flood level is the sink rim or overflow hole.)
The main drain
Plan drain lines to minimize the possibility of clogs. The general rule is that smaller drainpipes -- 1-1/4 inches for bathroom sinks and 1-1/2 inches for kitchen sinks, for instance -- lead to larger branch drains. These in turn lead to the main stack, which is the largest pipe of all -- typically 4 inches. Because the main stack is also vertical, it will rarely clog.
The main stack leads down into the ground, then out toward the municipal sewer. The underground horizontal pipe, or main drain, that runs toward the sewer line can sometimes get clogged, especially if it is an old drain made of clay pipe.
Typical Venting Alternatives

A true vent pipe must remain dry while water runs down the drain. A wet vent also serves as a drain line but is large enough that it never actually fills with water.
Venting with an Air Admittance Valve (AAV)

Window framing in this wall prevents installation of vent line, so an Air Admittance Valve (AAV) is installed instead.
Other Venting Options

Here are other common ways to tie vents into a stack. Check local codes to see which methods are accepted in your area, then choose the method that requires the least number of holes or notches in studs.