How important are vent stacks and what is their purpose? — Angie's List member Mike Shadoan, Oak Harbor, Ohio

"They introduce air into the plumbing system so there's no vacuum," says Ralph Geiselman of highly rated Ralph E. Geiselman Plumbing in Pawtucket, R.I., which services Providence. "It's like when you put your thumb over a soda straw, it creates a vacuum and you can pull the liquid out with it. When you take your thumb off, it introduces air and liquid flows out."

Vent stacks are an integral part of any home and several may be needed to adequately vent multiple fixtures.

"Every plumbing system needs to have that air vent," says David Duck, owner of highly rated Mr. Rooter Corp. in Oklahoma City.

They usually come up through the roof, George Salet of highly rated George Salet Plumbing Inc. in Brisbane, Calif., says, and are made of cast iron, copper or plastic, depending on local code requirements.

"Plastic is the best selection because it lasts forever," he says. "However, where the vent penetrates through the roof, paint it with latex paint, so the UV light doesn't deteriorate it."

Salet recommends an open cover or screen on the end of the vent stack pipe to prevent debris from causing a stoppage. Vent stacks also help prevent gas or odors from sewer or septic lines from entering the home.

Experts agree they're generally trouble- and maintenance-free too.

"It's pretty rare to have any type of problem with them," Duck says.

Geiselman agrees: "They can get clogged," he says. "But since it's usually only about a 4-inch diameter pipe, it doesn't happen that often."

Mike Curran, vice president of highly rated Jarboe's Plumbing, Heating, Cooling & Electric in Buckner, Ky., which services Louisville, says if an obstruction did occur, it'd likely cost less than $200 to clear.

Geiselman says it's more common to find poorly installed plumbing vent stacks in new-construction homes that don't extend through the roof, which can lead to excessive moisture and odor in the attic.

If you suspect a problem, Geiselman suggests calling a plumber to assess it. Warning signs stacks aren't venting include a pungent smell. "Also, if you sink goes 'glug, glug, glug,' or someone flushes a sink and another sink makes a noise, that's a really good clue," Salet says.

Do you have service-related question? E-mail us at mailbag@angieslist.com.