Q: We have a gurgling sound that erupts from our bathtub drain every time we flush the toilet. No water comes up from the drain, and it drains well when we shower. Could something be wrong with the roof vent?

A: For waste water to properly drain, there must be air behind the water, which requires a plumbing vent above the roof. In some instances, an automatic air vent or air admittance valve can be used to provide the air needed without allowing sewer gases to enter the home. Plumbing vents provide air to allow the water in the pipes to drain; it is not a vent for odors.

If you think of placing your thumb over a soda straw immersed in a glass of soda and lifting the straw out of the glass, the liquid will remain in the straw until you remove your thumb. That’s an example of a vent at work. Because the tub drain and the toilet drain are at the same height above the main sewer pipe, the vent may be partially blocked, too small, not pitched properly, or it may not extend high enough above the roof. When the toilet is flushed, the rush of water into the sewer is pulling air through the tub drain causing the gurgling noise. First, make sure the plumbing vents extend above the roof. In cold weather, simply run hot water to the sinks and look outside to see if steam is coming out of the roof vent pipes. In an attic, make sure the vents are not blocked or covered. If everything appears to be correct, you will need the assistance of a licensed plumber.

C. Dwight Barnett is a certified master inspector with the American Society of Home Inspectors. Write to him with home improvement questions at C. Dwight Barnett, Evansville Courier & Press, P.O. Box 268, Evansville, Ind. 47702 or e-mail him at [email protected].

By C. Dwight Barnett, Tribune News Service (TNS)