Q: Four of the five toilets in my house overflow frequently—two of them clog up every month and it's costing a lot of money to snake them out. I've used three different plumbers and none has been able to come up with a solution that lasts. Two of the toilets are on the second floor, two on the first floor, and one is in the basement. Some are of the low-water-consumption type, but not all of them. I'm at a loss.

— Andy, Livingston, NJ

A: Richard Trethewey replies: The fact that this is happening to more than one toilet on more than one occasion tells me it's probably not a toilet problem. And it's not the main drain leading away from the house; if something were plugging that, you'd see overflows at all the lower fixtures as the pipes backed up. So I think what's happening is more likely due to a blocked vent, a vertical pipe connected to all plumbing fixtures. The vent pulls in air from the outside, typically through a roof vent, replacing the air that is sucked down the drainpipe with every flush.

To see what happens when a vent is plugged by, say, a dead animal or construction debris, put your finger on one end of a water-filled straw. No matter how you turn the straw, the water won't come out until you lift your finger and let air in.

You may also have "wet venting," a configuration where the drain of one fixture (or more) serves as the vent for another. This setup could clog more than one in which each fixture has a separate connection to the main stack. Either way, the solution is to hire a plumber to inspect and, if necessary, clean out your vent pipes as well as the main vent stacks, either from the roof or from the main drain's cleanout.
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