Q: At irregular intervals this winter, the downstairs toilet would gurgle violently and then all the water would drain out of the bowl. This seemed to happen when the washing machine, also on the first floor, discharged its load of wash water, and when the upstairs tub was being drained. What's going on?
—Richard Bowman, Waldorf, MD.

A: I'd bet that one of your plumbing vents is blocked. Most people think that those small pipes that stick out of a roof are there to exhaust sewage smells, and that's true.

But they also equalize the air pressure in the drain pipes so waste water can flow freely. You can see how the principle works by sticking one end of a straw into a glass of water and then covering the other end with your finger. As long as you keep your finger in place, you can remove the straw, and water inside won't drain out. But when you remove your finger, the straw drains immediately.

The giant finger blocking your roof vent could be ice or snow or debris or even an animal carcass. The result? Whenever a lot of water goes down the drain, it sucks the water out of your toilet and maybe out of the traps under your sinks. The blockage probably makes your tub drain pretty slowly, too. A plumber can remove an obstruction by running a snake from the roof down your vent system, but to keep it from happening again, you have to figure out what's causing it. If it's ice, you can replace the top section of pipe with a bigger one, up to 4 inches in diameter. Or you can insulate the pipe below the roof, or even heat it with electrical cables, but that's usually not necessary except in arctic climates. If snow buildup is causing the blockage, add an extension so the vent pipe clears the snow pack. And if animals are to blame, cover the end of the pipe with hardware cloth held in place with a hose clamp.
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