It's natural to suspect a clogged waste line if your toilet overflows, and nine times out of 10, that's the problem. If you can't clear the toilet clog, it may be too far down the waste line for your tools to be effective. The possibility also exists, however, that it isn't the waste line that is blocked but the vents. If you have a septic tank, it may be full or not functioning properly.
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Waste Line Venting
The plumbing code requires all waste lines (drain pipes) to be vented. Without venting, the vacuum that water creates as it flows through the pipes can suck water out of drain P-traps and may even be strong enough to stop the flow of water altogether. A toilet releases a large amount of water in the pipes in a short time. If the water can't flow, it has nowhere to go and backs out of the toilet. It may also come out of other drains connected to the toilet waste line, including the shower and tub.
Other Drain Clogs
Drain clogs aren't always located near the toilet. There can be clogs farther down in the pipes, possibly near the point where the toilet drain meets the home's main drain, which ultimately leads to the septic tank or city sewer main. A clog here will not likely be cleared out with a plunger, and it may be too far away from the toilet to reach with a toilet snake (most toilet clogs occur in the toilet itself).
You may have some success clearing a deeper clog with a commercial drain cleaner, but if there is a lot of standing water in the drain, you'll probably need to pour in a large amount of cleaner to make the water acidic or alkaline enough to dissolve it. Therefore, the best solution is to snake the main drain with a drain snake or, if necessary, a sewer auger.
Septic System Problems
If your house uses a septic system, check your records to find how long ago it was pumped. If it has been more than seven to 10 years, the tank may be unable to accept more water. If your tank is located on lower ground than the leach field and has a pump to transfer water to it, the pump may be broken, the electricity disconnected, or the float, which gives the pump the signal to come on, is faulty. If the pump isn't working, water will back out of the tank and may overflow from your toilet.
Septic problems usually require professional attention. If your tank is full, you'll have to get it pumped before you can make any other repairs.
Clogged Plumbing Vents
If you suspect a plumbing venting problem, check the vent pipes on your roof for obstructions. Clear obstructions farther down the vent lines with a jet of water from a garden hose or by running down a plumbing snake. If you suspect a clog deep in the waste line, find a cleanout opening and try to snake the clog from there. There's usually one just outside the house or near the point where the main drain and sewer drain meet.