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 it, 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.
The plumbing code requires all waste lines 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.
Waste clogs aren't always located near the toilet. There can be one deep in the pipes, possibly near the point where the toilet stack meets the sewer. It may not clear out by plunging, and it may be too far away from the toilet to reach with a snake. You may have some success clearing it with commercial drain cleaners; 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.
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.
If you suspect a venting problem, check the vent openings on your roof for obstructions. Clear obstructions farther down the vent lines with a jet of water from a garden hose or a plumbing snake. If you suspect a clog deep in the waste line, find a clean-out opening and try to snake the clog from there. There's usually one just outside the house or near the point where the waste stack and sewer meet. 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.
Chris Deziel is a contractor, builder and general fix-it pro who has been active in the construction trades for 40 years. He has degrees in science and humanities and years of teaching experience. An avid craftsman and musician, Deziel began writing on home improvement topics in 2010. He worked as an expert consultant with eHow Now and Pro Referral -- a Home Depot site. A DIYer by nature, Deziel regularly shares tips and tricks for a better home and garden at Hunker.com.