A single clog in your toilet is usually easy to diagnose: You probably used too much paper, or you may have dropped something by mistake that doesn't belong in the toilet. It's a different matter when the toilet clogs frequently. The fault could be in the waste pipe or the vent pipe, and it can be due to hard water. Sometimes the toilet itself is the problem; some early low-flow models are prone to clogs.
The vents are the part of the plumbing system that is most often overlooked, but they can cause major problems, and clogging is one of them. If leaves, ice or a bird block the main vent opening in the roof, and air can't get into the pipes, negative pressure slows down the flow of water, and sediments can settle out more easily. If the blockage is total, water may drain from your toilets and other drain P-traps every time you flush a toilet, empty the bathtub or use the washing machine. This undermines the purpose of the toilet's internal P-trap: to keep sewer gases from entering the home. Frequent toilet clogs may be your only indication of a partial blockage -- until the blockage gets worse.
The Stuff You Flush
It isn't difficult to figure out what you should flush down the toilet, but not everyone in your house may have a handle on what you shouldn't flush. Hair can create major problems by hooking onto pipe joints and snagging solid objects. Anything that congeals in cold water, such as grease, may make it through the toilet as far as the waste fitting, only to settle out and become a semipermanent part of the plumbing. To keep your toilet flushing properly, watch what you put in there, especially if your house is on a septic system.
Federal law mandated low-flow toilets in 1992, and for a few years after that, manufacturers were still experimenting with designs. Toilets manufactured between 1994 and 1997 -- identifiable by a date stamp inside the tank -- are known to clog frequently. If you have one of these, the best strategy is to replace it. If your toilet is old enough to be heavily stained with mineral deposits, it's likely that the inlet holes around the underside of the rim are clogged with lime deposits. This makes the toilet flush slowly and incompletely and produces clogs in both the toilet's internal trap and waste pipe.
Frequent clogs in a plumbing system served by a septic tank and drainage field can indicate the need for system maintenance. If sediment has collected inside the septic tank, and it's time to pump it, water drains slowly and clogs develop. The same thing can happen if the tank freezes. Another cause of clogging in a septic system is leach field failure, and it's usually a major problem. Incursion of tree or shrubbery roots into the plumbing waste lines also causes clogs; roots affect septic systems most often, but they can be a problem even for homes served by a municipal sewer.