The plumbing system that services your home includes an interconnected series of drains and drain lines that remove the water used by all of your bathroom fixtures. The drains that service your shower and tub enclosures, lavatory sinks, and toilets must have a consistently unobstructed pathway to the main sewer lines that lead to your septic system or your municipal sewer system. Problems anywhere along these drain lines or sewer lines will cause the toilet to back up into your shower.
Basic Bathroom Plumbing
The drain lines that service your bathroom fixtures each have what are known as P-traps, which are required by plumbing codes to prevent sewer gases from entering your home. Used water from the shower and toilet pass through the drains and P-traps and proceed along the drain lines to the sewer lines. This plumbing system will continue to remove used water from the bathroom fixtures without backups as long as all drains, P-traps, drain lines, and the sewer lines remain unobstructed.
Bathroom Drain Problems
Shower drains are often clogged with hair and soap residue. Toilets become clogged from insufficient flushing and foreign objects that either don't dissolve, such as paper towels and personal hygiene products, or large objects that find their way into the toilet bowl, such as toys and personal items like combs and brushes. However, drain clogs in showers and toilets are not the real cause of a toilet backing up into the shower. This problem indicates a clog further down the system, such as in the drain lines or in the sewer system itself.
Sewer System Blockage
Used water from your bathroom fixtures are eventually deposited into the sewer system, which consists of your own septic tank or outdoor pipes that drain into your municipal wastewater system. Malfunctions in the sewer system, such as pipe blockage from debris and tree roots, clogged septic tanks, or septic drainfields that become over-saturated from excessive wastewater input, rainfall or failures, will lead to backups within the plumbing system of your home. This will prevent proper draining, leading to inner system backups.
System Backup Results
Problems with either or both of the interior drain pipes and the exterior sewer system are among the causes of a toilet backing up into a shower. Plunging toilets and removing blockage from P-traps won't resolve the issue of a toilet backing up into a shower. Blockage in drain pipes may be removed using plumbing augers to "snake out" the clogs. Augers are also useful in exterior sewer lines for removing tree roots and debris. However, septic system repair may require flushing out septic pumps and tanks, repairing pumps and valves, or the drainfield itself may require repair. All of these repairs are best left to qualified plumbers or septic repair contractors, or even your municipal wastewater department.