If your shower drain is backing up or draining sluggishly despite having no surface hair clog, the issue is typically caused by a ventilation stack blockage on your roof or a partial blockage further down in the main branch drain line. When a roof vent pipe becomes clogged with leaves or bird nests, air cannot escape the plumbing system, creating a vacuum that severely restricts wastewater flow across household drains. Verification: Check if other plumbing fixtures gurgle when you flush the toilet, which indicates a venting problem.
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