Yes, a shower and a toilet can share the same main drain line, and this configuration is extremely common in standard residential bathroom layouts, provided it is installed in strict compliance with local plumbing codes. In modern plumbing systems, the toilet typically connects directly to the larger 3-inch or 4-inch main vertical stack, and the shower drain ties into that same drainage run downstream through a properly sized sanitary tee or combination wye fitting. Crucially, the system must include proper venting to prevent the shower trap from being siphoned dry by the heavy water surge of a toilet flush.
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