Tub Drain Frozen !!better!! Link

Ice expands as it freezes. This expansion exerts tremendous pressure on pipe walls. If your drain pipe is older cast iron or thin plastic, the ice may have cracked the pipe or separated a joint.

It seems counterintuitive. Your bathroom is inside your heated home, so how does the drain freeze? tub drain frozen

If the ice plug is thick, heat alone may take too long. Ice expands as it freezes

Once the water begins to flow, your job isn't over. You must assess if the freeze caused damage to the pipe. It seems counterintuitive

Once you've thawed the drain, take these steps to ensure it never happens again:

If your tub has an access panel on the wall behind the plumbing, open it during extreme cold snaps. This allows the warm air from your home to circulate around the pipes, raising the temperature in that cavity and preventing the trap from freezing.