I'm surprised it damaged the sheathing. I found in my previous house that
they were just dumping into the attic for many years with no apparent
damage. Your attic probably has inadequate ventilation.

I believe the previous owner likely ran the shower at a high volume setting.
This is because when I moved into the house, I noticed that my shower faucet
has no volume adjustment, but the shower head itself (Speakman 2000) has an
adjustment and happened to be set at what, to me, seemed a REALLY high
volume setting. Granted the showhead, even at full blast, is supposed to be
regulated at 2.5gpm but at the setting the previous own had, I can guarantee
this generated a LOT of steam if he took a shower for more than a few
minutes at anything above lukewarm.

J.