We have a problem with our bathroom ceiling. The portion that is missing is due to a leak that was happening on the roof around the chimney. The previous owners paid someone to fix it, but it was not fixed properly. It leaked down and eventually ruined the plaster, and I'm assuming the lath that's up there too, which has been exposed for some time. I'm concerned about the steam from all the showers we've taken while it's been exposed. Well, my question is, how would I go about repairing this open space? Do I tear the whole ceiling out and replace with drywall? Can I just patch the hole with drywall then paint over it to conceal the seam? Thanks!