No problem. As for the staining, there is a product called Kilz, it's a primer and stain-stopper. If you just paint over the stain it'll come through again so treat it with Kilz first, a couple of coats, then paint over it. If it's a non-painted coating on the ceiling you will probably have to paint the whole hallway so it doesn't look odd but the other rooms should be ok if the colour difference isn't too noticeable. An airless sprayer is your best bet for painting the ceiling if the coating is too delicate for a roller. Over the crack itself I'd use fibreglass tape then mud it. If there is mould up there, you'll want to remove all affected insulation and spray the affected areas with a mould treatment. At least we know what it is now, and it's not structural. When I bought my house we had a mystery ceiling stain but the valley on the roof was quite obviously done for, as the previous owner had attempted a repair. We replaced the roof a year or so ago. You should be ok just replacing the boot, or caulking it. In the meantime tape over the joint might even suffice if you can get up there.

I love all of your comments, thank you so much! You are all so much smarter than me at this. J I now have clarity on which direction to go. I completely agree—and hadn’t thought about—making the marble into a piece that is a non-permanent part of the house we can take with us. I may also try to incorporate it into at least one element in the house but will make that decision down the line. I am going to purchase a few of my favorite pieces now and worry about it later. I told my husband also that I would love a dining room table if we can get a large enough piece. Once this is all said and done I will definitely let you know what we did and post final-product pictures. (haha in a year or so I would think) And msmeowàI live in Tampa so take solace knowing that not all in/near Central Florida use a scorched-earth approach!

Thanks for that info. I probably should have mentioned my odd vent arrangement. Laundry is on the second floor. Our vent goes straight up the wall about 18" then through a secondary lint filter. After that, straight up into the attic, about another 5'. Makes a 90-degree turn and runs 3' laterally into a Fantech booster in the attic. Exiting that, a lateral run of 3' another 90-turn, and runs about 6' up and out the attic roof. Yes, complicated -- thus the booster. I kind of though the booster would hold back the damp like that damper. It just seems like a lot of vent for damp to travel down.

Katie, is this Plaster? If it is, it is the cause could be anything. Unkeyed plaster separated from the lath due to a seasonal movement, lumber drying and shrinkage or a stress crack caused by to much stuff in the attic, etc Most common cracks in plaster are cosmetic unless you see more signs of failure, on the floor below or in the basement like foundation crack, beam/floor settlement, etc. Cracks in plaster less than 1/8-inch thick are considered to be stress cracks and pretty much a normal occurrence and cracks over 1/4" considered to be more serious and should be investigated... On your picture, it looks cosmetic, if this was the connection point where the new extension was connected to the main house, most likely the new lumber dried up and made the plaster loose the grip and over time with seasonal changes it got worst...I would add some screws to secure the plaster if its lose, clean out the crack, fill it with caulk and leave a slight indentation, let it dry and tape and patch the ceiling and keep an eye on it. Good luck