Vermin or wild life of different kinds seem to love setting up house under the roof and in the ceiling cavity. In probably half the roofs I have opened up in Brisbane, I have found Red Back spiders, so I guess that the dry, warm conditions must suit them perfectly.
Back in the 1980s I ended up taking 3 ute-loads of birds nests to the dump from a house in Carina. If there had been a spark from wiring or a fire for any reason in that house, there would have been no stopping it because of all the straw and twigs in the ceiling. The amount of bird lice was also incredible and I'm sure that I scratched for the next week just thinking about it. Thankfully today sparrows are virtually non-existant in Brisbane so this kind of thing rarely happens.
Carpet snakes seem to like living in a ceiling, and the record for Stark Roofing is 13 at a house in Sherwood. This was also back in the 1980s and we took them to the RSPCA that was at Annerley at the time. If they need to be removed today however, a licenced snake handler will be called at the owner's expense to take them away.
Ordinarily we check with the owner to see if the snakes should be left where they are or removed. Some people like to have them in the roof cavity to keep down rats, mice, possums or birds that might potentially live up there. If they are left in the roof cavity, we have to first of all identify their entrances or exits, which are usually near a tall tree that is close to the house, and make sure that they remain there so that the snake can get out. Some owners assure me that snakes can climb up and down downpipes, but I've never seen this happen.
Once I had a couple of Kiwi blokes, who were new to Australia, working for me. We were all on the roof and when we lifted up the first sheet I spotted an old snake skin snagged on a rafter. I said to them "If you see a snake when we lift up the next sheet, put it straight back down again." As I started to lift the sheet, I wondered why it was so heavy and so turned around to find them both bolting down the ladder.
Possums also seem to like living either in a roof cavity, or under the eaves so its not surprising that we often disturb them when re-roofing or re-guttering a home. If it is in the roof, we aim to possum-proof ceilings with screws, wire-netting and flashings to prevent re-infestation. If they will not leave the ceiling while the the roof is off, a possum catcher must be called, otherwise they will be trapped undert he new roof and die.