Concrete tile roofs have gaps between the trim tiles and the fascia that are just big enough for the bats to crawl through. There are other ways too probably. Whoever said it must stink, you bet it does, you'd be able to smell the urine just standing on the roof. When I first started roofing we had to go change some broken tiles on what we called the bat house. You could smell the piss and hear the little fuckers under the tile (there's an inch and a half gap near the headlap under the tile). The homeowner said that all the neighbors would get together at dusk to watch them come out. We were supposed to pull the tile off and replace the felt but my uncle got into it with the superintendent in the tract and told her to shove the house up her ass.