If a large community of bats invades an attic there is much more concern for diseases from the animals infecting the humans living in that household, but there is also more structural damage that can be done. Bats defecate where they roost which means mounds of guano build up over time as a community of bats grows. This guano can cause molding of wooden structures in an attic as it moistens the wood. It also adds a relentless weight that can eventually break structures. As bats cluster together in an attic space they look for more room to perch and they don’t hesitate to land on important wiring. Bats may not be a very heavy animal, but a multitude of them hanging from the same wires every day can cause those wires, or even thin beams, to break.