It's normal to have birds on the roof. As I type I can look over to Matalan's roof and see the private life of pigeons. I've seen a heron on there, and a falcon at one point. Birds sit on roofs.
DH was upstairs in the top room and heard what sounded like a determined effort to steal the slates from our roof. He said it sounded like a full on act of destruction. It was actually two juvenile crows rummaging underneath the edge of the slates where it meets the gutter and making a right racket. The crows ignored DH when he opened the window and reluctantly left when he told them to clear off.
I am rather worried that there may be something under there, like a mouse or a sparrow that means we will have problems. However crows are full of mischief so they could have just been enjoying the noise.
This is normal for the crow tribe. I think I have mentioned the raven that hangs around the bird of prey displays and nicks the food meant for the vulture. They are relentless, shameless and inventive. I find them far more interesting than sparrows.
For anyone interested, a YouTube video of a crows bullying a lion is here.