I learned that people put a big stone on their chimney in Norway to prevent from trolls and I am wondering about the myth behind this story.
I don't know that this is the origin of the practice, but the story of Butterball (Smørbukk) makes reference to Trolls being defeated using a large stone placed on a chimney:
And when that was done, he scrambled up over the door, dragging the pine root and the stone with him, and one he placed over the door and the other on the Troll's chimney pipe.
But when they got to the door, Butterball threw the pine root and the stone at their heads and killed them all.
You can read the whole story here: Norwegian Folk Tales, by Peter Christen Asbjørnsen, pgs 52-56