Is it permissible for this woman to own a dog to protect chickens from weasels, bearing in mind that the dog will be outside the house or on its roof? Does this prevent the entry of angels? And is a dog only permitted for the purposes explicitly mentioned in the religious texts?

It is not permissible to keep dogs except for hunting, guarding livestock, or protecting crops. Scholars have differed regarding the ruling on keeping a dog for guarding homes, and the more authoritative opinion is that it is permissible due to necessity. The dog whose keeping is authorized does not prevent the entry of angels, but it is better to keep it outside the house if possible.