If you consistently hear scuffling overhead, you could have squirrels nesting in your roof or chimney. Squirrels will often cross roofs without causing any problems. They are only considered threats if they stop to chew on electrical wiring or somehow enter your house.
To keep squirrels at bay, many homeowners resort to homemade repellents. However, many of these repellents, like mothballs, are dangerous and toxic to humans, not to mention illegal. It's against the law in all 50 states to use mothballs to combat animal pests. Mothballs are designed to rid spaces of insects and moths, not rodents. If you were thinking of using mothballs, don't.
If you really need to get rid of the squirrels on your roof, consider other options such as traps, screens and proper roof, soffit and chimney traps.