Off the top of my head -
1) cut all vegetation close to or taller than house height back at least 6-10 feet from the roof, so they cannot jump to it. Most squirrels will not jump more than 6 feet horizontally unless spooked, but some can do 10 feet. Of course, if the roof is within about 4 feet vertically of anything they can jump up off of like a shed or fence, you have a lost cause.
2) spray the roof edges where they land with wolf or fox urine, or spray with pepper spray (though those only last till the first good rain)
3) get an electrician to sleeve your power line for at least a 3 foot length with a loose-fitting slit piece of PVC pipe if the squirrels take the powerline freeway to your roof - ditto for phone/cable lines if above ground
4) get out your pellet gun or .22 and dig out your recipe for squirrel stew
Personally, unless the skittering sound bugs you, I don't know they will do much damage unless they start storing their nuts in your downspouts or you have exposed wires on the roof for them to chew, as long as the attic and chimney and exhaust vents are all screened so they cannot get in there.