1) READ the directions. Some brands are 'self regulating' and can cross over themselves; most will self-destruct if you do this.
2) WATTS the problem? Receptacles are usually placed under the eaves. But- make sure there's enough capacity .... don't be surprised if each length needs it's own circuit.
3) Make sure there's another, separate receptacle, on a different circuit, for the Christmas lights.
4) Make SURE melt water has some place to go. It won't help if it all freezes up in the downspout.
5) Have a roofer INSPECT the existing roof for damage .... or you'll be blamed.