What roofs hate about December
Foot traffic on cold tile, fasteners through shingles, decorations weighted with sandbags that sit in a valley, and cords run through a gap under a tile that then acts as a wick for rain. Each of those produces work for me in the new year, and each is avoidable.
The roof does not care how the lights look; it cares how they are attached.
Use clips, not fasteners
Every fastener through a roof covering is a hole, and holes leak. Plastic clips for gutters, shingles and tile exist for every common profile and cost very little. Buy the right type for your roof and use nothing else.
If a clip will not hold something, the something is too heavy for the roof edge anyway.
Rooftop figures and frames
Wire-frame reindeer and sleighs on the roof need to be tied to something solid, which usually means a vent pipe or a chimney, not a tile. Spread any weight on a flat piece of foam or carpet so the frame does not concentrate load on one tile or one shingle. Keep everything out of the valleys where water runs.
On a tile roof, honestly, put the reindeer in the yard.
Cords and power
Run cords along the eave and down at a corner, not across the roof surface. Never lift a tile to pass a cord underneath. Use outdoor-rated cords and connections, and put connections where rain cannot pool on them. A timer that shuts the display off overnight saves the cords and the power bill.
Inspect cords for sun damage each year; the valley sun is hard on plastic.
After the season
Take everything down in dry weather, check the eaves and ridge for anything cracked, lifted or missing, and clear any debris the display trapped. If you see a problem, get it fixed before the winter rains rather than after.
A five-minute look in January is worth a lot more than a ceiling repair in March.
A word on timers and safety
Set the display on a timer so it is off overnight and while you are away; it protects the cords, cuts the power bill and removes one fire risk from a dry December. Keep a second person at the base of the ladder for every trip up, and stop when the light goes, because the roof does not get safer in the dark.