You're gonna need your own well and backup power for the pumps to pull this off. I know for a fact that it works, assuming that you start the sprinklers many hours before the fire comes, and water much of your lawn too, not just the house itself. The house what was saved was a stone house, no flammable exterior material like plastic, drywall or chinesium.
With the right set-up and preparation, cleared/doused vegetation, filled gutters etc. a roof sprinkler can absolutely save your house. Not relying on mains water though - 50,000L tank and a diesel pump with ample fuel stored in a block/brick/cement enclosure.