A pipe froze while I was away. Am I responsible?
Possibly, if the heat was turned off or down and the policy conditions required it to be maintained. The owner’s insurer pays for the building and can seek recovery from you. Your liability cover is what answers, and the figure is a building repair rather than a contents claim.
What should I do before leaving for a week in winter?
Leave the heat at a reasonable setting rather than off, let the owner know you are away, and ask whether they want anything specific done. Those three steps cost nothing and prevent almost every version of this claim.
Does my renters policy cover the damage to the building?
Through the liability section, if you are held responsible for it. It does not cover the building as property, because the building is not yours. That distinction is why a minimum liability limit is a poor idea in a cold climate.