Will home owners insurance cover a detached garage with a caved in roof that needs replacing?

It depends on what your policy says, and what caused the roof to cave in. If it caved in because it was not properly maintained, then the answer is most probably no (say for example the garage is 50 years old and the roof caved in because it just rotted away). If it caved in because it was covered with heavy snow and ice, or a storm or other act of God caused it, then it may be covered if your policy covers outbuildings or other improvements on your property. Start by talking to your insurance agent, but don't give up if they tell you it isn't covered. Read your policy. If it sounds like there is any possibility that your policy might cover it, appeal the denial of the claim and consult an attorney.