Being in the Roofing trade, you will know that there are many risks which you should insure yourself for using a public liability insurance policy. It can be harder to obtain public liability for a roofer if you use heat or work on industrial areas.
If you were to be working on a roof and dropped a tool off the building which was to hit say a car then a public liability insurance policy would usually cover you for this, or if you were to drop a tool which hit the customer and caused injury.
Insurance for a roofer can be expensive so you would probably want to insure with a broker who specialises in liability insurance who could give you cheap insurance for a roofer.