1. Builders risk would cover structure and materials during the course of construction.
2. Not sure if your homeowners would cover or even have an add on for this type of work. You should review the policy and discuss with your agent.
In general, having a certificate is worthless unless it is required in a written contract that your are added as an additional insured. I think your best bet is to have the contract REQUIRE that the roofer is responsible for any damages to your property as a result of his work. This should apply to both his ongoing & completed operations (GL policy). If they refuse to add this in the contract then I would consider finding a different contractor to perform the work. You shouldnt have to obtain a separate policy (such as builders risk) to insure these operations; the roofer should have the proper insurance in place and add the applicable costs to his proposal for the work. If you have a contract that has already been signed then request the wording be added via change order.
This is how it is done for commercial construction (I work in risk management for a global developer). You can request a million additional insureds on a certificate but if there is no requirement in the contract... open your check book.
Hope this helps
