Umbrella and excess liability insurance are a type of policy that is designed to help protect you from major claims and lawsuits. It does so by taking care of the additional amount in excess of a General Liability or other Commercial auto policies limits that one may have. Thus, if you had a $1 million general liability policy, with a $1 million umbrella, you could cover up to $2 million per any one claim as the Umbrella is in addition to the General Liability.
An umbrella is advisable if you are going to be working on very large buildings or projects. Even if you portion of the work is very small. For example, you were working on rehabbing on room in a School. You accidentally started a fire and the entire school burned to the ground. It would cost you far more than $1 million to rebuild the school so unless there was an umbrella policy to take care of the excess you could be liable for additional charges.