An umbrella (or excess liability) policy provides additional liability coverage on top of the contractor's primary policies (commercial general liability, commercial auto, employers' liability). When a claim exceeds the primary policy limit, the umbrella picks up where the primary leaves off. Standard umbrella limits in commercial construction run from $1M to $25M, depending on project type, contract requirements, and the contractor's underwriting.
Umbrella coverage is increasingly required by owners on commercial projects: $5M umbrella is common on mid-size commercial, $10M+ on larger work. The umbrella sits above and follows the primary policies, meaning it generally covers the same incidents but at higher limits. Some umbrellas are "true" excess (they pay only after primary is exhausted) and others are "drop-down" (they replace primary if primary is invalid). Read the umbrella terms carefully; assumptions about what triggers coverage frequently turn out to be wrong at claim time.