A Certificate of Insurance (COI) — usually the standardized ACORD 25 form — is a one-page document from a contractor's insurer proving the business carries active liability coverage. It summarizes the coverage types, policy numbers, limits, and effective dates. Importantly, the COI is proof of a policy, not the policy itself: it doesn't grant or change coverage, it just attests that coverage exists.
In commercial work the COI is table stakes. General contractors and property managers almost always require a current COI before a subcontractor can mobilize on-site, and they typically ask to be named as an additional insured (extending the contractor's liability coverage to them, with notice if the policy is cancelled) and for a waiver of subrogation (the contractor's insurer agrees not to come after the owner to recover a claim payout). General-liability limits of $1 million per occurrence and $2 million aggregate are a common contractual ask — though that's a norm set by owners and GCs, not a statutory requirement, and large facilities may demand more.
For a building owner, the COI is a quick, verifiable filter. A contractor who can produce a current certificate naming you as additional insured — and whose coverage limits match what your lease or property manager requires — is operating like a commercial outfit. One who hesitates, or whose certificate has lapsed, is a risk you'd be absorbing yourself if something goes wrong on your roof. It's worth confirming the certificate is current and issued directly by the insurer or agent, since the document is only as good as the live policy behind it.