Primary and noncontributory is an endorsement provision that makes your insurance policy respond first and without seeking contribution from the additional insured's own insurance. Without this endorsement, if a claim arises from your roofing work and the GC is named as an additional insured on your policy, both your policy and the GC's own policy might share the claim payment. The primary and noncontributory requirement eliminates that sharing arrangement.
When a GC requires primary and noncontributory language on your COI, they want assurance that your policy pays the entire claim before their insurance is even notified. This protects the GC's loss history, keeps their deductible from being triggered, and prevents disputes between insurers about who pays what percentage. For roofing subcontractors, this is a standard contractual requirement on virtually all commercial projects.
The endorsement is typically added by including ISO endorsement CG 20 01 or through blanket primary and noncontributory language built into your policy. Many modern CGL policies include automatic primary and noncontributory status when required by written contract. Check your policy language carefully, because some forms only provide primary status (your policy pays first but can seek contribution) without the noncontributory element (your policy pays first and cannot seek contribution). Your certificate should specifically state "primary and noncontributory" to satisfy most contract requirements.