A waiver of subrogation is an endorsement on your insurance policy that prevents your insurer from seeking reimbursement from another party after paying a claim on your behalf. Subrogation is the insurer's right to "step into your shoes" and recover money from a third party who caused or contributed to a loss. When you waive subrogation in favor of a GC or property owner, your insurer cannot sue them to recover claim payments, even if their negligence contributed to the loss.
General contractors and property owners commonly require waivers of subrogation in construction contracts because they want to avoid being dragged into subrogation disputes between insurers after a project is complete. On roofing projects, this is particularly relevant for builders risk claims where multiple parties may share fault for a loss. If a storm damages a partially completed roof and your insurer pays the claim, a waiver of subrogation prevents your insurer from going after the GC's insurance to recover part of that payment.
Most commercial insurance policies allow waivers of subrogation by endorsement, but the waiver must be in place before the loss occurs. If your contract requires a waiver of subrogation and you fail to add the endorsement before a claim happens, you could be in breach of contract. Some carriers charge a small additional premium for waivers of subrogation, typically 2% to 5% of the policy premium. Always disclose waiver of subrogation requirements to your agent when you sign a new contract so the endorsement can be added promptly.