A supplement is a documented request to correct an insurance carrier's estimate after the carrier has already issued one. It is not a second claim, it is not a dispute, and it is not a favor. It is the part of the process where the scope of work gets corrected to match the work the property actually requires.
Carriers write their estimates from an adjuster's inspection. That inspection is fast, often from the ground or a single trip up a ladder, and it is priced by software using default assumptions. Anything the adjuster did not see, did not measure, or did not think to include is simply absent from the number. Absent does not mean denied — it means nobody asked.
What a supplement is not
It is not asking for more money on the same scope. A carrier will reject that immediately and should. A supplement adds items that belong on the estimate and were left off, or corrects quantities and prices that were entered wrong. Every line has to be justified by something: a photograph, a measurement, a code requirement, a manufacturer specification, or a policy provision.
When it applies
Almost always. Most storm claims are underpaid on the first estimate, not because anyone is dishonest, but because a single inspection cannot capture a full scope of work. The common gaps are code-required upgrades, items concealed until tear-off, quantities calculated from rough measurements, and trades adjacent to the roof that were never examined.
What decides the outcome
Documentation. A supplement is only as strong as the file behind it: the carrier's full estimate, a complete photo package, and the policy conditions that establish what is covered. Without those three, there is nothing to argue from — and a request with nothing behind it is the one kind of supplement carriers are right to reject.