When an insurance carrier's adjuster inspects storm or wind damage to a roof, they write an initial estimate covering what they observed. On many roofing claims, that first estimate misses line items or quantities that only become clear once a contractor is actually on the roof, or once repair work has started and reveals damage the adjuster couldn't see from the ground or a single inspection pass.
A supplement is the mechanism for correcting that gap: a contractor documents the missing or underscoped work — with photos and, where relevant, the specific building-code requirement that applies — and submits it back to the carrier as an addition to the original estimate. It isn't a dispute over price on items already agreed to; it's a claim that the original scope of work itself is incomplete.
In practice, a supplement package restates the estimate in the carrier's own format (Xactimate is the estimating software most carriers use) with the missing line items added, each with a note explaining why it belongs — a code citation, a measurement, or a specific photo. That justification matters: an adjuster reviewing a supplement is looking for a reason to approve each new item, not just a longer estimate.
The carrier then reviews the supplement and decides how much of it to approve. What happens next — how the contractor gets paid for approved work, and on what terms — depends on the specific arrangement between the contractor and whoever is billing for the work; that's a separate question from how the supplement itself gets built and submitted.