Coat it, rework it, or replace it
A shingle roof gives you one decision and one moment to make it. A metal roof gives you three, and the difference between them is not a percentage — it is a multiple. Working out which one you are looking at is the whole exercise.
The three roads
Renew the surface. The panels are sound, the fasteners hold, the seams are tight, and what has gone is the coating. Cleaning and recoating buys years and costs a fraction of anything else on this list.
Rework the details. The panels are sound but what holds them is not — fasteners backed out, washers gone hard, laps opened, closures missing under the ridge cap. This is slow, unglamorous work over the whole roof, and it is still far short of a replacement.
Replace. The panels themselves are compromised: perforated by too many fastener holes, rusted through at the eave where water sat, or laid over an older covering that has to come off anyway.
What tells the three apart
The panel is the deciding thing, not the appearance. Surface rust and chalked paint look alarming and are usually the least of what is wrong up there. Rust through the panel — particularly along the eave, where meltwater sits longest — is the one that ends the conversation.
The second question is how many times the roof has been fastened. Every re-screwing enlarges holes, and there is a point past which no fastener holds reliably any more.
The third is what is underneath. Panels laid straight over an old shingle roof sit on something that is still ageing, and a recoat only postpones dealing with it.
Who to ask, and what that tells you
A contractor who works only in shingle has one of these three options available. That does not make him dishonest, but it does mean his answer is decided before he arrives.
Ask any bidder to say which of the three he thinks it is and why, in terms of the panel rather than the paint. The ones who can are the ones who have worked metal in this valley, and there are more of them here than in most places.
Nothing here is a substitute for someone getting on the roof. The point is to know what the three answers are before anyone tells you there is only one.