Putting metal over the shingles that are there
It is done here all the time, and it is not automatically wrong. Laying metal over an existing shingle roof saves a tear-off and a dumpster. It also puts a fifty-year covering on top of something nobody has looked at.
Why it happens
Removal is a real share of the cost of a roof — labour, disposal, and the risk of weather while the house is open. Skipping it is a genuine saving, and on a sound deck with a single layer of shingle beneath, it can be a defensible piece of work.
It also goes quickly, which matters on a farm property where the crew is fitting the job between other things.
What it costs you later
You lose the one moment when the deck can be inspected. If boards are spaced, soft, or previously repaired, nobody will know until the metal comes off — and that may be decades away, or it may be when a fastener will not hold.
You also add weight and trap a layer that can hold moisture. Whether that matters depends on how the metal is set: furring strips over the shingle create an air space and let the assembly dry; panels laid flat on the old covering do not.
And a roof already carrying two layers of shingle is a different proposition entirely. That is generally a tear-off, whatever goes on top.
The questions that settle it
How many layers are under there, and how does the bidder know? A tape measure at the rake edge answers it in a minute.
Is the metal going on furring, or straight down? If straight down, ask what happens to the moisture that gets into the assembly.
And what happens at the eaves and the penetrations, since everything that was flashed for the old covering now has to be reflashed for the new one at a different height.
This page is about a common local practice, not a recommendation either way. A good crew will tell you honestly which of the two your roof is a candidate for.