I've got a feeling you know what you're talking about.The solution to holes through the underside of conventional sheet metal roofs is to stick to air rifles (if .177 caliber) that don't exceed about 500fps.
This is why inside the barn, pump guns (the Crosman 2100 is a very good one) and pumping only five to six times instead of a full ten pumps are the way to go.
If outside and with a backstop that can withstand it (circular shaped much thicker steel corn cribs, cement silos, etc.), you can step up to even the likes of a the more powerful pre-charged pneumatics that go up beyond 1000fps.
As a side note, it's a mistake to go for the most powerful air rifles possible that go beyond the 950fps-1000fps level because airgun pellets destabilize when the shock wave catches up with them soon after they go out of the barrel, which in turn throws off your accuracy quite badly.
You should no no faster than 900 fps for that reason.
Also, you can use a pre-charged pneumatic even indoors with a sheet metal barn roof if it has a regulator that allows you to limit the velocity down to the 500fps level, which many (Benjamin Marauder) can indeed do.