Older embossed tin shingles and standing seam panels were installed over slats that sit on beams. Sometimes there is only an inch or two between slats, or in this case, 6 or 7 inches.
The finish on the underside is the original application, and is usually in good shape, thereby protecting the underside of the shingles/panels from rust.
This 100+ roof still is free of leaks, and "looks good"-- only requiring a periodic maintenance coating on the top side of the shingles
