Original finish in good condition on the backside of roof

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.

Modern decking did not appear until after 1900.

This 100+ roof still is free of leaks, and “looks good”– only requiring a periodic maintenance coating on the top side of the shingle.  Only rarely have I seen rust on the underside, except around chimneys where all styles of roofing wear first.