Looks like the roof is wavy and is sagging, or maybe that is just the metal? Might need to strip it and re-sheath it.

New roof over the old one adds weight so depending on how it was built that may not be wise.

If the new addition plugs into the side of it with taller walls you'll need a roof with less slope to it.

Look better? Add some high windows down the sides to let light it. You could also add a masonry wainscot to the outside perimeter.