We have a gabled-roof single-story detached garage with horizontal 2x4 beams spaced four feet apart and a 20' span. There are two vertical 2x4 beams connecting the two horizontal center beams to the roof. There are also 1x6 diagonal braces connecting the roof to every joist.
I'd like to install plywood flooring above most of it for storage. Nothing that we put up there will be too heavy, but I'm certain I'd need to provide additional support. I would plan to walk on the floor to store and retrieve things.
What size and spacing of supplemental joists would be reasonable to support a floor? How should they be safely secured to the existing structure? And what thickness should the plywood floor be for reasonable support? Should I consult with a structural engineer?
The following may be duplicates.