It does not seem possible to collect only one surface area info of a solid.

It is not a big issue when dealing with one type of part or even an array of, say, a plank for a table or joists for a house.

But, if the thickessses vary for parts, and the widths and lengths do, too, it can mess up calculations for, say, paintable areas, wetted surfaces, and so on. If one is covering a wall or a floor with decorative and custom tiles or pieces of something, it is not necessary to have the thicknsses involved. Subtracting areas of sides of irregular pieces would make for a lot of work.

Suggestion: Built into the logic of solids an attribute or feature that is "plane aware". The user could then group or meta-layer assign to an additional layer (but not changing the parts' parent/sensible/adminstrative layers) that can also be used in BOMs. So, if the user lassos or groups slab or built-up solids, and hits the shortcut for surface area, the user could select from notional choices such as these:

-- skyward face
-- earthward face
-- outward face
-- inward face
-- mutually-facing each other faces
-- mutally-away from each other faces
-- toward a specified plane faces
-- away from a specified plane faces

Or whatever Tim or others think is better than what I postit here now.