Yes, JAE is correct. A typical 3 or 4 ply built-up roof with embedded gravel surfacing has about 2 psf for the membrane and 4 psf for the gravel ( 2 psf of the gravel is typically embedded in an asphalt flood coat and 2 psf is "loose" on top). This type of roof system is not often used anymore (though it is certainly one of the better performing roof systems available). For roofing terminology, this is not referred to as a "ballasted" roof since the built-up roof is bonded to the substrate.
A "ballasted roof" generally refers to a single ply roof membrane that is held in place against wind uplift by loading the surface with either stone ballast or pavers. The membrane for such systems is usually not attached to the substrate, although some systems do have supplementary perimeter attachment. Since the ballast is the primary resistance to wind uplift, there has to be a lot of ballast installed on the roof. Most systems have 10 to 12 psf for ballast. If you do the wind uplift calcs, that's woefully deficient for higher wind load areas so if that's where you are, recommend against its use.
A fully adhered or mechanically fastened single ply roof system is very light. Most of them that are commonly installed these days (PVC, TPO or similar membranes) only exert about 2 psf in additional dead load. They have no gravel surfacing and no supplementary ballast.
If the roof membrane is a modified bitumen system with ceramic granule surfacing, use about 4 psf for the loading, since those systems use heavier membranes that are bonded to the substrate, but do not have aggregate ballast.
