How do we get 17 psf roof dead load for structural calculations

Roofing = 6.5 psf
Re-Roofing = 2.5 psf (this is because re-roofing is done without removing existing roofing)
1/2″ plywood (3 x 0.5 psf) = 1.5 psf
Insulation = 0.5 psf
Framing = 5 psf
Suspended Ceiling = 1 psf

Total Dead Load = 17 psf

Mechanical Equipment Load must be included.