Whoops, I meant 2 x 4 chords. After looking at the existing loading on the structure, here are the loads i've generated that are being supported by the joists:
ACTUAL ROOF DEAD LOADS (DL):
(Estimated from visual site review)
Built-up roofing 2.5 psf
Insulation 1.5 psf
1/2" plywood 1.5 psf
Joists at 2'-0" o.c. 3.0 psf
Suspended Acoustical Ceiling 1.5 psf
Mech, Plumbing, Elec. 1.0 psf
Fireproofing 1.5 psf
Sprinklers 1.5 psf
Miscellaneous 1.0 psf
15.0 psf
EXISTING ROOF LIVE LOADS (LL): (Reducible) 20.0 psf
TOTAL: 35.0 psf
So it looks like a 15 psf dead is currently being supported by the joists, but I'm just not sure what they were designed for. The additional load on these joists will be around 5 psf, so I'm optimistic that the framing will be able to support the joists, but still want to try to find some capacity of the existing joists. Some jurisdictions are allowing roofs supporting solar panels to have a reduced live load down to almost zero where panels are covering the roof and no live load could be applied to the areas. The project is in Phoenix, so Snow loads are nil to minimal that they wouldn't add anything or much to the panel loading. Therefore, the roof seems to be ok if it was designed for 20 psf live load, but I just dont know if it was because of its 20 year age (its in good shape though).