want to add PV panels onto older house. it was build in 1960.
To what I understand the code requires structural upgrade to meet current code requirements whenever new items are installed onto roof.
The city requires 100 psf load (i.s. snow-load) + dead load the structure has to support. The dead load is all the materials mounted on incl. rafters, plywood, roof top and PV system. In my case the total load is about 118 psf.
My first estimates on the beam, span and beam support I calculated is that is supports about 65 psf, guess that must have been the code when it was build.
Don't really want to modify anything on the structure of the roof.
Any thoughts?
Maybe use long racks (need a span of 18') to support itself and those racks are resting on the supporting walls and not on the roof - therefore not adding any load to the roof?

You can't guess at this, you need a PE to design and stamp plans that you would submit for the permit of the PV system. This is not a DIY job.

If this was my project, I would just build a ground mount system for the panels