[TheMoyers]

I am a retired licensed professional engineer (PE), and I did have a system installed on our home here in TV. No permit was required. I have a MAJOR recommendation. Get it UL approved. Underwriter's Labs has specific standards for these systems, and if you get it approved, you will have a UL inspector come and verify that rods are properly spaced, and the grounding system mentioned by other people who responded to you, has been properly done and tested. The UL inspector took actually readings (in ohms), and ensured mine were well within the required standard. And yes, it is not required, but if you want to keep the UL approval, you will need to have it checked periodically. I do ours every 3 years.
The UL stamp costs a few hundred dollars extra. But here is what it is worth it, and why rechecks are important. There are numerous electrical connections between the roof rods, cables, and grounding rods. A cheaper system might use connectors and components that give acceptable ohm readings on installation, but as time goes by the different metals corrode at different rates, and you might find that 5 years later you have a system that has too high an ohm reading, hence ineffective.. The UL inspector checks those materials, and a recheck periodically ensures it.