You don't have to install the electrode vertically. If you have 6' of soil over bedrock, you can drive that 8' rod at a 45 degree angle. In an extreme case, you can bury it in a horizontal trench.
For a house built on solid rock with no topsoil, I'd venture that a ground rod is probably not the best choice of grounding- other methods are acceptable, too, and installing bonded rebar or a copper loop in the concrete foundation is probably going to be a better bet.
Honestly, I don't know why this isn't done more often. It's trivial for the masons to embed 20' of #6 in the footer, and leave a pigtail for connection at the panel. Why don't we ever see this? It would be well protected and offer an excellent ground- even better if it were also bonded to the rebar.
[This message has been edited by SteveFehr (edited 11-30-2006).]
