Rick,
It ought to 'work fine 'n last a long time'! Well,
maybe not a real long time, but a couple of years or
so (don't tempt Moma Nature LOL).
Your metal roof should make as good a ground as any
dirt/radial system, especially where you live, with the
volcanic 'stuff'. Just how 'good' will depend on the
mounting location, the pitch of the roof, and so on.
There's no way of predicting exactly what your results
will be as far as the input impedance since it will
depend a lot on how well you tune the antenna.
'Doc

PS - If it doesn't work well, send me a bus ticket and
I'll help you 'fix' it???