I have a Butternut Vertical (as a secondary antenna) and a regular radial
system set up with it. I also have a metal roof on my house (here in the
country in Western Washington). I have been wondering if I can use the
metal roof as the radial system/ground plane for this antenna. Haven't
tried it yet. Would this work or would it detune the antenna?

I did that once with a HF6V and a metal roof about 40' above ground.
It seemed to work fairly well. The tuning on the Butternut is fairly
touchy on several bands, so yes, you will probably need to retune it
between roof mounting or ground mounting (I did).

Tor
N4OGW