I have the Can-Scan-III the base is the Larson High frequency magnetic mount for this antenna designed basically for mobile use but for a short time maybe a week I have it placed on my roof as a base antenna & I have it placed on a dryer vent (Metal) is this sufficiently also grounded ? Could this become a problem in bad weather Lightning? Damage to my radio !
RF grounds and lightning grounding are two different things.
For your RF ground, the magnetic mount is capacitively coupling to the metal in the vent. Not ideal, but you are getting something. Ideal would be an NMO mount permanently installed in a metal sheet at least 1/4 wave in length in all directions under the antenna. On VHF frequencies, this would be about 38-40 inches diameter.
Lightning ground is creating a path to a ground rod. As prcguy said, it's difficult to lightning proof any sort of residential antenna install that won't result in some level of damage.
To do a lightning ground, you'd need to have the antenna base grounded directly to a ground rod under the antenna. You'd also want a lightning suppressor on your coaxial feed line where it enters the house. The ground rods need to be bonded to your homes electrical ground rod. Complex, costly and takes some design work.