I have an inverted-V, approximately resonant on 40, 20, 15 and 10m.

I have tried numerous verticals for comparison, but on these bands the
inverted-V always receives signals on these bands better or equally
well, at best. The verticals have been well grounded (using a "large,
good quality" metal sheet roof as the ground) and I have done my best
to minimize losses. The verticals have been anywhere from 1/4 to 5/8
wavelengths long. But they are always outperformed by my inv-V on
these bands. Only on 80 and 160m are the verticals better.

Now, this leads me to wondering if the radio waves arriving at my QTH
tend to be largely horizontally polarized on the higher frequencies?
Or can there be any other explanation that "good" verticals, even 5/8
wavelengths long are never better than the inv-V. Any thoughts?

The verticals and the inv-V were all mounted on the metal roof of my
house. The house is located in a poor-quality ground area (rocks,
shallow vegetation), the direction to Europe is across a low hill
approx. 1/2 mile away while the direction to America is across the
open sea approx. 1/8 mile away from my house.

73 - Kristinn, TF3KX