The RF resistance of the galvanized steel roof will be sensibly zero
due to the very large area of the 'skin'. (relative to diameter of
radial wires).

If you wish to use radials they may just as well be attached to the
edges of the roof. This will effectively increase the area of a small
roof.

The difference in received signal strength compared with an inverted-V
is probably due to the difference in polarisation. A vertical on the
roof may be better for DX.
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Reg, G4FGQ